biasing preamp tubes?

[daz]

i built a 18 watt and the 4 stage preamp is cascaded with 2 volumes and a master and has a good bit of gain. I have a bit of a issue with the stages in V1 tho. It's a bit bright and buzzy when the first volume is cranked. as it's turned down it goes away at about 12:00, but then of course so does the gain. I don't mind losing some gain, but not quite that much. So I thought maybe the cathode and plate resistors might need to be tweaked so that the tube is biased correctly, but i know nothing about doing this on a 12ax7. The plate is 100k and the cathode is 1k with a 1uf bypass. How low can i go on the plate resistor? i tried larger cathode resistors but they tend to cause it to sound a tad dead by the time i try one large enough to kill the buzz.

so i'd really like to know how to tell when a preamp tube's bias is withing ok specs, plus whats the difference in effect of changing cathode resistors to reduce gain as opposed to changing plate resistors to do the same?

[tbryanh]

Please post a schematic or a link to one. Thanks

[cbarrow7625]

Sounds like you may need more voltage swing in the cathode and/or plate circuits. Distortion in generated in a few ways in a tube preamp, the ugliest sounding of which is plain old clipping of the DC rails. Good, smooth harmonic distortion created by compression & soft clipping of the signal is the most musical, desired sound. "Fizzy" distortion is often a sign that you are clipping the DC rails and/or just driving one of the stages further down the signal path too hard (and causing it to clip when you don't want it to). The highest gain distortions require the clipping but it is done in such a way that the tube is driven into full on square wave clipping and tons of harmonics are generated (including odd order). When you are just getting into clipping you get increasing amounts of odd order harmonics that can be out of balance with the sound you are going for.

Counterintuitively, you may actually need to increase the values of the plate resistor & the cathode resistor to increase the voltage swing before clipping. THis will also result in more gain in that particular stage. Depending on what you have downstream, this may cause you other problems.

What happens when you turn down the second volume control (which I assume is between stage 2 & 3)while the first one is cranked? Does the bright & buzzy sound go away? If not, I suppose that you have localized the problem to the 2nd stage. That's good! Tweak away on the second stage and see what you get.

As far as biasing is concerned, there is not much you can do to hurt the tube. For a 12AX7 plate resistors as small as 27k are used sometimes to reduce gain (Marshall SLX preamps) and plate resistors as high as 470k get used regularly in many circuits. The most common values lie between 100k-220k (all along the range). Try differnet values and see which ones work in your circuit.

For the cathode circuit, a lower value resitor is going to give the sound more of a "hard feel" and "attack". Higher values will make the sound mushier. For the second stage in a hi gain preamp, I think 1k is probably too low a value (typically) and to get the sound you want you may have to comprmise a little bit on the "feel". I would recommend trying 1.5-1.8k to start. Those values will feel a little "softer" than the 1k but not so much that it just feels like mush. Anything much higher than about 3.3k will usually begin to feel ovely soft but those higher values can be used for "effect" to actually create more non-linear distortion in that tube stage. The 10k Marshall cathode resistor & the world famous 39k cathode resistor found in the SLO100 (and all of it's wannabe knock offs) are used for just that. The high cathode resistor values greatly reduce the gain of the tube stage but the increase dramatically the distortion created withing the tube itself (not a bad trick really). The trade-of is the reduced "attack" and "feel" of these kind of circuits.

Hope that's enough to keep you tweaking. A schematic would be helpful if you want to dive into more specifics.

Chris

[tbryanh]

The highest gain distortions require the clipping but it is done in such a way that the tube is driven into full on square wave clipping and tons of harmonics are generated (including odd order).

Transistors can provide full on square wave clipping too. Why do you think tubes are still used for this?

[daz]

Great stuff, thank you !!!! Really, thats some of the best detail i've found so far. I tried your suggestion of a higher (3.3k) cathode R on the second stage and it did indeed seem to soften up the tone, which was another issue i had that was really bothering me. However, now theres too much gain and i'm wondering what you would recommend to lower ? The plate is already 100k. I guess the smart thing to do would be lower the stage 1 gain because i'm sure it's stage 2 thats being hit too hard. reaosn being is that when i crank the first volume i hear blocking distortion starting to happen. But with stage 1 down a bit, even with the seond volume cranked the blocking isn't there anymore. So the 3.3k worked great on stage 2 to soften the tone, (even thinking to go higher because it's no where near mushy) but i now need to figure out how to lessen the gain in the first stage. But with the plate at 100 and cathode at 1k all i can do is go up from there, not down? correct? And so how do i reduce gain? Should i use a fairly large grid R into the second stage?

EDIT: I also just noticed that the softness i gained seems only to affect the high gain tone. But when i turn the guitar volume down to clean up, the bridge p/u sounds very muddy and dull and hard still. Yet the single coils are totally opposite when i turn down. On other amps usually they are both affected the same way.

Sounds like you may need more voltage swing in the cathode and/or plate circuits. Distortion in generated in a few ways in a tube preamp, the ugliest sounding of which is plain old clipping of the DC rails. Good, smooth harmonic distortion created by compression & soft clipping of the signal is the most musical, desired sound. "Fizzy" distortion is often a sign that you are clipping the DC rails and/or just driving one of the stages further down the signal path too hard (and causing it to clip when you don't want it to). The highest gain distortions require the clipping but it is done in such a way that the tube is driven into full on square wave clipping and tons of harmonics are generated (including odd order). When you are just getting into clipping you get increasing amounts of odd order harmonics that can be out of balance with the sound you are going for.

Counterintuitively, you may actually need to increase the values of the plate resistor & the cathode resistor to increase the voltage swing before clipping. THis will also result in more gain in that particular stage. Depending on what you have downstream, this may cause you other problems.

What happens when you turn down the second volume control (which I assume is between stage 2 & 3)while the first one is cranked? Does the bright & buzzy sound go away? If not, I suppose that you have localized the problem to the 2nd stage. That's good! Tweak away on the second stage and see what you get.

As far as biasing is concerned, there is not much you can do to hurt the tube. For a 12AX7 plate resistors as small as 27k are used sometimes to reduce gain (Marshall SLX preamps) and plate resistors as high as 470k get used regularly in many circuits. The most common values lie between 100k-220k (all along the range). Try differnet values and see which ones work in your circuit.

For the cathode circuit, a lower value resitor is going to give the sound more of a "hard feel" and "attack". Higher values will make the sound mushier. For the second stage in a hi gain preamp, I think 1k is probably too low a value (typically) and to get the sound you want you may have to comprmise a little bit on the "feel". I would recommend trying 1.5-1.8k to start. Those values will feel a little "softer" than the 1k but not so much that it just feels like mush. Anything much higher than about 3.3k will usually begin to feel ovely soft but those higher values can be used for "effect" to actually create more non-linear distortion in that tube stage. The 10k Marshall cathode resistor & the world famous 39k cathode resistor found in the SLO100 (and all of it's wannabe knock offs) are used for just that. The high cathode resistor values greatly reduce the gain of the tube stage but the increase dramatically the distortion created withing the tube itself (not a bad trick really). The trade-of is the reduced "attack" and "feel" of these kind of circuits.

Hope that's enough to keep you tweaking. A schematic would be helpful if you want to dive into more specifics.

Chris

[cbarrow7625]

Yeah, they can but its still a different transfer function. Transistors tend to accentuate odd, higher order harmonics. Tubes still clip softer even when hitting the rails so they tend to accentuate lower order harmonics and the even harmonics are ususally higher in amplitude than the odd harmonics.

Its also a matter of voltages too. If you could run transistors on 250-300V rails, I'm sure the clipping would be much "softer". 100 VAC peak signal hitting 80 volts of headroom clips a lot softer than 100V peak hitting 15 volts of headroom in a solid state circuit.

[cbarrow7625]

If you have blocking distortion going on, chances are that your coupling cap going into the second stage is too large and/or the grid resistor is too large. What matters with blocking distortion is just the time constant formed by the cap/resistor combination.

A neat trick for this is to put a lower value resistor (like 100-250k) from the wiper to ground on the volume pot. That way, when the volume is turned all the way up you reduce the time constant because the impedance of the pot seen by the coupling cap changes from 1M to 100-250k (depends on the resistor you use, of course). This will also reduce the bass repsonse of the signal when fully cranked (something you typically want anyway to avoid farty-ness). When the volume pot is turned down, the cap sees a larger resistance and more bass is passed through the cap/resistor combo.

Now, if that doesn't work you will need to add a voltage divider before the stage (not just a series resistor). A series resistor into a tube grid does not change signal level. you need to put something like a 470k series resistor + a 470k resistor to ground following it into the grid of the stage. That will reduce the level going in.

Iwould recommend keeping the first stage plate resistor at 100k but increasing the cathode resistor to 1.5k-1.8k (maybe even up to 2.7k if you want a Marshally kind of grittier sound.)

Increasing the cathode resistor up to about 3.3k will usually end up giving you maximum gain for the stage. Any higher than that and the gain starts to go down again. You can use that to your advantage.

If you are not fond of the sound of the resistor on the wiper of the volume pot then just reduce the value of the coupling cap between stage 1&2. That will have a similar effect but it may get too thin sounding as you turn down the volume.

Keep us updated.

Chris

[daz]

Ok, i tried the voltage divider and wasn't happy with the resulting loss of gain. It dropped way too much. however, i may try a 1 M for the grid>ground R and see how that does. The pot resistor i already tried. The coupling cap would cut too mych high end because i just finally got it to where it's got just enough highs as is. So the quest goes on. But your first advice has already helped me a lot so your help is appriciated in any case. thanks. I'll keep tweaking and let you know if i find the magic bullet.

EDIT: i tried a 470k at stage 2's grid, tho w/o the ground resistor. i tried this before with a 100k and for some reason i didn't like it. this time it seemed to work quite well tho. maybe placebo effect? Who knows, but it seems good now. I'll just have to play with it a while to be sure. but don't get me wrong..the amp never sounded bad. It's as Nigel says, "nitpicking really". :D But seriously, it's been a killer tone since i cascaded the stages. Just trying to perfect it is all.

[cbarrow7625]

Cool. Sounds good. Yeah, I suppose the 470k series resistor reduced the current enough to the tube to stop the blocking distortion. I forgot about that effect. Glad to hear it worked.

I know the feeling though...it's all about the tweak. The final tweak & the next "final" tweak, and the next one....... I have a 5 channel amp with 12AU7's as output tubes that I have been "tweaking" on fo about 5 years now. I just need to wring out that last little fraction of perfection and I'll be done...no, really.......

[daz]

I know the feeling though...it's all about the tweak. The final tweak & the next "final" tweak, and the next one....... I have a 5 channel amp with 12AU7's as output tubes that I have been "tweaking" on fo about 5 years now. I just need to wring out that last little fraction of perfection and I'll be done...no, really......

LOL!!! So true and funny. I tell myself i want to get it nearly perfect and be done with it, but i fear that may never happen. On a serious note. being my first amp i am quickly learning that tweaks that accomplish what you wanted may after all not be quite so great as you 1st thought. After a day i'm now wondering whether i like that resistor there ! It got rid of that blocking distortion, which by the way was just beginning to happen at 10 but would go away by turning the 1st gain down a bit. But it also left me with a less elastic feel to the string....a bit less sag i think, which was the first *symptom* i would get as i turned the 1st volume up even before any of the nastiness began. So maybe it was GOOD after all and i just needed to use a smaller pot to limit the output. Or maybe that 470 just needs to be smaller, or maybe or maybe or maybe.... LOL. :) crazy stuff man, but fun !

On a serious note, at this point there is really only one thing i desperatly need to put the amp squarely in the "best i've owned" category. Thats would be the midrange. The cathode resistors you suggested really helped. But it's still there to a degree, and theres something else that i think is probably caused by the same thing thats causing the hard mids. that would be the tome you get when using a strat with high gain and playing single notes up around the 10th fret. That fat hollow sound that sounds sorta flute like. Thats just not there at all, and it's a sound i use and love a lot. I don't know if it could have something to do with the output section, but i have tweaked the pre ad nausium so i wonder if it does. I've had other EL 84 amps that do it so it's not the tube type. I'm certain tho that if i could get that tone the hardness would be gone and visa versa.

[cbarrow7625]

I hear ya on the strat thing. It's difficult. "The Tweak" is why I have 5 channels. It's really hard to make asingle channel do everything you want.

With 4 gain stages you end up with a fairly symmetrical clipping. Each stage basically clip hard on half of the wave. The negative swinging part of the wave can only swing down to the cathode voltage. The positive going half of the wave can swing up until it hits the DC anode voltage. The second stage only provides the beginnings of a soft clip. The 3rd & 4th provide a hard, symmetrical clipping of the full wave.

A 3-stage preamp will have a tendancy to give a more assymetrical cliiping. That is where that real nice strat tone often lies. It's the assymetry of the clip that can make a strat sound like its full & singing. Not having a 4th stage to completely clobber the signal and turn it into full on high harmonic squished hi gain glory helps too.

It's hard to get that great strat sound with that kind of setup.

For your other issues, have you tried just putting a 100k-470k resistor in series between the coupling cap & top of the volume control? It sounds like you may just need to knock down the signal a little bit to get rid of the nastiness. Remember to try combinations of all of this stuff too. Quite often subtle changes in many differnet locations (rather than one big change in one place) can make all of the difference ingetting the "glory tone" and missing it altogether. Tweaking is all about stringing together a bunch of subtle changes to get that perfection squeezed out.

[daz]

Well maybe i should try eliminating a stage, tho then i won't have the amount of drive i'd like to have. that would be fine for gigging, as i don't use near as much as the amp has and i always get more if i do need it with a clean boost.(not to mention with the output stage driving at stage level the gain would have to go WAY down anyways) But since i don't play out anymore with rare exception, it's a home amp and the gain is more important to me there because the low volumes i need at home, the only decent sound i get are gain tones.

So i suppose i could try it, tho i'm not quite sure i want to unless i could be sure the tone i'm looking for lies there. But i'll take it into consideration. I'll try the resistor between the pot input and cap too. Thanks.

[daz]

I just realized it was a simple move of a couple wires to remove a stage, and i did so with no sucess as to that sound. Sounds the same but less gain. So i'll give that resistor on the pot a try.

[daz]

I just realized it was a simple move of a couple wires to remove a stage, and i did so with no success as to that sound. Sounds the same but less gain. I just that resistor on the pot a try to and no go. But i'll try anything else you can think of, so if you have any more suggestions, the iron's still hot.

[Jag]

After a day i'm now wondering whether i like that resistor there ! It got rid of that blocking distortion, which by the way was just beginning to happen at 10 but would go away by turning the 1st gain down a bit. But it also left me with a less elastic feel to the string....a bit less sag i think, which was the first *symptom* i would get as i turned the 1st volume up even before any of the nastiness began. So maybe it was GOOD after all and i just needed to use a smaller pot to limit the output. Or maybe that 470 just needs to be smaller, or maybe or maybe or maybe....

For your other issues, have you tried just putting a 100k-470k resistor in series between the coupling cap & top of the volume control?

+1 on what cbarrow7625 said. I've got an SE amp I built that was too nasty above about 8. I put a 220K in series with the coupling cap and the top of the 1M volume control and it sounds great. I think, in your case, 470k is just a bit too big. Just out of curiosity, what is the value of your volume control?

I hear you on the "day after the mod blues". No matter how good a change sounds to me, I always have this little devil on my shoulder saying "are you sure you like that" :mad:. It's the same feeling I get the day after I buy a new guitar. The little devil says, "you've got three Strats, a couple Tele's, a PRS, and a bunch of acoustic instruments, do you really need that 335?".

It's an addiction!:)

[daz]

Just tried a 220k. No good still. I'm really finding that it's a big problem determining what really works and what doesn't. I have to leave things in and after a while, a day is best, replace it with the original part. most of the time i end up keeping the original ! there are only maybe 2 or 3 things i've left permanently out of a hundred things i've tried ! I do enjoy the hunt, but to be honest not so much that i can spend months on this. I hope to nail it withing a few weeks and button it up for good. (really ! :))

Oh.... the volume is 500k

[Jag]

I've got a 1M. You're using a 500K, maybe try a 100K series resistor? Maybe bypass some highs around the 100K with a small value cap?

[daz]

i tried a lot of stuff there including 2 other pots, a 1 meg and a 250k. But the thing is i'm really no longer looking to improve the blocking distortion issue, as it's barely starting with the 1st volume at 10 and al i have to do is turn it down a bit and it's fine. What i'm really after now is removing that hard midrange tone. In another thread i said....

theres something else that i think is probably caused by the same thing thats causing the hard mids. that would be the tone you get when using a strat with high gain and playing single notes up around the 10th fret.

However, i forgot to mention i meant with the neck p/u. A rather important detail in getting the tone i was describing! In any case, i know that tone and the hardness in the mids are both caused by the same thing whatever that may be, and thats now what i'm looking for. Once i find that then if i want i can work on the 1st stage level thing. But thats unimportant to me by comparison.

[daz]

Ok, things have gotten much better with some of your suggestions. i finally found that by doing a few of them simultaniously they seemed to work together and clear up the buzziness and thin tone. It's sounding much better. But the issue of the midrange still remains. Heres whats going on. the mid and treble knobs are VERY close together in terms of center frequency. I need to find out how to go about moving the treble up in range and the mids down. I have messed with the tone stack and even used that tone stack calculator with no success. Can anyone explain to me exactly what components do what in the typical marshall tone stack? i have increase and decreased the slope resistor to as low as 33k and high as 220k. However i'm not sure which way does what. seems to give more or less signal to the mid and bass controls as far as i can tell. And one thing i'm really curious about is the treble cap. It shows 470pf in most marshalls, but i had a 390pf available so i used that. What exactly does that cap do......does it determine the frequency center of the treble pot, and if so, should i use a larger number (say a 560pf which i happen to have) or a smaller number (i have a 250) to move the frequency center of the pot UP?

If i'm off base, then what SHOULD i do to move the treble center up and mid center down? Like i said i have used that duncan calculator, but if i follow it's advice and use the values that give a EQ curve like i'd want, it doesn't seem to reflect that tonally when i try it.

[tbryanh]

Do you have something against posting a schematic? Afraid somebody might steal your trademark sound?

[daz]

Not at all, but i didn't have one because it's considerably modified from what it originally was. It was a regular 18 watt that i cascaded. so there was no schematic. However, i did edit one to reflect my changes (albeit sloppily) friday to post at the 18 watt forum. But the host's bandwidth limit must have been exceeded so it no longer works. I'd put it elsewhere and post it again now if i could, but it's on my work PC so i'll have to wait till monday to upload it somewhere else. Doesn't matter anyways because it's at work where i can work on the amp. at home i can't turn it up enough to hear any changes. (apartment....nothing louder than LOW tv listening levels)

[lowell]

hey guys, diggin the thread here. i'm also a bit confused. doesn't reducing the cathode resistor increase cathode current thus leading to a thicker and sometimes squishier sound? I thought increasing this resistor would provide increased clean headroom AND larger voltage swing, but a harder thinner tone too. correct me if i'm wrong.

[daz]

Well, just going by what cbarrow7625 said i increased it and it seemed squishier to me. Then again as i said, after a lot of tweaking you can have a hard time keeping your judgment in hand. I should probably try putting it back to 1k again and see how i like it, especially after all the other tweak i've done since. the 3.3k cathode may well have turned out to be a band aid for other values that needed changing and then were changed. This tweaking stuff is truly never ending due to the infinite amount of combinations.

[cbarrow7625]

OK, lot of expalnation for the tone stack:

1.) Treble cap - The treble is a "shelving filter". That is, everything above a certain frequency (set by the cap value & potentiometer settings) gets raised & lowered at once. Since the tone controls are all interactive, the exacty frequency where the shelf starts will move up & down a bit depending on how you have all ofthe tone pots set (something you should be able to see in the Duncan Calculator). Mainly, the treble cap is going to set how much upper midrange you have in your sound. On a high gain channel a 250pf cap will sound kind of "hollow" like it is missing some meat in the sound. If the cap is much above 600-700pF, the treble cap will start to dominate the tone and will really reduce the effectivity of the Mid & Bass pots. The Mesa DR uses 560pF & 680pf (I think? That may be higher than the actual value - going from memory). Anything higher than about 500-560 will start to give you a real thick upper midrange presence. 390pF is probably too small a value for a hi gain channel. Try the 560pF, I think you'll be happier. This will likely take a bunch of esperimenting. I like to clip in 47pF caps one at a time until I get right into the range that sounds right to me. For a hi-gain amp like this you will probably want to use a 250k LINEAR pot. For a Fender-type clean channel a 250k AUDIO taper pot is more appropriate. You can use an audio taper pot in a hi-gain amp but you will find that the useful tone range is limited to about the 7.5-10 range on the knob. The linear taper will give you a useful range from about 5-10.

Bass Cap - Anything between .02uF & .1uF will work in just about any amp. It just depends how deep you want the bass to osund. Typically hi-gain amps will use a .02uF. That gives a nice, tight aggressive bass. If you want a warmer, smoother bass response step it up to .047 or even .1uF. It may begin to sound too flabby or farty if you have the PI & Power stage set up to pass a lot of bass as well. 1M audio pot is almost always used (sometimes a 500k audio but that really just limits the amount of LF available.)

Mid cap/slope resistor - The mid cap & slope resistor work together to set the MR lowpass frequency (all the MR cap does is create a lowpass in conjunction with the LF cap to basically make an LF "bandpass"). The typical Fender/Marshall circuit has a MR dip built into the frequency response between where the MR cap rolls off the lower mids and where the HF cap picks up and adds back in the upper mids.

The slope resistor & MR cap are a series lowpass circuit. For a given cap value (like .02uF), the range of frequencies where it starts to roll off the lower mids can be varied by adjusting the slope resistor. 100K + .02uF cap creates a MR roll off pretty low in frequency (creates a lower MR dipthat is pretty deep too). A 33k + .02uF cap allows a wider range of lower mids through the Bass/MR section of the tone controls & can give you a "thicker/chunkier" sound. Changing that cap to .033uF or .047uf just moves the range of frequencies down from the .02uF.

The slope resistor, unfortunately, also affects the amount ot bass signal allowed through the LF/MR portion of the circuit. The slope resistor + the setting of the LF pot creates a voltage divider for the LF. Higer value slope resistor = less LF. This is where you can get into tweaking the value of the slope resistor + the MR cap. If you find a MR tone setting you like using a .02uF cap but the slope resistor ends up being to high a value & you don't have enough LF, just change out the cap to a lower value (.033-.047uF) and tweak the slope resistor back to where the MR sounds right. With a larger cap, the slope resistor will end up smaller. (80k + .02uf cap gives roughly the same frequency response as 33k + .047uF).

That being said, .02uF is usually in the proper range for a hi-gain circuit. In my amp I have a 100K micro-pot across a 220K slope resistor (roughly 68k combined at max pot rotation). It allows me to fine tune the slope resistor for exactly the midrange sound I want at any time....and I do mean fine-tune. Sometimes the slightest tweak of the pot is the difference between the "right" sound and something that sounds just awful to me. I highly recommend this to anyone trying to fine tune a tone control circuit. You can install it permanently or just use it to tweak, then substitute fixed resistors for for the pot once you find the right combo of slope resistor / MR cap.

The MR pot can be audio or linear (again, just a useful range question. Whatever works best for you. I think Marshall is usually linear). The MR cap can also be attached to the wiper only or can be attached to the wiper with the wiper & ground lug tied together. I prefer the latter (the "Fender-Style" MR pot connection) in most of my amp channels, even the hi-gain ones. To me it always feels like I have more useful range of the MR control with this connection. It's just a tast thing. Do whatever works for you.

Last thing to mention is that the "treble" pot is really a "balance" control between the Bass section of the tone controls & the treble section (treble cap). If you think of it this way, it is easy to visualize the "high-shelf" tone control created by the treble cap & the "low shelf" tone control created by the combination of the LF & MR section. The treble pot just selects if you accentuate the high-shelf or the low-shelf portion of the controls. Of course, the two halves of the tone controls sum somewhat through the treble pot as well.

I suppose I could keep going but that's probably more than sufficient to answer your question.

Chris

[daz]

Thanks a lot for that ! I will have to study it and apply some of those things. i recall putting a pot across the slope years ago too. I think i may do that again. The one thing thats worrying me tho is that it seems like no matter what i do that hard midrange and lack of sparkle never changes to any degree that brings me close to what i'm after.I know it's possible because even my modded classic 30 gets it, tho the overall tone quality isn't as intense which is why i like this amp so much. I have tried paralleling treble caps to 250pf and gone up to 560 pf and i still don't seem to get a high enough range from the treble pot. I tried a lot of things this weekend but being at home i was unable to play it loud enough to hear. So today, monday, i get to work and fire it up only to hear i made it a lot worse ! One of the things i did was use carbon films on the cathodes which i was told to do elsewhere. i changed everything back this morning except those and it still sounds no better than when i left work friday IF AS GOOD. So much for that idea. Arrrrrg. Funny thing is, after all the tweaking i'm beginning to think it was better before i started. :(

[cbarrow7625]

Lowell,

Like most things in audio, the answer really is "it depends". It depends on how you define "thicker & squishier". My definition of that may be different than yours. It depends on other things in the circuit. What comes before / after that tube stage. What are the voltages of the circuit, etc., etc.

In my experience, reducing the value of a cathode resistor ( to a reasonable value like 820 Ohms) from something like 1.5 or 1.8k gives what I think is a "harder" feel, less dynamic. It feels to me like too much tube compression.

You can't get increased clean headroom though. You are reducing the grid to cathode voltage. Reduced voltage = reduced headroom before clipping. The plate voltage gets lowered too because of increased current being pulled across the same plate resistor (larger voltage drop across the plate resistor).

I don't claim to have all the answers, just throwing out my experiences to try to help, get some feedback & learn some more myself. If you have a differnet take on it, I'd be happy to hear it.

Chris

[cbarrow7625]

Do you have a cathode follower before the tone stack? If not, try & add one. If so, try to bypass it. I am not a fan of the "sound" of a typical cathode follower in a hi gain amp. I try to avoid them whenever possible (which is almost all the time). The sound of the tone stack can be affected a lot by an anode vs. cathode connection.

Do you have a master volume or a voltage divider immediately following the tone stack? What value(s). You really need to have a 1M or greater pot / voltage divider combo after the tone stack or you can really get some tone sucking going on.

Changing the treble cap will not give you "more treble". As I expalined in the previous post, the value only controls the amount of upper midrange you will pass through it.

If you need "more treble" or "more sparkle", then you will likely need to go back through the rest of your circuit & determine where you are losing treble. Any caps to ground? Caps around the plate resistor? Etc., etc.

If you are just talking about the range of rotation on the pot (see my previous post as well) use a linear taper pot. The audio taper pot in the treble position in a hi-gain amp will limit the useful range of the pot severely. What exactly do you mean by "range"?

[daz]

Do you have a cathode follower before the tone stack? If not, try & add one. If so, try to bypass it. I am not a fan of the "sound" of a typical cathode follower in a hi gain amp. I try to avoid them whenever possible (which is almost all the time). The sound of the tone stack can be affected a lot by an anode vs. cathode connection.

it's a marshall cathode follower that feeds the stack, is that what you mean? Thing is, my all time fav amps are like that so it should be capable of sounding as i want.

Do you have a master volume or a voltage divider immediately following the tone stack? What value(s). You really need to have a 1M or greater pot / voltage divider combo after the tone stack or you can really get some tone sucking going on.

It's a 1 meg master after the stack.

Changing the treble cap will not give you "more treble". As I expalined in the previous post, the value only controls the amount of upper midrange you will pass through it

I seem to find nothing i put there seems to matter much, tho oddly i find the lower value 390pf i had there to begin with seems best. I looked at the duncan calc and it seemed like a 100pf would do what i want. But thats so low i can't see why i should need that unless i'm putting a band aid on the real issue.

If you need "more treble" or "more sparkle", then you will likely need to go back through the rest of your circuit & determine where you are losing treble. Any caps to ground? Caps around the plate resistor? Etc., etc.

My thoughts exactly, tho i seem to have already tried everything. Only cap to ground i believe is the one on the 1\/2 of the PI thats not used. I will post a edited schematic of what i now have after tweaking in a few minutes.

If you are just talking about the range of rotation on the pot (see my previous post as well) use a linear taper pot. The audio taper pot in the treble position in a hi-gain amp will limit the useful range of the pot severely. What exactly do you mean by "range"?

No, the rotation seems fine. To make sure i'm clear and to sum up where i am....

1-the mids still seems hard sounding

2-The low mids (say 400-500Hz) seem dull

3-there is little very high sparkle/high harmonics

4-the mid control seems to boost a range too high...probably 1k where as my marshalls used to seem to boost around 500Hz or thereabouts.

I will post a schematic soon, tho i will have to edit it first to reflect where the amp is now. Thanks very much.

[cbarrow7625]

Alright. A schematic would be really helpful at this point. I'll wait to see what you have down on paper and see if I can't tell you something more useful.

100pF for the treble cap? I doubt that is what you really want. How are you determining from squiggles on the computer screen what you "want". The TSC is a cool tool but I think you may be a little off base in what you think you want and what you really need in this case.

Post the schematic!!

[daz]

Here ya go...

http://img406.imageshack.us/my.php?image=18dq2.jpg

[cbarrow7625]

Ahhh....now we're getting somewhere!

So, cathode biased, no feedback, verly large PI tail resistor, 56k cathode follwer resistor, 22uF cap on the second stage, no HF "emphasis" cap/resistor combos in the preamp and all ,022uF coupling caps.

With no negative feedback, EL84's and cathode biasing I would totally expect this thing to sound very midrangey & "honky". Going for a hi-gain amp, feedback can be your friend. An arrangement like this would typically be seen where you are trying to overdrive the power tubes into saturation (which I don't think you are trying to do - correct me if I'm wrong). Feedback around the power amp is going to go a long way to "flattening" out your sound and getting rid of some of the ugliness you are hearing out of your tone stack. If you add a "presence" knob (ala Marshall) you may find that you now have a very tight, balanced low end & an additional Upper Mid/HF tone control for your sound. I have a setting on my amp that allows me to tweak the feedback resistor and remove it altogether and I find that I really, really, prefer it in (at even lower than normal stock values, usually). Everything is just much smoother & more balanced.

I'm not really sure why you have a 56k tail resistor of the PI. Did you copy that from somewhere or was it the only value you had? You might want to try just copying a standard Marshall-type PI + feedback setup in this thing. I think you might like it.

For a cathode follower, you typically want to make the cathode resistor in the follower the same value as the preceding plate stage. This helps with symmetry of the voltage swings. A 56k here can affect the tone of the amp. Give that a try.

I find that having a cathode cap on the stage directly preceding the tone stack usually gives the tone a much-needed "boost". It will also lower the output impedance of the cathode follower by lowering the output impedance ofthe preceding stage. Put a 2-10uF cap (I usually find 5uF works well here) in parallel with that 1.5k Cap.

In doing that, you may find that you need to ditch the cap around the 3.3k resistor in the previous cathode.....speaking of which....for a hi-gain amp, 22uF in this spot in the circuit is almost always going to be way too large. No wonder you are getting blocking distortion going on. This very large cap (in combination with the large coupling caps & no HF emphasis) can explain more of the "midrangey" tone stack problems. You are emphasizing way too much lower-mids for a typical hi-gain circuit. If you want to keep that there I would recommend taming it down to 5-10uF at the most.

If you are going to use 0.022uF copupling caps in a hi-gain amp, you really need to emphasize the HF somewhere in the circuit or you will end up with the dull highs that you have been complaining about. Try just putting a 500pF cap around the 470k series resistor. How about 470k/470pf caps before one of the volume controls? That would help a lot as well. The fact that you are running each stage into a 500K load instead of a 1M-1.5M load (like is seen in many, many high gain amps) will tend to contribute to the lack of harmonic sizzle in the sound. To make nice harmonic distortion, tube stages like to "run free", that is they like to have a very high load to run into as to not load down the output of the tube (and create a voltage divider with the internal impedance ofthe tube itself).

If this was my amp (and these are totally just oersonal preferences based on trying out a lot of stuff over the years), I would have the 1.5K cathode resistor on the first stage & the 820 ohm resistor on the 3rd stage as well.

I feel like I'm telling you that everything is wrong & to start over from scratch....sorry. Without actually hearing it it is hard to make the right recommendations. I have tried a bunch of things over the years to see what I like & what I don't. I have tried preamp cirucits very similar to this & I have always had the same complaints about them that you are talking about here. Not enough HF presence, too much lower midrange, blocking distortion...etc., etc.

Take these thoughts for what they are worth. I hope they help send you down a different path where you end upwith exactly what it is you are looking for in the tone. Let me know what you think.

[daz]

Thats a lot of info ! I have a lot to try now so i'll be busy for a while. As to NFB, i tried adding a presence before but i really had no one telling me what would be best and the values i used or way i did it wasn't productive. as to the 56k tail, it was in the schematic i used. I'll look at a jcm 800 and swap to whatever value that is. thanks sooo much. I'll let you know how it goes.

[daz]

I've tried several things so far. Things that seemed to help were replacing the tail with a 15k, changing the 56k cathode follower resistor to 100k, and removing 2nd stage cathode cap. (which by the way was a 1 uf...my mistake on the schematic) replacing the 1st stage pot with a 1 meg seemed to be worse, but i will have to swap it out again and do more A/B testing because it seemed to give less gain which is opposite of what it should have done. Didn't have two 1 meg pots so i couldn't do the second pot. Also, putting a 500 pf across the 470k didn't seem good, and neither did a 470k/470pf before a pot. I assumed you meant to the input of one of the first 2 pots after the coupling cap. thats where i tried it anyways.

Of course there is always ear burnout and placebo effect. But at this point i thing some NFB would maybe be the thing to try. But i'm not sure what values and how to best implement it. So i may try a marshall copy. By the way, i used 220k instead of 470k on the PI for more headroom, which is something i was told to do when i asked elsewhere how to get a cleaner power section.

EDIT: tried nfb and didn't like it. I however then tried the hi pass filter you suggested (470k/500pf) coming out of the second pot and i liked it. Now the amp is sounding pretty good, but i think i may have to try nfb again

[Bruce / Mission Amps]

Wow Daz... are you aware that a cathode biased pair of EL84s is only going to need about 12-15v of drive signal to turn them on full blast and beyond?
You have quite a bit of gain structure there and I suspect the probability of preamp gain clipping has more of an effect on the actual tone then the tone controls. I mean there will be so much square wave energy that the whole thing will sound crunched and compressed with much less definition, or tonal spread, between the bass and treble.
In other words, I think you can smash the cathode follower driven tone stack with what you have now... and all that with no negative feedback.
I think the first thing I would do is limit the gain of this thing by about 50%.
And not just be reducing the gain of the phase inverter as you are doing with the 15K cathode and 56K tail resistors.
Reduce the gain of that first stage by a bunch to start.
Dump that 820 cathode resistor in favor of a 1k5 to 2k2 bypasses by a 1uF cap.
Try a little smaller value coupling cap too.
4n7 to 10nF comes to mind here and then use a 220K to 470K resistor in front of your gain pot... but smaller, maybe a 100Ka to 250Ka pot would be better.
On that triode in front of the CF triode... try a 4k7 cathode resistor instead of the 1k5.

etc...

[cbarrow7625]

Try a bunch of differnet values of NFB resistors. Remeber, the NFB resistor and the long tail resistor (the one that goes on the bottom of the PI tail that is not currently in your schematic) form a voltage divider for the NFB.

Just because you don't like one voltage divider setting doesn't mean there is not a combination that will work. We can only give you some basic guidelines here. It is up to you to take that info & run with it. Try a bunch of variations of things before saying you don't like it. Try a 100K series NFB resistor going to 5k to ground (at the bottom of your current PI tail). Remember, the .1uF cap & 15k tail resistor both go to the top of the 5k resistor (the .1uF doesn't keep going to ground, it gets "lifted" so that some NFB goes into the 2nd half of the PI).

Try different cap/resistor combinations in series with your volume controls. I can't list them all out here but suffice it to say that adding any combination of these won't hurt your amp so don't be afraid to just stick stuff in on a whim & try it.

[cbarrow7625]

Good advice Bruce. I assume that he is using it with the MV turned way down, which will work OK. If he opens up the MV it would probably not only sound bad but squeal like a pig.

Since it is cathode biased,it would be easy enough to put a voltage divider between the output of the PI & the EL84's. I'm thinking start with 150k series, 47k to ground to start (or just replace the 220K resistors with some 250K audio taper pots to find the right level, then substitute fixed resistors later).

[daz]

Well, it's obvious i'm no amp designer. :) But you guys are giving me a good education here ! Lemmie try some of this and get back.

[daz]

Heres the schematic as the amp is now after the last few things i tried. These are the ones that seemed to help in some way or another, tho i think lowering the 1st gain pot wasn't good and i may go back to 500k. The amp was better a couple tweaks ago i think. losing the gain seems to have hardened the mids even more, and i'm not sure what to do about that aside from going back a few tweaks. But there is also some good stuff happening that i can't quite put my finger on. So maybe i'm on the right track but in getting there it is causing other circuit values to need changing.

http://img142.imageshack.us/my.php?image=18bre8.jpg

[daz]

This is driving me nuts ! you can pretty much ignore that new schematic because i've changed a lot of things since then. I think i need to set everything back to where it was to begin with and start over again by trying things and slowly listening and digesting it all before i move on. this was all too quick and i think i did too much and sorta lost the magic i was hearing before even tho i was making progress in other ways.

One thing tho....does it matter which tap i use for feedback?

[Bruce / Mission Amps]

Go ahead and put it back where you last liked it and then just do one mod at a time...
My hat is off to you home builders that can and do this stuff without a good signal generator and O'Scope!! With one or two small mods at a time, just looking each stage, with a scope and clean signal injection can be incredibly revealing!!

If you have a feeling the basic tone was there but it was just to hard to control, you can also try a split load resistor on the first or second gain stage. That would be your 100K plate load resistors.
Try something like two 47K resistors with the coupling cap between the two or for less output to the next stage a 27K sitting on top of a 56K resistor. etc.
Or even better, stick a small 100K trim pot in place of the 100K plate load and connect the coupling cap to the wiper. Turn the trim pot to taste.
(I've done this many times only to find 82K to 100K was still the really good ones...:))

With respect to your phase inverter, your 56K tail resistor is OK and it will help create better balance of the PI output.
But I'm not sure why you picked the 15K cathode biasing resistor in the phase inverter.
I think it might have been instinctive thinking on your part that it would help reduce the gain of the PI a little.
This really is easy to do but not the perfect way to achieve gain reduction.
The negative aspect to using both of these large value resistors is that the further you take that phase inverter colder in bias, by using your 15K cathode biasing resistor and holding the whole thing up further from ground by the large value tail resistor, the harsher and harsher that stage can sound and it does impose it's will on the output, plus it will also clip easier.
I think that stage really wants to be idling in fat solid class A for better tone with higher idle current... way better then higher bias voltage, cold idle current (nearing class B or C)... in such a way that it will run into cutoff.
Hint: The power tubes will follow what ever the phase inverter does.

Remember, with respect to where the triode's grids and grid load resistors, (see the bottom of the 15K cathode resistor), the common 15K resistor you have installed is sharing cathode current to both triodes.
Typically, when this is done you would see a lower value cathode biasing resistor.
You've seen it in many Fender and other amps who's dual triode's cathodes are sharing a common cathode resistor... 820 ohms vs 1500 ohms for each cathode.
So, try a 1k5 ohm to 2k2 resistor and work on gain reduction in front of the phase inverter.
The long tail phase inverter is a little tricky to really understand sometimes it's a good idea to work on that theory in order to see what you are doing with by changing the supporting components.

********
Adding to those thoughts, read up a little deeper on what the term "bias" really means with respect to the three basic elements of the tube and how there are different ways to change the bias!
It will reveal some interesting things to you about the circuit.
As an example, with that big 56K and 15K Rk resistor there (Rk means cathode resistor), you could be adding some other negativity to the total amp circuit, because the actual plate to cathode voltages are lower, creating less headroom of the PI and additional colder bias of those two triodes... which can result in more odd harmonic ugliness when drive so hard by your preamp stages.
With high relative bias voltage (with respect to the higher cathode to lower plate relationship)... both of the triodes can be driven right into cut off much easier.
In the wrong circuit I think this can sound very ugly and even worse if you then decide to use localized negative feedback in the phase inverter.

[daz]

Bruce, i wasn't sure at first, but now i am.......you think i know more than i do ! Pathetic, but true. I didn't understand much of what you said, so i don't know exactly what you were suggesting i do to the PI values. i've played with electronics in my amps and pedals and all for years, but i never built an amp till this one. I can easily read a schematic and know some basics. But theory is greek to me. I understand some basic things such as how NFB works, how filters work and such. But when you start talking about swing and cutoff and those kinds of things i'm lost.

I didn't do this for fun, and i never work on gear for fun. I do it because i am a slave to good tone ! I can't play without it because i cut my teeth on amps that were touch sensitive and without that i may as well be playing one fingered. So i never learned much, just what i needed to know. But building and tweaking an amp requires more knowledge and i guess i'll just have to take it a day at a time and hope i get lucky. The amp had so much magic at first, and i could have easily lived with the imperfections if i could have just gotten that smooth transparent midrange. But at this point i feel theres something inherent in it that won't allow that to happen. maybe the tranny or something, i dunno. I figured as good as it sounded at first it would only get better.

in any case, i will put it back to where it was assuming i can remember all the values, and i was thinking of maybe trying to reduce the level before the tone stack so i don't have to lose the amount of preamp distortion it had before. I can afford to lose a bit, but not 1/2 or more which is what happened. Then if i'm stuck with those hard mids i'll just have to deal with i suppose. Thanks much for your time.

[Bruce / Mission Amps]

Bruce, i wasn't sure at first, but now i am.......you think i know more than i do ! Pathetic, but true. I didn't understand much of what you said, so i don't know exactly what you were suggesting i do to the PI values. i've played with electronics in my amps and pedals and all for years, but i never built an amp till this one. I can easily read a schematic and know some basics. But theory is greek to me. I understand some basic things such as how NFB works, how filters work and such. But when you start talking about swing and cutoff and those kinds of things i'm lost.OK Sorry... this will be very general and so as not to upset the overly sensitive, hard core theorists here, I apologize now for the looseness of this.

All I was trying to point out was that as the input signal (a sine wave) is imposed on the grid, the plate output will be 180 degrees out of phase and when linear, should be a complete duplicate of the driving sine wave on the grid but at a much larger voltage swing and that pretty much is the definition of voltage amplification.
The swing is the total up and down of the signal from a zero point up positive to a peak, back to zero and then down to a negative peak and back to zero again.

In high gain amplifiers, with what is usually too many gain stages and little attenuation, ... it is super easy to get ambitious with "over the top" and unneeded gain. That builds in lots of problems with tone control, smearing and a buzzy sound that reminds me of hundreds of bumblebees in a paper bag.

At any given moment, a moment in time with respect to the signal being a sine wave riding on the actual DC bias voltage, the grid sees that as varying bias voltage.
The AC signal added to the DC bias is a momentary total bias voltage.
The difference between those, with respect to the actual bias voltage, is the audio component.
Positive going signal means less momentary bias voltage and negative means more momentary bias voltage.
But that change is what makes the cathode and plate conduct in variable DC, hence forth called AC or audio, and creates the AC voltage amplification on the plate which you want to pass through the low impedance path of the coupling capacitor... onto the the next stage's grid.
You've copied, or duplicated, the AC signal on the grid at a much higher output voltage on the plate, or if a cathode follower, in phase with the nearly the same AC voltage as on the grid, but
much much lower impedance and no gain.

If the input signal to your phase inverter is so large as to be higher then the tube's bias voltage, the output of the tube will be either cut off in the negative going part cycle or roundly flat topped and saturated on the positive going part of the cycle.
And that goes with the power tubes, especially!.... even though a little of that sounds really cool.

More "apparent" total negative voltage on the grid will drive that tube into cut off during the peaks... which means the tube is temporarily biased too far negatively and it stops amplifying.... to what is called cut off.
In other words, the tube gets driven negatively to a point where it stops conducting in that peak part of the negative cycle and there is no signal coming out anymore. Transition is UGLY.
The other part of the sine wave is more positive then zero level, and with heavy drive levels will push the output so far as to reach high current tube saturation. Not quite so ugly but not linear.
The negative going one will sound like a class B amplifier, which in most audio amps, is not really linear and can be very ugly sounding, while the positive one will still have output but be massively distorted with all kinds of even, odd and compliments of those frequencies... all mixed up which we call harmonics.
That can sound OK to some level but very dirty.
The power tubes will try and duplicate that exact same distorted waveform up until the driving level on it's grids does the same thing too and they get driven into cutoff or saturation. That's a complicated waveform now and the poor output transformer and speaker has to deal with it... nasty.

I guess what I'm really trying to say is, it is a matter of balance throughout the entire amplifier to get a great cleanish sound and still be able to kick it in the ass to get some of the lovely preamp and power tube distortion... especially where the tubes start screaming for mercy and want to take off on their own a little.
Other wise you just end up with an amp that does everything equally poor and has very limited use.

OK I just re-read all this and I'm rambling on and on.... sorry.

[daz]

No need to say you're sorry....you're spending your free time trying to help me and anyone looking at this whom it may help ! But still it's beyond my understanding to a large degree. i just need to know where my amp may be out of balance and what values to use where. at this point i went back and didn't like it. But i DID get that gain back. So what i did was to leave the cathodes close to how i had them, tho a bit less gain. then add many of the suggestions you both had for the rest like it was yesterday. at this point i like it best but it still needs tweaking and still has that hard mid sound. But it had that even after i did all of your suggestions, so it's something i guess i haven't altered yet.

One thing i wasn't sure about that you mentioned a couple times was that you questioned the use of a 15k cathode resistor in the PI, but i never had one noted there or used one that i recall. It was 56k, which was on the original schematic, then on cbarrow7625's advice i changed it to what i saw on a marshall schematic which was a 10k. Right now i have a 470ohm from the cathode to the same 10k to ground. that 470 WAS a 820, but i again changed it to reflect a marshall.

Anyways, it's not bad right now but i'm still desperately trying to find the source of that mid sound thats no where near as smooth and transparent as any of the marshalls i've had. NFB helped a tiny bit, but it also made the overall tone suffer. And i'm sure i wired it up exactly like the marshall i copied. (actually i tried 2 different NFB circuits i copied from 2 different marshalls)

[Bruce / Mission Amps]

See if you can spot the changes I made to your schem.
Try something more like this as a start and give me some plate voltages... it makes a difference.
I still think it will still have too much gain but from reading your posts, it sounds like that is what you want and the cleanish usable sound isn't that important.

[daz]

Geez man, you live and breathe this stuff dontcha?! i know you probably loved editing that, huh? :) Well, yeah....there are a ton of changes. what i wanna know tho is, what do you think that will sound like? i ask because i know you are into fenderish circuits, and i'm 110% marshall all the way. I lust after certain aspects of marshall tone and thats my #1 objective.

As to my gain fetish, you won't believe my answer. But the fact is that i like a lot of gain, but not so much for the high gain tones themselves. For whatever reason i have always loved the sound of a high gain channel turned only part way up. Say 1:00-2:00 on the pre, then turning the guitar down to get semi cleans with a single coil, and i use guitars with a hot hum in the bridge. So i can go to almost clean to cranking distortion without pedals. And the key for me is that for some reason i much prefer the sound of a high gain circuit turned down than one which i have to turn the pre to 10 to get the amount of gain i want. that may not make sense, but a high gain amp set for 1/2 the distortion it can generate sounds much different to me than an amp that with the pre gain on full generates the same amount. So while i don't need that much gain, amps that have that much just seem to have the type of tone i like even at lower gain levels. It works for me too, i assure you. And as far as turning it up, if i DO use it for gigs, which i rarely do anymore anyways, i would tuen the gain way down when the master goes up. so it should work even then. After all, the preamp only generates a ton of gain if you twist the knobs that way.

In any case, i may try some of those tweaks, but if i decide to do them all i would have to make another schematic of where it is now so mi don't lose that. It's got some tweaking to be done before i'd be happy, but it's close emough to be a good starting point. so i don't want to lose that. Took me all day to get it to that point from the way i left it yesterday. thanks Bruce.

[cbarrow7625]

Daz,

Bruce made some really good recommendations. However, if it is a "110% Marshall" kind of sound you are going after then what I think you maky need to do (in direct opposition to what Bruce posted - sorry, Bruce) is increase the values of the volume pots to 1M & put in some series resistors before the pots of 220K-470K with some bypass caps around them. I am not saying "just copy a Marshall circuit", what fun would that be? What I am suggesting is that you have a relatively "small" load that each of the tubes is terminating into (500K pots). Part of the Marshall sound is the fact that the tubes are not loaded very much at all by the ciurcuitry following each stage. They will typically have a 1M-1.5M load after each gain stage (series resistor + volume pot).

Following Bruce's recommendation for a 470K series resistor and a 100K-250K pot would actually be good for the gain structure but it will likely miss the "something" you are going for with the Marshall-type sound & harmonic structure.

A good place to start would be to use Bruce's values everywhere except for the volume controls. Make those 1M. At that point you will have too much gain, some squeling, amybe even some blocking distortion again but where you say you like to have the knobs set it may sound really good.

Since you are building this amp you don't have to make it "go to 11" on the knob. You can just use fixed resistors in the circuit once you determin where you like the volumes set & move your 1-2 O'clock knob position to actually be at '10' on the volume pot. If the max gain you are going to use is not "fully dimed", then make the amp so that it isn't allowed to put out that fully blazing high gain sound. If you do this, you will give yourself a much, much wider window to work in tone-wise and in the harmonic structure. Where you seem to be getting in trouble is getting that "fully dimed" sound out of the amp when it sounds like you won't use it anyway. If you won't use it, don't build it in. You are making a custom amp for you, don't worry about making it do everything. make it do only what you want & you'll probably be much happier with the results.

An amp that is jack of all trades & master of none is dreadful. Make yours a master of one.

[Bruce / Mission Amps]

Well, actually I did change the volume and MV pots to 1M audio.
Maybe you didn't see the modified schem I attached to my reply.

But, to be really usable, I think the first stage pot needs to be a good gain control and less then 1Ma.
You are probably right that 100K might be a little low for a thrasher amp but I wouldn't use more then 500Ka.
No harm in doing it with a 1Ma pot but when turned down so the total output would sound good to me, that extra 500K to 750K portion of the pot above the wiper will act a bit like a low pass filter and it could sound a little muddy in high gain.
Hence the reason I used the classic +700Hz -3dB Marshall filter of, 470pF||470K or for a little more gas, 220K||1nF, still a +700Hz filter.

Keep in mind, you must take what I say as worth a grain of salt because I am not one of those super high gain buzzy chainsaw tonebox guys. I hate it.
I think the first stage should be tuned with less bass and with a max output boost of around +12dB to 15dB, ...not +30 dB, which might be available if left unattentuated.
It would be an annoying gain monster to me with so much distortion before the MV that the MV wouldn't be much more then an additional distortion boosting device. Daz likes this!!.... ;)
I do have a feeling that Daz is not that terribly interested in what I think is a good usable tone range but is way hooked on the preamp square wave distortion and then just amplifying that with the PA.:D
Of course all that, with out serious bass reduction and tone shapping throughout the preamp stages, ... will net you a very fuzzy and middy sounding amp....:eek:
Wasn't that one of the complaints?
******************************

I'll leave it to some others to work it out now.
*************************************

[cbarrow7625]

Bruce, sorry if I missed the change tothe 1M pot. I did look but pretty quickly. You are right about the pot being turned down acting like a lowpass. I'm all for the resistor/cap combo you suggested.

I think, ultimately, what Daz should end up with is something like a 500k pot in that position but I think he may want to have about 1M in front of that pot as well (the Marshall 1.5M total I referred to). To really simulate the Marshall settings it would be a 470K/470pf followed by a 470K/.001uF in series (to simulate turning down a marshall volume control with it's .001 bypass cap followed by the 500K pot (probably with anothe rbypass cap on it (250pF - .002uF, chooses to taste).

It's hard to say without hearing it.

[daz]

So much to think about ! I'm going to take it a bit at a time and see what each tweak does. But trust me guys, i'm not a chainsaw buzzy preamp kind of guy. I hate boogies and other amps that are made to do that. They are generally too cluttered to do anything else well. But a jcm 800 or even 900 can do high gain and still sound like SRV's cleaner tones. (tho i never use TOTALLY clean...much too dynamic, and in a bad way to my ears) If you don't believe it i have plenty of band recordings of myself using a 800 or 900 with some gorgously clean and semi clean sounds that i got from using the right tubes and speakers, setting my amp a certain way, and manipulation the tone with my guitar's pups and controls. i could never get the same thing from all these vintage fenders and things that people go nuts over. As Eric Clapton once said in a song, "it's in the way that you use it". I used to always get compliments on my tone playing in a band that did everything from soft ballads to hard rock. As Eric Clapton once said in a song, "it's in the way that you use it".

That said, i'm fine with less gain than i have now, but once i did all those tweaks the other day it was all but gone. I had about the same with both pre pots dimed than i do now with both on 12:00. Maybe 2/3 that of a classic 30 if that. Anyways, on to the amp...the ron is hot. let ya know what happens in a bit....

[daz]

This is hard. theres no way to say this w/o sounding ungrateful, and trust me, I AM VERY grateful for all the help. You can't hear the amps so what you're attempting to do isn't easy i'm sure. But in any case, I did everything up to the PI. The 100k resistor on the master was the last thing i did. But i didn't go any further because at this point it sounds much worse. Again, i'm sorry ! I have to tell you what i found tho. (i hate this !) Anyways, the mids are harder, the amp sounds thinner, and the squishiness has gone away. I appriciate your help and i don't expect you to be able to work any magic over the web w/o even hearing the amp.

So i guess i'll set it back because i'm sure doing the PI mods isn't going to change this voice in any major way. Please don't take this the wrong way!

[cbarrow7625]

Why, you ungrateful........just kidding.

It's all part of the fun. It's got to be in there somewhere. Keep reporting back. A sound clip would be cool at any point if you have the technology to post one.

[daz]

I can't here at work. At home i'd only be able to turn it up to LOW TV level, and to get that i have to use a Lpad. So that probably won't happen. I tore all the morning's tweaks out and i'm going to change a lot of values to pure marshall values. Of course with EL84's i'm not sure how that will turn out. But the PI is now there. I'll have to see how far i can go on the pre w/o running into any issues. Then theres the P/S which is different. I have a 32/32 where a marshall has a 50/50. And my lows are just not tight which may have to do with that ? Also wondering about fixed bias for tightness?

[cbarrow7625]

Switchable bias is a wonderful thing. I switch from Cathode to fixed bias depending on the mood I'm in usually.

Fixed will definitely tighten up the bottom (it will also "tighten" up the top, but that's the tradeoff you make).

Haveyou tried larger screen resistors on the EL84's? It will lower your overall output power somewhat but it can also soften the tone of an amp. Just another random thought foryou to try (or not).

[daz]

Haveyou tried larger screen resistors on the EL84's?

No, but what value would you suggest?

[cbarrow7625]

Up to about 2.2K would be considered in the "normal" range. Up to about 4.7k could be useful if you like the way it sounds. You could probably even go larger.

Somebody please correct me if I am wrong here, but I don't think there is anything you can do with these resistors that will hurt anyhting, except making them too small. The values you have now are probably smaller than they should be. You really want to limit the screen current to help protect the screens.

As you reduce the screen voltage, I beilieve the output impedance of the amp will go up somewhat.

[daz]

ok, hweres what i did. while i awaited your answer i looked at the schematic and realized i had a 470 ohm there and that the schematic shows a 100 ohm. this because i used another schematic when i built it that was modded by someone. also, theres only one. Should i have one for each screen? ayways, i decided to see what happens with a 100 ohm like this schematic shows. i only had a 150 ohm so i tried that. It helped a lot ! I realize i went down instead of up like you said, but it got tighter sounding.

Then i put a 100k pot for the mids and it acts very different. When i turn it down it gets softer sounding but still sounds resonant, where with the 25 it got soft in a bad way,(dull) and turning it up to a workable level made it hard again. anyways, the throw is now of course too touchy, so i'm wondering how i could put the 25k back in and still get this effect. I realize that because the pot is wired using all 3 lugs it's not simply a variable resistor, but more or less two. And somehow that changes things balance-wise. in any case, the amp now sounds far better than ever, tho i still want more of everything i've been moaning about. But it's much closer than ever now.

[Bruce / Mission Amps]

Up to about 2.2K would be considered in the "normal" range. Up to about 4.7k could be useful if you like the way it sounds. You could probably even go larger.

Somebody please correct me if I am wrong here, but I don't think there is anything you can do with these resistors that will hurt anyhting, except making them too small. The values you have now are probably smaller than they should be. You really want to limit the screen current to help protect the screens.

As you reduce the screen voltage, I beilieve the output impedance of the amp will go up somewhat.I hope everyone reading all this understands that my comments and suggestions are what I like and only my opinion based on years and years of blowing stuff up and starting over!
I really at am at a loss as to why the Daz amp is sounding worse and worse.
Although not really my cup of tea, I have built this kind of amp over and over quite a few times and actually designed one VERY similar to what I re-drew for Daz and beta tested/tuned with another amp builder here in Colorado (SwanPro) when he and I were called DYNA Q Amps and this amp was called the BB20... BB was Bruce and Bob. I still have a few chassis and face plates for the damn thing too.
Anyhow, Bob has been selling them as the Swanpro RCS20 now.
The transformers are all salvaged from the old Bedrock company where we bought out all their old stock when they dropped out. As far as I know Bob is still using them as we must have bought 150 amps worth of trannys.
B+ is about 375vdc and the El84s are idling at 9-10 watts each.
I did not build this amp but the circuit board in the other picture below is my design after about three or four prototypes with Bob tweaking as we'd go. He has very good ears.
The big difference is that Daz's volume control is the boost pot and his first gain stage is the gain pot. The LED is a foot pedal switched FET circuit that switches the high gain first preamp from very low "fixed gain" (specially selected for cleaner) to full bore but controlled by the boost pot... set it where you want it and hit the boost pedal to get there.
Most of the exact Vintage amps Bob talks in this clip have been tuned by me and him together over a few years to some extent or another....

http://www.swanproamps.com/MP4_sounds.htm
http://www.swanproamps.com/images/Amp-Wiring-LG.jpg

With larger value screen resistors, the amp starts sounding a little slower and less lively. With low screen to plate voltage, it seems like the EL84s compress quite a bit easier too.
On El84 amps with cathode biasing, I like small screen resistors myself, 220-470 is what I've used...anyhow, less then 1k ohms on each socket...BUT, I do use an additional 1K5 to 2k7 2-5 watt dropping resistor.... dropped down from after the B+ screen rail, regardless if it is a choke or 1K resistor, etc., that's to get the screen supply voltage at least 5-15v less then the plate voltage.
I just like the way it sounds.


Oops I just remembered that we went to a dual gang, 250K master volume... after the phase inverter in these in the final configuration.

[daz]

Bruce....i know at this point you're probably thinking i don't know good tone is. But i've been playing thru mostly good touch sensitive amps for years and gigged constantly from 80 till several years ago when my left hand went south on me and I all but destroyed my right hand thumb socket at the wrist. But in any case i do know what good touch sensitive tone is and my favorite tones would include guys like SRV, Brad Paisley, and the reverend Billy G. i also like high gain boosted with pedals too, but even then it pays to have a very good touch sensitive amp because even with high gain they will display touch sensitivity. something most production amps could never do.

In any case, the 470 5w is now 150k 5w. But do you think i should leave it like that or should i put one on each tube instead of them sharing. and if you think i should do that, do i half the value to 75 ohms or double it or would it stay as 150 ohms for each resistor? Sorry for the ignorance, but like i said before, i AM ! :)

[cbarrow7625]

Cool! Good news. Each screen really should have its own resistor (again to protect the screens). If you don't mind an occasional power tube blowout & a fuse popping and possible OT damage, I owuldn't worry about seperate resistors.

If you like the sound of a lower screen resistor (see above comments, they apply) go ahead & use it. I'm really not trying to be a smart aleck here, I've done some things for tone that I probably shouldn't have but have been lucky so far.

For the Mid control, determine what the range is that you like to use on the 100Kmid pot then measure what those settings are. You will have to disconnect the wires & measure the resistanc from the wiper to each end of the pot at the max/min settings that you like. Then, replace the pot with a pot value that is = to the difference between those min/max settings. Add fixed resistors to each end of the pot = to whatever you measured in that direction so that the pot + 2 resistors comes back out to approx 100k. Sounds complicated but it is pretty simple.

As with the volume control, I am recommneding that you set a usable range limit onyour controls. That way the sounds you like will be available for 0-10 on the control rather than just from 4-7 (or whatever it really is).

Smells like progress.

[daz]

I'll add a second resistor, but should i use 150 ohm for each or does seperating the screens with thier own resistor mean i have to double or half the value?

the mid control i think is a syptom of a problem elsewhere because it sounds best all the way grounded, and thats not right. I've always like the mids up a good bit, but something seems to cause that control to add those hard 1k sorta mids i've been trying to get away from. and the hint is that with either value pot turned all the way to ground, it's a different sound. I guess because while the cap is now grounded, the bass pot is now 100k from ground instead of 25k. But what does that mean in terms of why it's not the case with any marshall i've ever had? And at this point everything in v2 onward is copied from a marshall.

[cbarrow7625]

the bass pot is now 100k from ground instead of 25k

Hmmm...well then, Bruce's suggestion (I think) to change the Bass cap from .022uF to .047 uF is probably the right thing to do. You could go as high as .1uF (which I did on a marshall-like channel in my own amp to smooth out the low range and it worked really nice). Once you do that, you may find that you can go back to a 25k pot.

Double the value of the screen resistor if you split them (to 300 Ohm). Bruce's suggestion about an additioanl dropping resitor is a great idea too.

Bruce, thanks for jumping in on this and adding all kinds of really good ideas (keep 'em coming). I love to find out what other people like to do in these situations.

[daz]

I tried both a .047 and a .1 but theres little to no difference. the mid pot still boosts what i hear as treble, and it's bright and hard as he||. I keep staring at the wiring to see if theres a wiring mistake but it all looks fine. You should see this board....it's so sloppy and full of solder blobs and all from endless experimentation !Total slop fest....it's amazing it's not noisy ! I can't wait to nail the tone so i can get a new board and components and wire it all up new.

[Bruce / Mission Amps]

Hmmm...well then, Bruce's suggestion (I think) to change the Bass cap from .022uF to .047 uF is probably the right thing to do. You could go as high as .1uF (which I did on a marshall-like channel in my own amp to smooth out the low range and it worked really nice). Once you do that, you may find that you can go back to a 25k pot.

Double the value of the screen resistor if you split them (to 300 Ohm). Bruce's suggestion about an additioanl dropping resitor is a great idea too.

Bruce, thanks for jumping in on this and adding all kinds of really good ideas (keep 'em coming). I love to find out what other people like to do in these situations.Well one thing I would do right now is dump the 100K mid pot.
I know I wouldn't like it or do it, But IMHO, the absolute largest value mid pot you could use is 50K.
Truthfully, I would just use a good old fashioned 25K audio taper pot for the mid control, 1M for the Bass and 500K linear for the treble with a much smaller treble coupling cap... as I mentioned before, 180pf to 220pF.
If you like more of that scooped out mid tone, use only two of the mid pot's lugs.
Wire the .022uF cap to the mid pot's wiper, like a 6L6 Fender Super Reverb amp, which also means at the junction of the mid pot wiper and bass pot far left lug, as viewed from the back of the pots.
And Daz, that mid pot would be wired as a variable resistor, again, which means as viewed from the back of the pot.... only the middle wiper is used and the left outside lug is grounded.
A 100K is really too large of a value as a mid pot in these and that 100K lifts the tone stack pretty far from ground, making all the tone controls less and less effective as you move further and further from 10K to 25K and on up.

[daz]

100k in series between the bass and mid pot works with the 25k giving it a smore linear transition and the sound of the 100k pot. however, still no meat when i turn the mids up. Marshalls always gave me the meaty part of the mids, not low treble. In fact, i may even say the mids are in the middle of the treble range while the treble pot is now pretty normal in it's center freq, maybe a tad higher.if i could center the mid pot at around 500 i'd be home. Amp is sounding well better than ever !

[cbarrow7625]

the mid pot still boosts what i hear as treble, and it's bright and hard as he||.

OK, maybe that's another clue. An alternate version of the standard tone controls places a .0047uF cap across the MR pot from the top of the pot to ground. This shunts all of the HF out of the MR circuit. Maybe that will help you.

[daz]

Oh my....that did the trick.......i had to stop playing it, as i got too darn excited !!! Thanks !!!

At this point i could stop and probably be happier with it than most any amp i've owned. But i think there are problems awaiting me if i use it to gig. Like i've said, i rarely do anymore, but i've been considering "coming out of retirement" if you wanna call it that due to the excitement this amp has given me. And now more than ever. But i believe at stage level, tho we never play very loud, it will be too much power amp wildness to go with mush preamp gain. i'd rather get more from the pre and just some compression from a power amp thats just starting to breath a bit. I've been debating whether to bother, but i figured once i got this right i'd build a new amp with the same pre and a couple EL34's ala DC-30, which i hear is a sort of "marshall 50 that sounds good low" sort of affair. My 50's were always justb sounding right at a tad over the level most bars would allow w/o pestering you all nite.

Anyways, this this is sounding freakin' great thanks so much ! I'll still likely have questions, but for now i think i'll just play. :)

[cbarrow7625]

You might want to try putting a post-PI master volume (especially easy since you have cathode bias). Just replace the power tube 220K grid resistors with a dual 250k Audio pot.

that way you get to hit the PI with more level & at least get it's tonal contributions in the mix, then tame it down right before the power tubes. Again, something I have done many times & really like. If everthing else in the preamp is done well, hitting the PI with more level should give you a "fuller" tone and simulate a little bit of what we all like in power amp distortion (after all the PI distortion is part of that sound).

Good luck.

[daz]

About week or so ago i got my hands on a dual 500k clarostat and did just that, but it sounded horrid. It was probably because of the preamp design tho. the PI was getting slaughtered and you could hear it. Theres still a lot of gain tho, so i'm not sure it would be any different.

[cbarrow7625]

I find that the trick is you also have to have the pream MV turned down a bit so that you don't hit the PI so hard. I agree that with the preamp MV on 10 it does sound horrid. Somewhere between 6-8 on the preamp MV I find a sweet spot in most of mine.

It just might not work for yours but I thought I'd mention it.

[daz]

Actually, i cranked it up just now and was surprised at how controllable it was. Whether it's loud enough i won't know till i play out with it. But if i have to build a new power section for the same pre i will. probably will anyways since it's rather fun hearing such awesome results.

[Bruce / Mission Amps]

Actually, i cranked it up just now and was surprised at how controllable it was. Whether it's loud enough i won't know till i play out with it. But if i have to build a new power section for the same pre i will. probably will anyways since it's rather fun hearing such awesome results.OK sounds like you are making progress... now go back to your art work, or mine, or someone's ....and redraw it so we can see what it is you like.:)

[daz]

Ok, i'll try. I'm now home and the amp isn't here so i'll have to go by memory, which shouldn't be hard after all this !! LOL...

I'll post it in a while if i can remember everything well enough

[daz]

Here ya go. the filters may be reversed.... that is, i can't remember if it was the 16 or 20 in front and back. I think i had those values already and just used them rather than order more. would i benefit by getting larger filters or would that possibly kill the squishy sag i get and love?
as you can see i pretty much just used marshal values throughout V2 and the PI. The screen was 470 and changing to a 150 was one of two things that made a huge difference. The 150 made it tighter and smoother. The other thing was the extra resistor and cap in the stack, the resistor being my idea and the cap cbarrow7625's. it was that cap that changed the mid knob to a usable range, tho i may try different values to hopefully center it somewhere around 500-700HZ (guesstimate) where i like it.
No cap on second cathode....it just didn't need it. Tied it to a switch so i could A/B with and w/o it and the tone was just much better w/o. The tone is meaty yet not muddy. The lows can still stand some tightening, but not via adding more high end. I think the key may be in the P/S, possibly by lowering the screen to 100 ohm like it shows in many if not most 18 watt schematic. Going from 470 to 150 was great, so a bit more may be even better. heres the way it is now to the best of my recolection...

http://img245.imageshack.us/my.php?image=18rh2.jpg

EDIT: I was wrong about that 2nd stage cap....i DO need it, and badly. Don't ask how i thought i didn't....very long story. in any case now there is a 1uf bypass on the second stage, tho the value could change. I'm also going to experiment with the .0047 cap in the stack cbarrow7625 suggested.