biasing preamp tubes?

[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