biasing preamp tubes?

[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.
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....
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.