A couple questions about Peavey Butcher mods (should also apply to VTM)

[Butcher52]

Hello, I am working on modifying a Peavey Butcher and am trying to decide all the modifications I want to do before ordering parts. I hope these aren't stupid questions, I just want to make sure I have everything right.

If you want to pull up the schematic, it is a few posts down on this thread

1.) I am planning on starting with recapping the electrolytics, replacing screen resistors with 400 ohm/10w (R3,5,7,9), Replacing R49 (400ohm/10W) with a choke.
Should this JCM800 style choke work?
Marshall JCM 800 & JMP Style 5 Hy, 120 mA DC Choke ClassicTone 40-18032

Does anyone know how many Henries the Mercury Magnetics choke people like to buy is?

2.) Some people have mentioned replacing the treble peaking circuit on the input with a 68 ohm resistor. Would I remove C15 and C16 and replace R37 with a 68 ohm resistor? The high gain input already seems to match the JCM800 high input.


3.) The capacitor that corresponds with the Bass tone control which people like to change to .047MFD is C19, correct? Upgrading this value would lead to "tighter" more controlled sounding bass response?

4.) In a post somewhere I read someone say that removing the 470pf grid cap from the second preamp tube would give higher gain, the grid cap in question is C6, correct? What are potential downsides/issues this would have, there has to be some reason this was there?

5.) The other thing I read that could affect gain would be to "Replace V1B's cathode resistor with a 1uf cap", I think he really is referring to V1B on the JCM800 schematic which would be R12 on the butcher schematic? how does this work? are there any other mods I can do to make the amp have better preamp distortion without being out of control sounding like a 5150?

6.) The only other question I have is that until I'm more acquainted with working with this amp, I'd like to try setting the amp up with 2 6L6 power tubes instead of 4, and try and do the EL34 mod at a later point in time, so that the amp is 60W instead of 120 so I can run it "dimed" most of the time to get that nice power amp breakup.

This site details changing R52 with a 1k locking potentiometer to adjust the bias current with.

My math tells me that 30W/485V(my plate voltage should be close to this, I haven't read it yet though) = 61.8MA, and if we want to be at 70 percent that would be .7(61.8)=43.26MA. From some things I've read about JCM800's though, you should add 5MA for screen current, is this true as well with peavey butchers?

What resistance value should I set my pot at in R52 that would give me around 86MA, it should have to be under the stock 470 ohm value since that creates about 110MA of cumulative current, correct? How would I calculate that other than just using a pot and measuring it?


I'm sorry, I know this is a lot. I've tried to organize and explain my questions as eloquently as possible and if someone can help me through my first tube modding adventure I would be so grateful. I've worked with solid state some but this is a new world to me. Thanks!

-Wesley S.

[nashvillebill]

I'm not familiar with the Butcher so I cannot comment on these specific mods.

However, the point must be asked: why are you doing these mods? Is there something about the amp you don't like? What specifically are you trying to accomplish?

I'm not trying to be a contrarian, but too many times, people jump into modifying amps just because "I read this on the Internet". Change cap XYZ and suddenly you will have the tone of the gods! Change the power supply filter to a choke and wow what an improvement! Really? Improvement in what? We must be cautious about modifying an amp (at the level you're talking about) unless we have a clear idea what we are doing, why we are doing it, and how to diagnose whether we are doing this right. One person may think that "changing cap XYZ led to a much brighter sound" when in reality they've just put the amp into high-frequency oscillation. Changing the power supply has ramifications on the entire amp. Also bear in mind that Mercury Magnetics has a vested interest in telling you how much better your amp will sound...because they want to make a lot of money by selling you a choke.

Often, merely upgrading the speaker can provide tonal improvements far more efficiently than massive circuit changes. It's important to ask "what do I want to achieve" and find ways to achieve it, rather than just throwing parts at an amp because "I read it on the Internet".

[Butcher52]

....

You know, I'm not going to justify myself to you solely because I don't care, I came asking for clarification with a couple things and if you can't contribute as far as I'm concerned you shouldn't be posting in this thread. I find your attitude to be higher than though and belittling and that is no way to welcome to someone to a forum. Please do not continue posting to this thread unless you have a relevant response to one of my concerns. I can't believe you just wasted your time writing that out.

It's very similar to a JCM800 and all of these mods (that apply) are pretty common and the improvements are well documented. the Bias mod of course should make a noticeable difference in tone, and removing the treble peaking circuits should lead to a flatter response and higher preamp gain. Multiple people have reported noticeable improvements in the power supply by installing a choke. I'm just trying to make sure I'm correct with all of my values and help understand what a couple of the other possible modifications could affect the amplifier so I can modify it to better suit my tastes.

If you must know I basically just want to get this set up where it's biased correctly to 60w because right now I feel like it's too loud and cold-biased with lots of raunchy harmonics, and try and add some gain and clarity. Recapping should have a noticeable affect since this amp is 30 years old, and there is no reason why I shouldn't try adding a choke to clean out the power some and make it closer to an early manifacture JCM800.

[nashvillebill]

Look, this was your very first post ever here, and that first post came across as basically "I'm going to do these mods because I read about them on the Internet". What do you expect?

There's some very helpful people here, even if you don't think I am, but you needed to tell us what your goals are. That's all I was asking. You did answer that in the latter half of the second post, and there was no need to cop an attitude. I'm sorry if you construed my answer as being "belittling" but we can't gauge your intentions, or your skills, over the Internet.

[The Dude]

My $.02

1) 10W screen resistors is overkill and if you do have a tube failure it'll cause bigger problems. 3-5W is fine.
2) Haven't seen the mod scheme- can't comment.
3) C19 would indeed be the cap for bass control. Whether it would lead to tighter more controlled bass- you'd have to try it and see.
4) R15 and C6 form an RC filter and also shape tone. Though you might get more gain removing the cap, it may or may not have a desirable effect on EQ/tone.
5) The corresponding tube in this case would be V5B. However, your mod info seems to me to be incorrect. Possibly the mod is to ADD a cathode cap rather than replace a resistor with a cap?
6) You can certainly run the amp with 2 output tubes. Be aware that it does change the output impedance and you'll have to adapt. In other words, your 8 ohm tap will no longer be 8 ohms, your 4 ohm tap will no longer be 4, etc. Also, it's not going to be substantially less loud of an amp. It's not going to be half as loud just because you pull 2 tubes.
7) I would purchase a bias probe or install 1 ohm resistors at the cathodes to actually measure the bias rather than guessing. I would also not recommend biasing the tubes higher than 35mA@.
8) nashvillebill is a very knowledgeable person on the boards here and made some valid points. It WOULD be helpful to know what you don't like about the amp and what you are trying to achieve. You'll get much better advice. And, he's 100% correct that the easiest most effective mod you can do is a speaker swap. But even then we need to know what sound you have and what sound you are after. He wasn't trying to belittle you. He was trying to help you.

[Enzo]

Butcher, welcome to the forum. You are new here, and coming out hostile to the first person who responds is not going to win friends. There are many of us here who are perfectly willing to help. And it is a forum, not a free design service, so not everyone will agree with your ideas.


I have been a professional amp tech for decades, and if someone comes to me wanting to mod his amp, the very FIRST question I will ask is "what do you intend the mods to accomplish?" You yourself said you hoped they were not stupid questions, so please expect honest answers.

1. I can think of no justification for 400 ohm 10w screen resistors, can you provide one? And if you are moving up from the stock 100 ohm, why 400 ohm instead of standard value 470 ohm? Fender has proven over the last 60 years that 470 ohms is a good screen resistor for 6L6. And just a hunch, maybe wrong, if you are thinking the choke will have very low resistance, and these new resistors will take the place of the old R49, it won;t work. R49 drops voltage from current to all four screens PLUS the entire preamp.

People like to stick chokes in amps, what improvement will you expect?

2. Replacing large R37 with essentially a wire will change the tone, sure. But that also destroys the voltage divider it makes with R38 control. SO the signal coming into the low gain jack will no longer be reduced by a third, the net effect is a 50% hotter signal. You may or may not like that. For that matter,m the signal coming in from the high gain input stage will be similarly affected. And you did not specify which of the JCM800 series amps you were emulating, but looking at the first stages of the 2203, it sure looks like the exact same circuit that Peavey used and you want to remove.

http://www.webphix.com/schematic%20h..._100w_2203.pdf

3. C19 is the bass cap in the tone stack. For more on what does what in the tone stack, go over to Duncanamps.com and download the free Tone Stack Calculator. You can change the component values in a selection of common tone circuits and see in real time the tonal effect. The stock Butcher tone stack appears to already be identical to the 2203 stack. For tighter bass I might want to increase the B+ filter caps.

4. I see no way for removing C6 to increase gain, in fact it will DEcrease the highs passing through that point.

6. You want 60 watts? Pull two power tubes and plug the speaker into one impedance notch lower.

To set this amp up for EL34 is not trivial, substantial trace cutting will be required. If you plan to do this, then 1k 5w screen resistors might as well go in now.


It is easy to make bias adjustable, but I have to disagree with the blog you linked. yes the 470 ohm R52 has to be enlarged. I'd run it up to closer to 3k. And I would make R53 the variable element.

ANY tube amp has screen current that flows through the cathode, so yes, you have to either measure and subtract it or just use a fudge factor of a few ma. It isn't critical either way.

What resistance value should I set my pot at in R52 that would give me around 86MA, it should have to be under the stock 470 ohm value since that creates about 110MA of cumulative current, correct? How would I calculate that other than just using a pot and measuring it?

If we knew what current was going to flow in a circuit we would never have to make it adjustable. If your target is 43ma per tube, that might need anywhere from -50v to -40v or who knows what. It all depends upon each tube. The 470 ohm R52 does not create any current, the tubes do. You vary the bias voltage until the tubes conduct the amount of current you want. Tube current does not flow through the bias circuit. Raising R52 lowers the bias voltage.