Amp Techs: help me with a JCM800 clone mods

peaveyT60 · Feb 1, 2007

i have a peavey VTM60 (60 watt, 6L6 version of the JCM800) that i want to clean up just a bit, nothing major though. The high end has just a bit of fizz in it that i would like to clean up. i have modded a valve jr and a classic30, but am still pretty green when it comes to how a circuit functions and etc etc. Its gotta be just as simple as changing the value on a cap in the preamp section, or something similar.

a secondary want is to have a clearer, more sensitive guitar signal coming into the amp.

any help or ideas are appreciated.

voodooamps · Feb 5, 2007

Check out the schematic and trace through the boost switches, my guess is that on average you will prefer most of those in the off position. You can try to mirror the JCM800 2204/2203 as much as possible (less fizz/buzz) and all that could be as simple as comparing schematics then making some changes (IE: You can move R5 to a 10K, R18 & R20 can be 1M's, R16 - 82K,. R17 - 100K, etc.)

Have a great day
Trace

kannibul · Feb 5, 2007

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voodooamps":a6ebf said:
Check out the schematic and trace through the boost switches, my guess is that on average you will prefer most of those in the off position. You can try to mirror the JCM800 2204/2203 as much as possible (less fizz/buzz) and all that could be as simple as comparing schematics then making some changes (IE: You can move R5 to a 10K, R18 & R20 can be 1M's, R16 - 82K,. R17 - 100K, etc.)

Have a great day
Trace

Welcome to the board Trace!

(sorry it was 14 posts later!)

Pepi · Feb 5, 2007

Try a simple approach and put a Jan 5751 tube in V1. Just may be your ticket :wink:

peaveyT60 · Feb 5, 2007

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voodooamps":6d779 said:
Check out the schematic and trace through the boost switches, my guess is that on average you will prefer most of those in the off position. You can try to mirror the JCM800 2204/2203 as much as possible (less fizz/buzz) and all that could be as simple as comparing schematics then making some changes (IE: You can move R5 to a 10K, R18 & R20 can be 1M's, R16 - 82K,. R17 - 100K, etc.)

Have a great day
Trace

i do prefer them off.

thanks trace for the advice!

8)

peaveyT60 · Feb 5, 2007

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Pepi":6d85b said:
Try a simple approach and put a Jan 5751 tube in V1. Just may be your ticket :wink:

thanks!

will definitely check that out.

you guys' help is great.

duesentrieb · Feb 5, 2007

I'd play around with the presence section, f.i. increase the presence cap, move the nfb loop to a higher impedance . . .

peaveyT60 · Feb 5, 2007

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duesentrieb":4b376 said:
I'd play around with the presence section, f.i. increase the presence cap, move the nfb loop to a higher impedance . . .

Edit: ok, the NFB loop. let me see if i can find out about that. i might need help if i can't find out.

thanks

peaveyT60 · Feb 7, 2007

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voodooamps":f31c7 said:
Check out the schematic and trace through the boost switches, my guess is that on average you will prefer most of those in the off position. You can try to mirror the JCM800 2204/2203 as much as possible (less fizz/buzz) and all that could be as simple as comparing schematics then making some changes (IE: You can move R5 to a 10K, R18 & R20 can be 1M's, R16 - 82K,. R17 - 100K, etc.)

Have a great day
Trace

Trace,
I matched the preamp section of my VTM to a jcm2203/2204 as much as possible (VTM has 1 extra preamp tube as well as an effects loop). I also lowered the high gain input's grid resistor from 68k to 10k. As a result, that little bit of high-end fizz is cleaned up (which i attribute to matching to the marshall's specs) and the amp seems just a bit more sensitive to input levels (which i attribute to the grid resistor). I was going to also look at the NFB loop as suggested by duesentrieb, but i am happy with the sound now. I am in that stage now where i know just enough to be dangerous. :lol:

Also,
If I were to choose a new MM output tranny, would you use the one for (read: what are the differences in) the 50 watt, 6L6 marshall jcm800 or for a 50 watt, blackface bassman? I am guessing the one for marshall would grind a bit better and the bassman would have more sparkle.

Thanks!