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Re: mega gain build suggestions (4 stages)...

10/29/2005 11:48 PM
CBRe: mega gain build suggestions (4 stages)...
I don't think you need more gain stages - just something different.  
Below are the particulars regarding a Marshall mod I've been doing for many years. It's a very heavy, percussive sound. Great for heavy palm-muted riffs, etc. You need a closed-back cab, of course...  
I've never gotten around to drawing this up, but if you are familiar with a typical Marshall MV preamp, this should make sense to you:  
Input jack to first stage - 34k resistance (the two stock 68k's in parallel), or a 33k will do.  
First stage cathode - 3.3k bypassed with 330mfd.  
Out from first stage, through .022 into a 1 meg pot. Bypass the pot with a .001 cap. Out from pot through 470k bypassed with a 570p cap, on to second stage.  
Second stage cathode - 2.7k bypassed by .68mfd (you'll notice this is the stock 1st stage values - you can save some time/trouble by just switching the wires on the first two stages).  
Out from 2nd stage, through .022, into 1 meg pot (use one of the now extraneous input jack holes). Bypass pot with a .005. Out from pot through a 470k bypassed with a 500p, on to third stage.  
Third stage cathode - 3.3k bypassed by a 330mfd.  
Change tone circuit cap from 500p to 250p.  
Everything else - leave as stock.  
The two "gain" or "preamp" knobs should be set in the 5-6 range to hit the sweet spot. You could replace one with fixed resistors, but I have found that tweaking the two pots in the 5-6 range allows for a good tonal variation (due to the bypass caps) that helps when switching between different sounding guitars, etc., so I put them both on the front panel.  
Amps with EL34's or 6550's both sound very good. The 6550's sound "heavier".  
I believe this is very close to the mods performed by Frank Levi around LA in the late 80's. I first started tinkering with amps at that time, and dissected amps done by Levi, Soldano, Lee Jackson, etc.... so, nothing original here!  
Hope this is useful to you.