# Can this mod be done ???



## Robert1950 (Jan 21, 2006)

I have a Gibson GA-5 RI. I was wondering if a Tone Stack could be added to it. It is a PTP tweed champ like amp that uses an El84 and 12AX7 or 5751 etc.

Here is a link to a schematic: http://www.diycustomamps.com/pdf/GA5_schem.pdf


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## Wheeman (Dec 4, 2007)

I don't know about the mod, but looking at the schematic it looks mighty simple and tempting as a summer project.

Looking through the website, I stumbled upon this: Its for the Valve Jr. but should work for the Gibson.


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## mhammer (Nov 30, 2007)

You proably could stick in a tone-stack. The problem is that tone-stacks eat up a lot of signal because their whole raison d'être is to selectively bleed signal to ground. If you had the option to add recovery gain somewhere else in the circuit, that would be one thing, but there isn't a lot of room to redesign for gain in there, given how basic the circuit is.

You may be better off simply adapting the tone control found on the old tweed Princeton and similar amps. It provides variable treble bypass of the volume control in one direction, and variable muting/treble-bleed in the other direction. So, essentially an improved version of the standard guitar tone control, except that it doesn't eat up as much signal as a tonestack. NOT "the shiznazz" as tone controls go, but easily adaptable to your amp, tried and true, and will not need a gain recovery stage to permit pushing the power stage hard.


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## Robert1950 (Jan 21, 2006)

mhammer said:


> You may be better off simply adapting the tone control found on the old tweed Princeton and similar amps. It provides variable treble bypass of the volume control in one direction, and variable muting/treble-bleed in the other direction. So, essentially an improved version of the standard guitar tone control, except that it doesn't eat up as much signal as a tonestack. NOT "the shiznazz" as tone controls go, but easily adaptable to your amp, tried and true, and will not need a gain recovery stage to permit pushing the power stage hard.


That's what I had in mind. I was thinking of replacing the 12AX7 with 5751 to tame the gain a little too. This thing starts to break up around 10:00 with HBs. I've also got a Weber 4 ohm 8F125 to put into it.


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## Greg Ellis (Oct 1, 2007)

Yeah, without adding another gain stage, or even two (one before the tone stack and one after) you're going to lose a lot of signal here.

Several years ago I tried to do this to an old three tube amp (12ax7, EL84 and a rectifier tube). It didn't go so well.

There ought to be lots of suitable pedals you can stick in front of the amp, if your goal is to shape the tone (and assuming your guitar controls aren't doing the trick for you).


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## Guest (Jun 9, 2008)

Actually the pedal idea is where I would take this. You could build an effects loop with an-op amp and stick something like a Boss GE-7 in the loop. You'll need to power your solid state of course with a separate power supply +/- 15V.


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## Adicted to Tubes (Mar 5, 2006)

If you want something simple that takes nothing away from your amp,get an EQ pedal like an MXR.It gives you all the tones and more at your fingertips.


www.claramps.com


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## Robert1950 (Jan 21, 2006)

I've used an EQ pedal (Danelectro) to add some lower mid and bottom end to a strat. Maybe I will just add a lower gain tube and do the speaker change first. Someone had a 12AT7 for sale here someplace.


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