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Old 26th July 2009, 05:22 PM   #11
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Join Date: Jun 2006
Default Hammond amp mods

From the circuit I'd guess you have way too much gain - you're probably over-driving the second stage with a guitar signal and a gain of 600+. After that, there's no way back.

The reverb spring output was about 2mV and a guitar is at least 200...

I'd add a volume control where you've reduced the resistor value to 220K - it will need a series cap to block the DC on the grid of the phase-splitter stage.

At least that will let you listen to what's being over-driven in the input stages.
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Old 27th July 2009, 01:09 AM   #12
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According to the schematic, I see no problem making a conversion for guitar.

You'll need to replace most of the resistors and caps, lose a few, add a few, but the tubes and transformers are probably ideal as is.

I would lose the feeback circuit from the output altogether.... but not UNTIL you reduce the gain of that second stage!!!

Check out AX84.com and look at their High Octane guitar amp project. I would modify your preamp stages to model the High Octane. It's a winner. High Octane Schematic

Or... you could opt for their S.E.L. model, known as the Single Ended Lead project, which has a little more crunch and grunge. (but your power tranformer is probably better suited for the H.O., since your schematic indicates 300v on the output stage.)

The H.O. and the S.E.L. are both "single ended" output models, and your's is push-pull, but your preamp stages should be all you need to modify to get her going, and you already have most of what you need.

Your hammond "pushpull output stage" looks pretty good to go as is. This should make a terrific little amp. I should be so lucky to come across junk like that!

AX84 also has some push-pull projects too if you want to compare stages. Look in the Classic Amp Projects section. You can marry just about any preamp to any ouptput stage, depending on your taste, and thurst for power.

I'd begin there... and then tweak it to suit.
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