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Old 9th October 2008, 10:04 PM   #1
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Default Mod it or scrap it?

Hey people
I have an old Hammond organ amp that I thought would make a good guitar practice amp.However,my guitar signal is way too high for the input and is not making a pleasing sound at all.
I have been looking at small guitar amp schematics on the web to see what the usual setup is and it seems my organ amp is a bit more complicated than it has to be.

The problem is,I don't know if I should mod the pre-amp to bring the gain down or scrap the whole thing except the power amp and PSU and make a proper guitar amp out of it.
It has EF86 input stage and 6BQ5 pushpull output,which I have noticed looks very similar to a Vox AC15(1959/60 version) and some other small amps.

What should I do?I am afraid that if I get into modding it,I might end up rebuilding the whole amp anyway and if I scrap it and make an AC15 from it,am I ruining a perfectly good amp?

I have good experience in Electronics but am new-ish to tubes so any advice is welcome.
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Old 9th October 2008, 11:54 PM   #2
Jeb-D. is offline Jeb-D.  United States
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Have you tried patching straight into the power-amp, bypassing the pre? From the schematic, it appears to be where the compression pedal output plugs into.

If you end up with too little gain trying that, you can try triode strapping that pentode on the input of the pre. And/or removing that cathode resistors bypass cap.
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Old 10th October 2008, 12:00 AM   #3
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I would try one of two very simple modifications first to see how you like the sound. First, make a simple two resistor voltage divider for the input to knock down the guitar's level. Or make the 470K input grid resistor a pot to adjust the input level.

The second choice is to bypass the first tube altogether by disconnecting the .1 coupling capacitor from the plate of the EF86 and feed the guitar into the disconnected free end.

Victor
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Old 10th October 2008, 12:13 AM   #4
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You may modify it for a guitar.
A first, a new phase splitter is needed instead of one that is there now: it is good clean sounding phase splitter when used for organ, but when overdriven it sounds harsh. Borrow one from Fender amp. Also, pay attention on capacitors from phase splitters to grids of output toobs: with big values when overdriven grid currents charge them causing far$%ng type crossover distortions, that's why Fender used much smaller caps and large values of grid stopping resistors.
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Old 10th October 2008, 12:16 AM   #5
Jeb-D. is offline Jeb-D.  United States
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Quote:
The second choice is to bypass the first tube altogether by disconnecting the .1 coupling capacitor from the plate of the EF86 and feed the guitar into the disconnected free end.
Good suggestion as well. This will lower your gain to some point between my two suggestions.

It will just depend how much you need to bring it down.
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Old 10th October 2008, 01:09 AM   #6
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I can't tell for sure but if the AF amp and Driver 12AX7 sections are in fact in the same bottle you can rewire them as a LTP phase splitter. If you do that the remaining preamp tubes may just be the ticket gain wise. Two or three triode gain stages is probably just about perfect.
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Old 10th October 2008, 04:09 PM   #7
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Thanks everybody for the advice.
I think I will go straight in to the second valve,I never thought of doing that.
The first valve seems to have a gain of about 100 but the second one seems to have gain of about 4.
I think this because there are AC signal voltage levels on the diagram.The input should be 0.011vac and the output of first valve is marked as 1.32vac.The output of the second valve is then marked as being 4.2vac.
When I scoped the input with my guitar plugged in and turned up full,the guitar seems to be producing almost 1Vp-p(Les Paul copy,both pickups on, volumes and tones wide open).
When I scoped the output of the second valve at the input to the volume control,I was alarmed to see about 138Vp-p(and clipped) instead of around 4.2vac as the drawings stated.I just hope this has not harmed the preamp or the rest of the stages.
I think also I will make a new phase splitter for it as Wavebourn suggested.
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