The Amp Garage

Any chance you could post a pic of the glowing screens in question? I've been wondering about my screen current too.

This probably sounds stupid, but I'm not sure I can tell what's the screen and what's the filament in there. It all looks very tiny compared to the 6V6s I'm used to.

FWIW, the OT in an older Hammond will drive a modern speaker just fine. The problem is that the speaker field coil gets power from the center tap on the PT secondary. Both that and the negative end of the first filter cap (weird) are grounded through it (plus that circuit supplies some oddball negat...

Well, I'll bite if nobody else does. I'll try it the next time I have the amp open. Thanks for posting.

I've had good luck rebuilding old PA and organ amps using proven guitar amp designs. You learn at every step: how the original worked, what bits still work for guitar (mostly power supply and power section, but never quite right), and finally how classic preamp sections work. As a bonus, since the r...

It used to, anyway. The field coil speaker is missing from ours so I replaced it with a resistor per an old Hammond service bulletin. I used the bolt-on, heat-sink type and put it on the outside because it gets seriously warm. It seems there are more complex ways to approach the issue, but this work...

Sort of helpful, thanks. The organ sounds great now so it's really just a matter of curiosity.

I am glad I used 500v ratings, though -- modern wall AC kicks all the voltages up a notch so it was more like 430 volts across a 55-year-old, 450v cap.

This weekend I spent some time reconditioning a beat-up Hammond M3 that's in my band's new rehearsal space. Unstuck and lubed the mechanicals; added grounded plug and fuse holder; replaced (missing) field coil speaker with a massive wirewound resistor; and replaced scary old electrolytic caps. I alm...

Very cool, thanks for posting.

mmmoser -- sorry to hijack the thread, but do you squeeze some sort of eyelet or turret board into the ao-44 chassis, or do it all PTP?

I recently did something similar with an ao-43, and I'm thinking about a smaller footprint for the next one.

Just ran across the thread -- do you mean the first coupling cap? I've seen values down to .001. I can't hear too much of a difference, but mine's .002 now and it sounds good.

This doesn't help with the drive channel, but there's an easy mod for the clean side that converts the fairly useless Presence pot to a simple master volume. Works like a charm and makes that annoying hiss go away.

Is there a good reason not to put a 12ax7 driving a tone stack, Lightning-style, after an EF86 preamp? Every EF86 example I can find basically goes straight to the PI, with just a volume pot and usually some sort of top cut option. The venerable AC-15. I understand that you can't drive a tone stack ...

+1. As it happens I just received one of these, along with a cab and some tolex, to make a custom head that matches the cab. Nice people to deal with, too.

That's a very cool idea, thanks. I always forget about phase differences.

I don't think it'll work in a Matchless clone, though, because both amps use two triodes in parallel as the first stage. (As I understand, this is to emulate the high gain of the old EF86.)

I was thinking of doing the very same thing on my project amp, which right now is pretty much a Spitfire with an empty socket and some space in the middle. It would need another pot to keep both the tone knob and T/B stack intact, though. FWIW Trinity makes a variation on the theme that's AC-15 on o...