Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Dave on February 01, 2026, 12:34:21 pm
-
Hello guys,
I came across an A-100 at a price that I could not walk away from. Got it home hooked everything back up like it was supposed to be. I'm not an organ guy, but I was impressed. Super low noise floor. Super quiet tone generator. It was fun to play around with. So... here comes the tweaking.
Most don't like the internal power amp or speaker arrangement. I have to agree. Although it came with Jensen P12N speakers, it was very quiet. I took some measurements and it was showing about 5 watts worth of output with big chords.
I looked at the schematics (attached) and the amp does not use an input transformer like a Leslie hookup would and, I expect for that reason, the signal is attenuated a lot lot lot.
I happened to have an input transformer for a Leslie 122, so I thought I would play around with it and try to get it to make a little more sound.
I replaced the power and output transformers with ones that would make similar voltages but would accommodate a quad of 6BQ5's instead of two. Although it worked perfectly, I wasn't thinking about the fact that it wouldn't make much difference in the volume with the same signal level coming in.
So, I started stripping out some of that attenuation. Almost immediately, it started overdriving the stew out of the driver tube (12AX7). I assumed (because I don't know any better) that without the input transformer that you just couldn't squeeze much more out of it.
So, I rewired the driver tube to the specs of a leslie 122 (schematic attached), swapped the X7 for a U7, got the voltages close to correct, hooked up the transformer, and... it works great, but, at volume, same thing... a freakishly over driven driver tube.
I have checked and rechecked everything and I can't find anything that I suspect might be the culprit. I don't know much about these balanced output/input circuits though. Maybe I have missed something one of you guys might point out.
Dave
-
I got it working. Not sure at all why it works the way it does, but here's how it worked out...
After much fooling about, I tried to bypass the input transformer. I wasn't thinking clearly and I just jumpered red to red and black to black. When I realized that it wasn't what I intended to do, I scrambled to undo it, but, I decided to hit a key anyway just to see what it would do. Totally clean! Very strange.
To make sure I wasn't crazy, I bypassed the transformer all together. Total distorted mess.
I wired it back up correctly. Total distorted mess.
I wired it up backwards just to see. Thin, mid rangy, ugly mess.
I wired it back up with the transformer in circuit, but "bypassed" red to red and black to black and everything worked just like I had hoped.
I went from 5 watts of output to a solid 16 watts of output.
Don't really know why that worked. I hope somebody sees this who can explain it. I can't.
Dave