Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Pinball on November 13, 2025, 08:25:04 am
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Trying to service a 60's GA-30. This particular amp has V1 and V2 as 12ax7. V3 is a 12au7 (Phase Inverter)
I don't like the harshness of the gain. When I put a nice sinewave in at the input, I am seeing a very sharp almost sawtooth at the output of v2b when I add any volume over 25% of the vol pot. All the tubes are good.
This circuit has the "tone expander" as well. NFB off the output transformer.
If I turn the tone pot down almost all the way, I lose gain, but the output waveform starts looking more like a sinewave again.
Do you think the issue is in the tone circuit?
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Trying to service a 60's GA-30. This particular amp has V1 and V2 as 12ax7. V3 is a 12au7 (Phase Inverter)
I don't like the harshness of the gain.
My guess is that's due to a positive feedback loop forming via the shared V3 cathodes.
Fully bypassing it might help with that.
When I put a nice sinewave in at the input,
Amplitude and frequency?I am seeing a very sharp almost sawtooth at the output of v2b when I add any volume over 25% of the vol pot.
Just to note that 25% on a linear pot is well over halfway on an audio taper pot.
This circuit has the "tone expander" as well. NFB off the output transformer.
It looks like a bass boost?
If I turn the tone pot down almost all the way
It's another linear track, all the change happens between 0-33% rotation.
I lose gain, but the output waveform starts looking more like a sinewave again.
With the volume settings you're using, the tone control is causing a treble boost unless it's turned pretty low, eg 10% rotation. Do you think the issue is in the tone circuit?
I suspect the issue is with the paraphase splitter.
Or just old ecsps :think1:
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Do you think the issue is in the tone circuit?
I suspect the issue is with the paraphase splitter.
did you probe "inside the TS"??
sometimes you get pretty ugly stuff from scope loading/impedance
I typically test a TS to the "left" of the "input coupling cap" then the plate of the recovery tube, if those are "close to normal" move on down the stream....
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Turns out it was the 220k resistors prior to the phase inverter. One was open and the other was reading extremely high. In total there was 8 resistors that were way out of tolerance. I replaced all of those. One note, I changed V1 to a 12at7. That actually made the amp much more usable. No clean headroom after 2,3 on the volume until I did that.
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Turns out it was the 220k resistors prior to the phase inverter. One was open and the other was reading extremely high. In total there was 8 resistors that were way out of tolerance. I replaced all of those. One note, I changed V1 to a 12at7. That actually made the amp much more usable. No clean headroom after 2,3 on the volume until I did that.
Well done!
So the overdriven tone is ok now?
The only 220k I can see on the post 1 schematic are the V3 anode load resistors?
I suggest you consider fitting volume and tone controls with a decent audio taper, eg Alpha Taiwan.