Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: topbrent on August 24, 2010, 02:31:11 am
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So, I was experimenting with some different treble cap, slope resistor and v1a plate resistor values in a 35w single channel, no reverb, no trem, AB763 amp of mine. It is a simple, low gain circuit, with what I would consider few places to induce oscillation issues.
This was designed to be a nice, punchy clean sounding amp that would be the ideal pedal platform amp.
The amp was working just perfectly fine until I swapped 3 components.
As I performed the mods, I had good clean solder joints, and was quick with the heat. Didn't move any wires to affect lead dress, ect.
I was returning the amp from slightly modded values of 330pf treble cap, 150k v1a plate and slope, back to the stock AB763 values of 100k plates, 100k slope, and 250pf treble cap.
I have 250v on the 100k plates of v1 fed by a 33uf filter cap in the b+string, 1.5k/10uf cathodes, standard 6.8k mid resistor. All resistors are carbon film Xicon 1w.
Now, the amp sounds awful. It has developed this strange oscillation that sounds like a ring-mod distortion effect as the note dies out. To make a note sound, it takes a harder attack than one would expect, and the volume is now greatly diminshed.
- Very scritchy and it just sounds like crap now.
I have re-flowed the solder joints, swapped every tube and generally glared menacingly at this amp for a few days now.
I cannot find the source of the oscillation.
Help me, please.
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I'd blame whatever you changed last. So put the old components back, one by one, testing each time. Even if it still sounds the same you'd know more than you do now.
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What about grid stoppers?Or a bad treble cap if you used silver mica.
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You changes could have exposed an unstable portion of the circuit. Swap tubes, grid stopper, and chopstick all the joints and the wires.