• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Bell 2122 volume drop off

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you either have a bad pot, or a parasitic oscillation that is being picked up at higher gain settings.

Does the amp have a resistor in series with the first grid?
Try 50-100k between input jack and first grid. Eventually you may want to put 1k series resistors on the power tube grids, but that's only if the oscillations continue. These resistors acts like a filter using the tubes internal capacitance to AC ground (B+)...they form a low-pass at fairly high frequencies (generally above hearing, def above guitar amp range).

Have you tried the "chopstick" method of pocking around with an insulated chopstick to see if you can affect the frequency/onset of the problem? This checks to see if stray coupling or capacitance is introducing a signal back into earlier stages leading to a positive feedback loop.

Try removing all cathode bypass caps, they're electrolytic, way old, and they're not always good for tone when overdriven. They will also tend to help the valve amplify radio frequencies spuriously as they get wonky.

Make sure all internal wiring is short, and isn't all running parallel with other wires, long parallel wires can couple better than short, perpendicular ones.

Experiment with different feedback cap/resistor combinations...you probably need less high frequency response somewhere.

If this thing is as old as I'm guessing, I would at least consider replacing all the resistors in the power supply with newer, quieter resistors, replacing the power supply caps, and possibly the coupling caps into the power tube grids. This will ensure quiet, hiss-free operation, making a much better guitar experience.

This is honestly the hardest part of design for me, since I always do point-to-point. It takes some practice to find the culprits...make sure you are using a big soldering iron, it makes all the tube stuff (esp. old tube stuff) easier to work with. The radioshack 30w is the worst thing ever done to newbies!
 
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