volume drop on Princeton Reverb build?

[Bananafist]

Hi, I've built a couple of basic amps, but as my knowledge is still far from extensive, I'm having a PR built for me here in the UK, with larger than stock trannies, otherwise stock. I've just learned the builder, who is an experienced amp builder, is finding the output volume drops after a while when running the amp. Otherwise it works fine. I think he is a bit puzzled, and I wondered if anyone here had any practical suggestions I could pass on to him.
Thanks

[Colossal]

This might be an issue with the power supply. Voltage is volume. Since the behavior seems repeatable (?) it's hard to suspect a preamp tube. But I would certainly replace the tubes one at a time with a known set of working tubes, just to rule out a variable easily checked.

My guess is that as the interior of the chassis heats up (and those Princetons can get hot) something is reaching a stress point, triggering the volume reduction. Definitely check the rectifier tube. Maybe an electrolytic cap is starting to go. A failing cap might manifest itself as a sharp hum however.

I would take the voltages throughout when the amp is first turned on and again just after the volume reduction. If there is a voltage reduction throughout following the volume reduction, you can easily track that down to the power supply, rectifier, or even PT (check the secondaries).

Hard to speculate without more background.

[Bananafist]

Thanks very much for the suggestions, I'll pass them on. If it persists I'll try to get more info.

[Firestorm]

If it's the stock circuit, I'd suspect the series resistor in the bias network. The weak link in Princetons.

[Firestorm]

If it's the stock circuit, I'd suspect the series resistor in the bias network. The weak link in Princetons.