Loud amp hum

[TeenageShutdown!]

I have a ‘66 Ampeg Gemini IV that was working fine when I bought it, but then it developed a loud hum. The hum isn’t a 60 cycle hum. It’s a baritone sounding hum at a voice speaking volume. Adjusting all the controls across both chanels does nothing to change it, nor does having the guitar unplugged. I’ve tried different 3 prong receptacles throughout my house with no difference. It doesn’t make the hum when in standby mode. I can still play guitar through it but I am not confortable doing so.

I checked the tubes when lit up & I don’t see any odd coloring. It should be noted though that the tubes look to be original (RCA’s). I tapped on each tube with a chipstick & nothing sounded microphonic. Any ideas? Caps or filter?

[Squirrel]

In my experience this issue is usually a sign that the smoothing caps are on their way out. These are the large electrolytics in the power supply stage. You may as well replace all the electrolytics in the amp while you're in there, as they're only supposed to last about 20 years or so.

Edited to add: it could also be a tube issue or a bias problem, but if the electrolytics are original that would definitely be where I'd start.