Had this ST70 for a while now. it was completely rebuilt in '96, only thing original left is the chassis and transformers, (I bought it 1/2 done and completed the rest between classes in college)
In any case it worked great, (well one 7199 died after 5 years or so, which radio shack replaced for free!) couple years ago bought new output tubes figured the ones I got back in '96 might be getting tired. Had it all set up in my office at work, all was good for a week or so. Than it started humming a bit. nothing out of the speakers, the main transformer was making the noise. found low bias voltage, turned it up.... after a short while hum became more of a buzz and could be felt on the table. shut it down, left it for a day. found it would work fine for a few minutes when cold, than start humming again.
Office got moved(downsizing YAY, now I share a tiny office) soon after so I just shelved the dynaco at home (I have small children, hot delicate tube amps are not a good mix.
Now I'm considering selling, and I don't do broken. What I read: low bias voltage, transformer noise = rectifier tube going bad. SO a brand new JJ GZ34/5AR4 arrived today, plugged it in and BAM instant LOUD buzz. shut it down pull out the meter (Note: bias pots were turned all the way down before I tried the new rectifier) turned the amp back on bias voltage starts to rise, doesn't stop at just a volt or so it keeps going, already got over 4v by the time I got to the off switch.
So after all that, Question is, is my current assumption that the 50uf bias caps gone correct? (in my case they're 100uf) or is something else pooched (note: selenium rectifier is long gone, diode in place and it checked out)