Some amplifiers used a selenium or similar (copper oxide?) rectifier to
supply filtered DC to the early stages, reducing hum. As these
rectifiers start to fail, their effective resistance increases and
output voltage decreases. It could be that's what's going on.


There's a 1.6-ohm power resistor in the filament line, Fred. This was
done on purpose. It's hard to believe that it is for reducing hum, as
it amounts to a reduction of just 2.5 dB. This trick is done to diodes
in radio circuits to lower contact potential and improve
rectification, but I don't see the connection with triodes. Maybe it
lowers white noise. If so, I'll bet more than 2.5 dB. Next time I have
the amp apart (a no-name console pull), I'll short the resistor while
listening and see if I can hear any difference.

Brian