Brian wrote:
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.

Well, I suppose that 2.5 dB could be considered significant.

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.

That's the ticket, the good old "scientific method." Ideally you'd
actually measure the hum/ noise, but a listening test would be better
than nothing.

One thing to watch out for - changing the filament voltage can also
change the transconductance, and hence stage gain. So to prevent being
misled, it would be a good idea to adjust the input signal for the same
output voltage under the two different conditions, before removing the
input to measure the hum and noise.

Let us know what you find, this could be quite interesting.

Cheers,
Fred
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