"Sander deWaal" wrote in message


"Arny Krueger" said:

Do tell us Arny, about the hum that accompanies vacuum
tubes.

Something about the fact that a lot of tubed equipment doesn't put
DC on all the heaters.

With indirectly heated tubes, there's no need for using DC on the
heaters, except for sensitive stages like a phono preamp.
The hum of my power amps is -80 dB with AC on the heaters.

Given that I frequently find SS power amps with all forms of noise including
hum 110 dB down, this seems like it could be a problem. Hum and noise in
power amps is more likely to be a problem than with say, a preamp, because
there is no volume control following the noise source in the power amp.
Therefore -80dB hum and noise referred to full output (say 20 dBW) is
effectively only -60 dB down when referred to a lower but common listening
level of 1 watt.

DHT amps *must* use DC on all heaters, as is the case for over 40
years now.

Because SS rectifiers and regulators are so common, so effective and so
cheap, it's hard to explain why a quality tubed amp wouldn't use DC filament
supplies on all tubes that pass audio. DC operation also extends the useful
life of tubes, as does effective voltage regulation.