> Great points, Dennis. I have often thought about adding a separate HV
> contactor relay to both Alpha amps for the purpose of applying HV at some
> reasonable time after lighting filaments. My gut has never been
> comfortable
> with HV being applied to the anode at the instant when filaments are lit.
Hello Paul..
There could be two schools of thought here. There could be some danger
to the filament as voltages are rising, cathode isn't fully hot yet,
maybe the grid isn't sufficiently negative yet and current flowing on a
partially heated cathode, I don't know. However, in the case of your
CV, the application of HV simultaneous with the filament probably
serves to soften the filament startup current as the power supply is
also loaded by charging of the HV caps. I would think this would help
tube life. The cold filaments probably also serve to soften the
charging current going through the rectifier diodes. I guess there is
no real comfort here unless you can start the amp up with a twist of a
240v variac. But don't turn *too* slow or you'll blow the soft-start
resistor.
> Of course, with indirectly-heated tubes, the real issue is to refrain from
> RF excitation until the cathode comes up close to filament temperature.
Yes, but I still like the idea of softening the startup current in
tubes like the 8877. I've seen hot spots develop in glass tubes where
some low spot on the filament momentarily flashes at turn-on then
immediately cools. Those weak spots are where the tube will fail. I
don't know if the 8877 is subject to this kind of failure, but soft
starting would delay substantially that kind of failure or prevent it
altogether.
Always good talking to you..
Dennis
0SP
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