I guess that we're all agreed that a cathode (as opposed to a filament)
is likely to be seriously damaged under the proposed bias/filament
supply failure scenario, and that some form of interlocking is highly
desirable (or a big box of spare tubes!).
I hear all the points about the filament tube under the circumstances.
OK, but I'm not convinced it's a good idea even for a filament tube to
treat it like that.
Do I gather from K5PC's comments that the SB220 has a transformer with
enough leakage reactance to limit the maximum inrush current at turn on?
That is, I believe, the most elegant way to deal with the problem. If
you have to live with the transformers to hand, and don't mind buying
60Hz (about 10c per KWhour here for 50Hz - it ought to be cheaper than
in the US because the frequency is lower!!! - hi) you can always use a
110v filament transformer running off the 220 volt line with a suitably
large w/w resistor (and perhaps a rheostat to allow fine adjustment of
filament voltage) in series with the primary. Even with a perfect
transformer, that would limit the inrush current to 2 times the working
current. In practice, it would be less.
Naive question: What happens to the power factor seen by the mains
supply when a choke input filter is used, especially as the choke
inductance decreases with increasing current? How different is it when a
resonant (or nearly resonant) choke input is used? I use a tuned choke
filter at the moment, but I've got an appalling power factor - pulling
about 17Amps at 234volts for a DC load of about 2300 watts. I haven't
measured to find if it's lagging or leading - I don't have access to a
power factor meter, and haven't got round to putting in sensors and
using the scope.
73
Peter G3RZP
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