>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!).
A fuse seems unlikely to prevent cathode stripping. A 723/opto interlock
is indicated--or use a 'instant-on' thoriated tungsten filament type tube
instead of a heater type tube.
>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.
Nothing is what happens. Even though cathode damage is not an issue, a
potential problem is hotswitching the RF relays Also, it is not good to
transmit before the step-start relay shorts out the pair of step-start
resistors, because doing so prevents the relay from closing, and burns
out the resistors. In the amplifiers I design, it typically takes under
One second to charge the anode supply filter capacitors to 2/3 normal
V--which is where I set the step-start relay to close and short out the
step-start resistors.
>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's what Phil and Rich have been trying to communicate to you.
>That is, I believe, the most elegant way to deal with the problem. ... ...
... with a suitably
>large w/w resistor (and perhaps a rheostat to allow fine adjustment of
>filament voltage) in series with the primary.
Oops, 'rheostat' + 'filament V' is a major faux pas, Peter. During the
grate vhf parasitics debate, the Rauchian Camp, repeatedly asserted that
rheostats should NOT be used to allow fine adjustment of the filament V.
If you guessed that none of Mr. Rauch's Ameritron/MFJ amplifier designs
use rheostats, congrats.
-Rich---
R. L. Measures, 805-386-3734, AG6K
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