Rich says:
>The only tubes which seemingly have this limitation are the 3-400Z and
>3-500Z.
I believe there's a limit on all filament type tubes, but it may be that
manufacturers figure that only on the ones for ham use do you have to stress
the point. In the days of tubed TV receivers here, when 99.99% were all
series heaters and half wave rectifiers (and live chassis!), it was quite
common to use a negative temperature coefficient thermistor in series with
the heater chain to limit surge current. By using it in series with the HT
rectifier, you also limited the limited the switch on surge ( that was for
selenium HT rectifiers - indirectly heatad tube rectifiers let the tube
cathode get hit)
> No further increase in output occurs when filament potential is
>increased above what is needed to secure full emission.
So what? We all know that, but this is a case where we aren't talking
science or fact or whatever - just 'mo' is better', to misquote you! So the
thing would get wound up. Just like bright emitter tubes in BC receivers in
the early 1920s. (No I'm not that old, but I have books that are).
>A simple rheostat would be a cheap fix.
Sure. Especially if round the back so that it wasn't too easy to get to. Not
inside, because then it might not get adjusted when it needed it. But I
figure it would still get misadjusted. You can't beat Murphy - especially
when he's assisted by a ham!
73
Peter G3RZP
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