Years ago when we used ordinary plate ammeters before going to
fast-responding LED bars, the bias supplies discharged quite rapidly at
power-off, perhaps T~100ms or so. Seemingly immediately upon pushing the
OFF switch, one could observe a brief, modest spike of Ip as the tubes
themselves discharged the HV filters in a fraction of a second. My bet is
that's still happening with current models, but the bargraphs go dark too
soon to show the discharge spike.
Dick
-----Original Message-----
From: Peter Chadwick [SMTP:Peter_Chadwick@mitel.com]
Sent: Wednesday, March 31, 1999 1:47 AM
To: 'amps'
Subject: RE: [AMPS] Re:HV on tube w/o filament voltage
Dick says:
>Don't you think the HV filter will generally discharge much faster than
>the cathode will cool down & lose emissivity, Peter...?
It's all going to depend, isn't it? The relative time constants of screen
and bias supplies, the time constant of the HV supply..........you could
get
the case where the tube is cut off and cooling and then the bias supply
falls before the HV has. It's a bit academic, though - the probability of
it
giving a problem is pretty remote. If you're running multiple amps and
switching the heaters as you change band, leaving the other voltages on,
it's possible. X ray tubes have back bombardment which heats the cathode, I
believe, but I don't think anyone on this reflector is running B+ voltages
up in 100kV region - not in amateur service, anyhow.
73
Peter G3RZP
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