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[AMPS] Re:HV on tube w/o filament voltage

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Subject: [AMPS] Re:HV on tube w/o filament voltage
From: w4eto@rmii.com (Richard W. Ehrhorn)
Date: Wed, 31 Mar 1999 22:11:15 -0700
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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