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From: (Peter Chadwick)
Date: Tue, 17 Jun 1997 14:01:00 +0100
SM3PXO wrote:

>On a tetrode (not with handles) working in class A
>what will happen if heater voltage and g1 voltage are
>removed at the same time and g2 and plate still would be
>there? At the time the tube would be in standby with class A
>bais current.

First, it would draw a lot of current, assuming that it's a  tube that
will draw lots of current with Vg at zero. Second, if it's a tube with a
cathode (4CX250 etc) as opposed to a filament (4-400, 4-1000), if the
excess plate current didn't trip breakers or blow fuses in the plate
supply, it could well damage the cathode by stripping away the space
charge as the cathode cools and the emission falls off. In the case of
the filament tube, the cathode cools proportionally more quickly because
of its lower thermal mass, but it would still have a chance to become
thermally limited in emission, which isn't good for the tube. It might
cool quickly enough that the plate supply didn't trip; it might not.
It's not considered a good idea to get the space charge stripped away on
cathodes or thoriated filaments, although it's acceptable on pure
tungsten filaments.

the definitive answer must be : it depends.
On:- the thermal capacity of the tube cathode,
       - the fusing of the HV and/or screen supplies (if the plate volts
go because the fuse goes, the
         cathode has some emission and the screen is fragile, it might
       - the current that would be drawn  with zero grid volts.



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