> NPN and PNP HV transistors suffer from 'secondary breakdown effect' when
> medium voltages and currents co-exist (as is the case in regulator
> service).
It's doubtful that problem would occur in the control grid circuit.
Perhaps your mind is still stuck on tubes with big handles, and not
on the tubes Paul asked about.
One design solution to 'secondary breakdown effect' is to
> derate the dissipation to about 1/8 and parallel additional transistors.
> Another solution is to use a power FET transistor. They are virtually
> immune to 'secondary breakdown effect'. Motorola and International
> Rectifier make a 6A, 600v, 150w FET that appears to perform
> satisfactorily in HV regulator service.
A little overkill, in the control grid of a small tetrode! Almost any
HV NPN power transistor would handle a few mA at 50 volts or so!
73, Tom W8JI
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