Rich Measures wrote:
>>An external B+ to ground short in a GG amplifier - the classic case
>>being powering-up with the B+ crowbar switch closed - will not send any
>>current through the cathode at all.
>
>? I have not yet powered up with interlock shorting the hv to ground.
>However, I have had intermittent B+ to gnd arcs, mainly during
>intermittent vhf parasites. If an amplifier were switched on with the
>interlock shorting the hv to ground, the filter capacitor would not
>charge, so the shorting current would be minimal.
>
Good point - I should have said, "powering-up and then closing the B+
crowbar switch."
>> The current flows back to B-minus
>>via the grid current meter, and then the anode current meter in the B-
>>minus rail. If the meter protection diodes do their job, there should be
>>no damage, and the cathode voltage should not change significantly.
>>
>? True enough provided the amplifier has protection diodes that can
>survive the full discharge current produced by the filter capacitors.
>
- moderated by a surge limiting ("glitch") resistor, all of which is
totally within our control.
>>On the other hand, if the short is due to an arc *inside* the tube, the
>>arc could go right through the grid and hit the cathode.
>
>? Have you ever seen an 8877 grid?
>
Yes, there are some nice photographs in "Parasitics Revisited". The
point being...?
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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