Peter Chadwick wrote:
>
>Rich says:
>
>>I mean that the heater should probably not be DC grounded as long as the
>>possibility of a B+ to ground arc exists. . If such an arc occurs, the
>>internal insulation between cathode and heater can arc and possibly burn
>>out the heater. .
>
>
>Do I understand you as advising a bifilar filament choke, and tying one side
>of the heater and cathode together? That stops the heater cathode insulation
>getting punctured with a B+ to ground short; it suggests that the heater
>xfmr may need to float for any bias, (which won't be very much), and zener
>catching diodes (of greater voltage than the bias) can then handle any
>excursions during the B+ short.
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. 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.
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. Gossling's
paper that Peter has referenced confirms it often can. I've also seen a
post-mortemed tube that had suffered several internal arcs, and you
could count the burns on the grid mesh (same as the number of bangs the
operator had heard). The worst of the burns on the grid also had a burn
on the cathode directly underneath, where the arc had gone clear
through. That was a GS35b tube with a fine mesh grid... we might
speculate that tubes with a more open grid structure might be more prone
to letting the arc through to the cathode.
If the arc does get through to the cathode, then the cathode is pulled
up positive. That in turn may break down the heater/cathode insulation
if they are not tied together, and the cathode bias components may go as
well.
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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