>
>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.
? 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.
> 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.
>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?
> 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.
? This would not describe a high-Mu triode.
>
cheers, Ian
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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