SV: [AMPS] Inrush Current

measures 2@vc.net
Wed, 26 Apr 2000 20:22:40 -0700


>
>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...?

Penetrating the grid seems quite unlikely.  
>
>
cheers,  Ian.  

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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