SV: [AMPS] Inrush Current

Peter Chadwick
Wed, 26 Apr 2000 15:08:38 +0100

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.

Of course, if you do not use a choke and then tie one side of the heater to
ground, an arc that punctures the heater cathode insulation but leaves the
heater intact still leaves you with a tube you can't use. You might even be
able to get enough RF amps into the heater (from the input) to blow it for

Don't sound an attractive approach to me................unless you're
selling tubes.


Peter G3RZP

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