[Amps] Parasitics & Filament Sag

[Steve Thompson]

>>> Any change in current causes a resonant circuit to ring. Ringing
>>> amplitude depends on how fast the current changes and on Q.   Faster  
>>> and
>>> more Q =s more V.  I have heard from more than a few TL-922   owners who
>>> report that a big-bang, flashover, and damage occurred  when their  922
>>> was un-keyed.
>>
>> Sure, but how fast does the current change? By the time the decoupling
>> capacitors have charged to allow the tube to drift into cutoff, I  doubt
>> the rise time alone is going to make the anode ciruit ring at vhf.
> 
> 
> During the Grate Parasitics Debate in 1996, a member of the audience  
> hauled his SB-220 to work and loosely coupled the anodes to a HP  
> spectrum analyzer.  When the 220 was keyed, he observed damped-wave  
> ringing at 112MHz at on And at off.
I wish I could reproduce that. I've tried repeatedly with spectrum 
analyser and 'scopes and only ever find operating frequency ringing, 
even with suppressors removed. I've tried with a couple of amplifiers, 
and also with a tank circuit I put together where I can apply pretty 
quick steps of up to 10A.

Can anyone help point me in the right direction?

Steve