[Amps] Parasitics & Filament Sag

[Steve Thompson]

R L Measures wrote:
> 
> On Aug 28, 2006, at 11:38 PM, Steve Thompson wrote:
> 
>>
>>> Mistuning apparently does not cause instability.  The circuitry that
>>> generates the VHF signal that starts the oscillation comes before the
>>> tank circuit.  This source is the series resonant circuit that
>>> consists of the anode-C's X (reactance) to gnd, the anode-lead's X,
>>> the VHF suppressor's X, the DC-blocker's X, and C-Tune's XC to gnd.
>>> Since the X of the anode-C is many times higher than the X of C-Tune,
>>> adjusting C-Tune from max to min typically changes the resonant freq.
>>> only a few %. Whenever anode current changes quickly, said resonant
>>> circuit rings and generates a damped wave signal.
> 
> 
>> What rise/fall time is needed to start something ringing at (say)  
>> 120MHz?
> 
> 
> 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.

Steve