[Amps] Parasitics & Filament Sag

[R L Measures]

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.
>
> Steve
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R L MEASURES, AG6K. 805-386-3734
r at somis.org