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
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