[Amps] Parasitics & Filament Sag

[R L Measures]

On Aug 30, 2006, at 3:21 AM, Steve Thompson wrote:

>
>
> 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.

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.

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