[Amps] Parasitics & Filament Sag

[Steve Thompson]

>>>>> 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.
>>>
>>>
>>>
>>> Apply no driving signal and rapidly key the T/R bias.
> 
> 
>> Did that, no result. Hence my post.
> 
> 
> ¿You found the driving signal, but you applied no driving signal?   
> Please describe the coupling method to the S.A.
I should have said 'ringing close to frequency the amp was tuned to'.

I tried various couplings - a few inches of wire into a high impedance 
fet probe poked in lots of places. 1pF directly from the anode into the 
analyser via a 30dB pad. A two turn coil of wire 1.5" dia. placed 
everywhere I could physically get it.

Steve