[AMPS] parasitics

[Steve Thompson]

In message <19980422012421.AAB25551@[205.231.11.82]>, Rich Measures
<measures@vc.net> writes
>>In message <19980421063130.AAD25156@[205.231.11.91]>, Rich Measures
>><measures@vc.net> writes
>>>At VHF, with a conventional Pi-output, there is No VHF load on the 
>>>amplifier, even if a 50 ohm VHF-rated termination is connected to the 
>>>output.  
>>>
>>
>>Presumably the usual resistive suppressor in series with the anode only
>>works to dampen the vhf resonance when the other end of the suppressor
>>has reasonably low Z path to ground.
>>
>Indeed.  Perhaps it would be best to use two Tune capacitors in parallel, 
>one variable and one fixed, to assure that the suppressor would always be 
>sure to see at least one reasonably low Z path to ground.  

I reckon this would be a very shrewd move, maybe 10-20pF or so, even
when you plan to use a L match. 

>
>>Is there a case for adding a shunt suppressor load to cope with when
>>there is a high Z anode load at vhf - a state where the voltage gain
>>will be highest? 
>
>This is essentially what the parallel Ls/Rs suppressor does when it is 
>placed in series with the anode lead, however, to visualize it one needs 
>to convert the Admittance [Y] of the suppressor (at the anode resonant 
>frequency) to a parallel equivalent resistance [Rp] load on the anode. 

The low value Rs transforms to a high value shunt R - if the tuning
network presents a high impedance to the output end of the suppressor,
then the impedance presented to the anode is that high impedance, plus a
little bit of Ls/Rs, so the Rs does little to dampen any Q or provide a
load for the anode. The suppressor R dampens/loads situations where the
resonance is series tuned or where the output C of the tube is a major
factor in the resonance.

Has anyone stuck a wideband network analyser onto a hf matching network?
I'd be very interested to see some results. I have the analyser, but no
gear.

Steve


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