>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.
>
Sounds good, Steve. 10pF would cause minimal problems on 10m. To do it
right, one would need to measure the range of (VHF) resonances in the
variable C, and measure the resonances in the fixed C, to assure they did
not coincide near the anode resonance. This would help to ameliorate one
problem. The other problem is design a VHF suppressor that exhibits
minimal VHF Rp at the anode resonance freq. Thanks to Wes' measurements
with the Hewlett-Packard Model 4191A RF Impedance Analyzer, we know that
(for a given suppressor design) making Ls out of Ni-Cr wire instead of Cu
wire reduces VHF-Rp by c.40%.
>>
>>>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,
Mostly-true. However, Rs is in parallel with Ls, so Ls enters the
picture as well.
>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
Indeed
>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?
Mr. Rauch measured resonances in some common Tune capacitors. Typical
resonances are apparently within the range of anode circuit resonances,
And within tube freq. ratings for "Amplifier or Oscillator Service". .
. . Welcome to Parasiteville, gentlemen.
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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