ORIGINAL MESSAGE:
On Wed, 07 Apr 2010 08:09:51 -0700, Vic K2VCO <vic@rakefet.com> wrote:
>
>I am also wondering whether I will need parasitic suppressors at all. Many 813
>amplifiers
>I've seen (GE SB handbook, ARRL HB's, etc.) do not use them.
REPLY:
The trick in avoiding the need for parasitic suppressors in the anode
is to make the input impedance (cathode to grounded grid) very low at
VHF. Make it low enough and the tube will not oscillate because what
feedback energy there is at VHF gets swamped out.
There are several ways to do it:
1. Use a hogh-Q input circuit, say about five or more. The higher the
Q, the lower the VHF impedance.
2. Use a 10 ohm non-inductive resistor in series with a 33 pF
capacitor directly from cathode to ground. Experiment with the values
if needed. At VHF this becomes effectively a 10 ohm load to ground. At
HF< the 33 pF cap merely becomes part of the input network, with a
small resistor in series which lowers the Q a little.
3. Use a series resonant trap from cathode to ground, tuned to the
frequency of the VHF parasitic. You may want to place a low value
non-inductive resistor in series with the trap to broaden it and make
tuning less critical.
I have personally used only #1 and #2. #3 should work but probably
won't be needed if #1 and #2 are done correctly.
Every amp is different and what works on one might need some tweaking
to work on another. Having an amp with no parasitic suppressors is a
blessing, well worth spending some time on.
73, Bill W6WRT
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