Hi Bill.
> are a little inconsistent. Can you clarify which amps were operated
> which way?
>
> Bill-W4BSG
You can make any Clipperton modified for ten meters oscillate
quite easily.
Set the band switch to ten meters. Mesh the loading control in and
tune the plate control near minimum C while the PA is keyed. You
can watch the parasitic resistors go up in smoke as the PA
oscillates above 20 MHz and below the six meter band.
Virtually all un-neutralized 572B or 811A amplifier using four tubes
will oscillate on 20 Mhz or somewhat higher because the tubes
have excessive feedthrough on upper HF.
There are even cases where two 572B's are unstable. The Yaesu
FL-2100 is one example. If you remove the load and driver, key the
FL2100, and rotate the tuning and loading controls it will oscillate
on frequencies above 14 MHz or so. That is why Svetlana 572B's,
which require more cut-off bias than other 572B's, will oscillate in a
FL-2100 when the PA is on standby. The tubes remain in
conduction, the antenna relay drops, and the input and output are
"open circuited". The PA takes off at or near the operating
frequency, and parts get toasted.
Svetlana's patch was to increase cut-off bias, but the core problem
is really that the PA needs neutralized to compensate for the high
feedthrough of 572B's.
You'll find, when driven, the plate current dip is not near maximum
output, and the maximum grid current is not at anode resonance
either. Both are indicators of uncontrolled feedback at the operating
frequency. Both clearly show up in the Clipperton and the FL2100
Yaesu.
Oscillations near the operating frequency are particularly bad,
because system Q is high at that frequency so the tank can store
a lot of energy. Opening the loading control up (while running into a
load like an antenna) loads the tank, and reduces Q and feedback
in the PA.
73, Tom W8JI
w8ji@contesting.com
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