Replying to this subject, I was using a small phone and took shortcuts
that I may have wrongly assumed were a little clearer than actually
written.
" Remove the connection between the tubes and the plate cap and see if
the tubes are stable by themselves" ....
Should have read, " Remove the connection between the tubes and the
PLATE TUNING CAPACITOR and see if the tubes are stable WITH ONLY
PLATE VOLTAGE AND SCREEN VOLTAGE APPLIED ( proper bias of course )
and NO RF DRIVE."
At just enough bias voltage to start drawing plate current, move the
pi-network plate tuning capacitor and plate loading capacitor through
all possible combinations. If you don't see a rise in plate current,
change the bias voltage to draw 100ma per tube and again, rotate the
plate tuning and loading capacitors through their range.
If the tubes are DC stable at this point,
THEN set the bias to a low value of plate current and slowly apply RF
to the input with the output into a 1KW dummy load.
Get the amp stable with only DC before applying RF drive.
Sorry for the confusion.
Quoting donroden@hiwaay.net:
If you have under 200 ohms hung off the grids , a quad won't turn
into an oscillator. Remove the connection between the tubes and
the plate cap and see if the tubes are stable by themselves
Don W4DNR
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