Hi Steve,
> With the usual switched resistor in B-, there is a big grey area between
> low anode current and true cutoff. I've no idea what the gain might be at
> (say) 1mA or lower. With the output end of the matching network open,
> there would be whacky impedances at the anode at both hf and vhf.
>
> Steve
Gain is primarily set by the load impedance and other factors. In a
ground grid amp, it is primarily set by the ratio of driving impedance
to anode load impedance.
One thing to remember is the VHF impedance doesn't change any
significant amount, assuming you have tuned inputs and outputs.
That's because the output and input ports are isolated by the
filtering action of the tank circuits.
Amplifiers that oscillate when on "standby" almost always oscillate
at the operating frequency and lower. The FL-2100 is one example.
If you put Svetlana 572B's in the 2100, the tubes are not fully cut
off when the amp is on standby. Since 572B's have an excessive
amount of feedthrough caused by the long skinny grid leads and
poor internal shielding, the 2100 will actually oscillate and can even
blow out the bandswitch when on standby.
That's because the anode impedance goes to extremely high
values when the tank is disconnected from the load, and the tube
is not in cutoff.
The oscillation is on the HF resonant frequency of the tank, NOT at
VHF.
73, Tom W8JI
w8ji@contesting.com
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