> Jon says:
>
> >How much does gain typically get reduced by?
The gain, in a grounded grid PA, is directly related to the driving
impedance and output impedance of the stage. That is because
the input is directly in series with the output, and 180 degrees out
of phase.
If you work back through the calculations, applying the same drive
and reducing the HV decreases gain at the square of voltage
reduction.
An interesting result is operating impedance remains fixed, so if
you load at 2000 volts and switch up to 3000 volts the tank is still
tuned. That's why the SB-220 had a CW-Tune position with
reduced voltage.
The capacitors were not rated to be mistuned at the higher voltage. There was
not enough headroom.
> I maintain that in grounded cathode, it doesn't. We have the complication
> here that I'm talking in terms of the plate swing during the RF cycle. As
> the grid goes positive, the plate current increases, and the plate voltage
> drops. At the peak of positive grid volts, plate current is a maximum and
> plate voltage is at a minimum. That voltage is set by the load impedance
> and the plate current at the peak of the cycle - which is fixed by the
> grid volts. The gain isn't directly related to the plate volts, but to the
> drop in signal handling - the gain compression point has come down. At
> lower inputs, the gain will be much the same - especially with a tetrode,
> where gm is very loosely related to plate volts.
Even in a class C grid driven PA, power output still changes as the
square of HV change. All the tube amounts to is a time-varying
resistor, controlled by the grid to cathode voltages. While there is a
small change in anode resistance caused by the HV change, it is
minor for small voltage changes.
That's why, as I said, a plate modulated stage has a peak enevlope
output four times the unmodulated output when HV is doubled by
the modulator. It doesn't matter if that change occurs at a once per
day rate, or a once per nanosecond rate, the change is a square
law function unless something saturates or falls out on the low end.
> Looking at GG, that will be appear to be a bit different, because if the
> drive and fed through power are the same, but the tube is producing less
> output because there's less plate swing, then the total output power is
> less. Effectively, the gain appears to be less.
It not only appears to be less, it is less.
73, Tom W8JI
w8ji@contesting.com
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