> I am in the process of adjusting the PI Network (inductor taps) on
> this old 4 x 811A amp that I inherited and was wondering what
> impedance I should terminate the PI network with when making my
> adjustments? Per Orr's Radio Handbook, the plate Z of a single 811
> at 1700V is 5200 Ohms. I am running around 1600 Volts across 4
> 811As. For my initial adjustments, I used a 1500 Ohms carbon resistor
> across the tuning cap. Is this close enough? Should I include some
> additional shunt C to simulate the tube capacitance?
Mike,
Leave the tubes in place. Tie a 4700 ohm or 5100 ohm resistor
from each tube anode to the chassis.
Do your tank that way.
Better to err on the side of too low an anode impedance than too
high.
Are you using a Q=12, or some other lower value in the tank? You
can often get away with a Q less than 12 as long as Q is larger
than the square root of impedance ratio plus one.
Say the tube loadline comes out as 1200 ohms at the peak power
you ever expect to see, and the minimum load impedance is 20
ohms. Q would have to be some value larger than the (square root
of (1200/20)) plus 1, or about 9.
As Q is increased beyond that value harmonic suppression slightly
improves (most notably the second harmonic) but efficiency
decreases and tank heating increases. You have to almost double
Q to get a significant improvement in second harmonic, and you
also will nearly double tank heating.
73, Tom W8JI
w8ji@contesting.com
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