[AMPS] Tank coils etc

[Peter Chadwick]

Doug, KF4KL says:

>Another question. I have access to a vector impedance meter. I know
>what the plate impedance will be for the tubes I've chosen. Can I
>terminate the tank output with 50 ohms and use the vector Z meter to
>select and verify the taps on the tank coil? It's an easy thing to do,
>but in practice will it result in the correct tap selections?

The Q  of the tank will be set by the input capacitance of the Pi or Pi-L
network. The correct capacitance has a reactance of  RL/Q, where RL is the
plate load, and Q is between 10 and 15. I'd suggest setting the capacitance
at the ends of the Pi to the right value before setting coil taps. 

A point to watch is the variety of vector impedance meter. The network
analyser approach uses directional couplers to measure amplitude and phase
of the reflection in a 50 ohm system. If you measure a 2000 ohm load, quite
large changes in the load have relatively small changes in return loss, so
the accuracy goes to hell in a handbasket. Thus depending on the way in
which the box works, it might be a waste of time. If you use a vector
voltmeter, that's a different approach - and it needs a signal source.

>Using a
>variac I increased the primary voltage with the secondary shorted by
>an ammeter until it was delivering 750 mA. I ran it like that for an
>hour and it barely got warm to the touch.

Sounds pretty good. As Steve G8GSQ says. watch the formulas - I assumed the
use of the right ones for capacitive input.


73

Peter G3RZP

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