ORIGINAL MESSAGE (may be snipped):
On Tue, 23 Apr 2013 09:52:15 -0700, you wrote:
>Measuring resonant frequency with GDO with parallel capacitor: seemed to read
>low compared
>to formula, but I think it may be the best method.
REPLY:
That method is good, but there is another that I think is better. This can
be done either in a already-built amplifier or breadboarded on the
workbench. If you breadboard it you won't have the tube's capacitance to
consider but as long as you pre-set the tune cap correctly, it won't matter.
First of all, DISCONNECT all power to the amp, even filament or heater.
Using a spreadsheet or the tables in the ARRL handbook, determine the
capacitance of the tune and load capacitors. With the coil OUT of circuit.
pre-set the tune and load caps to the values indicated. The tube must be in
the socket and connected, but the plate tank choke should be disconnected if
you are using a low frequency capacitance meter such as the AADE unit. Also,
disconnect the coax leading from the load cap. You want ONLY the capacitance
of the load cap, not the coax at low frequencies.
With the capacitance values pre-set, re-connect the coil and connect a small
non-inductive resistor across the tune capacitor. The value should be the
same as in the spreadsheet or tables. Now, using an SWR analyzer connected
to the output of the tank, adjust the coil for SWR of 1:1.
I believe this method comes as close as possible to the actual operating
conditions when the amp is powered up.
Don't forget to remove the resistor before applying RF or you'll get a very
exciting flash-bang. Don't ask how I know. :-)
Bill, W6WRT
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