At 05:45 PM 6/26/2006, Peter Chadwick wrote:
>OK, probably a stupid question. Are we sure that the tuning point
>with minimum input capacitance is actually tuned to 50MHz, and not
>100MHz, with the stage acting as a power doubler?
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Au contraire, Peter, I think this is the best question asked so far.
I would recommend that you buy or borrow an MFJ-259 SWR analyzer or
the equivalent and use it "backwards" to be sure your tank is tuned
to 50 MHz and properly matched. If you've never done this before, here's how:
1. With power off and the HV caps fully discharged, connect a 1/2
watt non-inductive resistor from the tube's anode to ground. The
value of the resistor for 3 KV and 400 mA should be about 4000 ohms,
+/- about 10 percent. This value is not terribly critical, but the
closer the better.
2. Close the antenna changeover relay, either by mechanically forcing
it closed or by applying the working voltage to the coil. DO NOT
APPLY POWER TO THE AMPLIFIER ITSELF.
3. Set the MFJ to 50 MHz and adjust the tune and load capacitors for
a 1:1 SWR. You should be able to get a perfect match. If you can not
get a match, either your tank components do not have enough tuning
range, or something is mis-wired.
4. Remove the 4000 ohm resistor, de-energize the relay and power up
the amp. The tuning should be correct with only a very minor touch-up
of the tune and load controls.
And now you can be confident that the tank is both resonant at 50 MHz
and correctly impedance matched. The one thing this procedure will
not tell you is the operating Q of the tank. Assuming you have chosen
the correct value of L, that will take care of itself.
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