ORIGINAL MESSAGE:
On Tue, 21 Apr 2009 10:04:43 -0400, "chuck" <ka1mwp@comcast.net>
wrote:
>some say leave the tube in.
REPLY:
Definitely leave the tube in - it is part of the tune capacitance, in
parallel with the "real" tune capacitor.
Here's how I do it:
1. Using the ARRL handbook or a spreadsheet, determine the values for
the tune cap, load cap and coil on each band.
2. Temporarily disconnect the plate choke and the tank coil. In the
next step you want to measure ONLY the tune capacitance, tube
capacitance and stray wiring capacitance.
3. Using a capacitance meter such as the one sold by Almost All
Digital Electronics (my favorite), pre-set the tune cap on each band
and record the settings on the turns counter or dial scale, whichever
you have.
4. Re-connect the plate choke and tank coil.
5. Temporarily connect a non-inductive resistor equal to the load
impedance of the tube between anode and ground.
6. Connect the MFJ SWR analyzer to the output of the pi-network.
7. Going band by band, pre-set the tune cap for that band and find the
correct place to tap the coil while adjusting the load cap and tap
point for 1:1 SWR. DO NOT adjust the tune cap during this step! There
is one and only one setting of tap point and load cap that will give
you a 1:1 SWR for a given setting of the tune cap.
You're done. Don't forget to remove the resistor before powering up
the amp or you will hear a loud pop. Don't ask how I know. :-)
Does that make sense? Feel free to ask questions.
73, Bill W6WRT
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