[Amps] Pi coil tapping

[Steve Flood]

What set-up precautions are necessary to run through a coil-tapping process?  Here is what I've done so far...

With my tune and load capacitors mounted in their final positions in the RF compartment, but isolated from the rest of the circuit (no tube, plate choke, coil, output connections), I worked up a chart with turn counter values versus capacitance values.  I discovered in this step that my load cap would not register a capacitance when the 2.5mH output safety choke was connected.  Should that be the case?

In the next step, I reconnected the entire circuit (tubes in place, plate choke connected, plate load resistor connected from top of plate choke to chassis, coil output safety choke).   I attached an MFJ259B to the output with no coax (using a male-male PL259) and went to work on coil tapping.

I used the PI-EL program to derive the intial capacitor settings.  I was easily able to find tap points for 50+j0 matches by tweaking the capacitors.

This is 20-40-80-160 amp, so here is what I ended up with (L and Q are back-calculated)

20m calc    Ctune=55    Cload=288    L=2.57   Q=12
20m real     Ctune=39    Cload=436    L=4.0    Q=9.5

40m calc    Ctune=109    Cload=573    L=5.1    Q=12 
40m real    Ctune=111    Cload=880    L=5.0    Q=12

80m calc    Ctune=209    Cload=1100    L=9.79    Q=12
80m real    Ctune=210    Cload=1360    L=9.87    Q=12

160m calc    Ctune=431    Cload=2261    L=20.1    Q=12
160m real    Ctune=343    Cload=1850    L=25    Q=9.54

So, what have I got?   40 and 80 look good, except for the higher than expected Cload values.  It looks like I need to rework the 20m tap.  On 160, it appears I got too much coil.

Am I on the right track here?

Steve KK7UV