[Amps] Strange pi network tuning problem

[Guido Tedeschi]

First of all, thanks to everybody for the answers.

Here are some tips:

1) Yes, I'm using a dummy load, which has a good SWR across the bands 
(1.2 max on 20m, less above and below, measured on the TX plug, probably 
given by a -20dB resistive coupler and a Nissei Wattmeter, in series 
between TX and dummy load).

2) With low power, I mean only 20W on 20m with 200mA of plate current 
and variables capacitors to much closed (too much capacitance) for that 
peak of power

3) Tubes voltages are correct (600V on plate, 150V regulated on screen 
grid and -42V on control grid for 60mA current). On 15 and 10m the PA 
tunes correctly.

4) I connected again the VNA, this time directly to TX SO239 with the 
antenna relay manually closed, and a 2200 ohm resistors between tubes 
plates and ground: I was able again to tune the PI network for 50 ohm on 
VNA on 80, 40 and 20m with plate and load capacitors set to "normal" 
positions, only on 40m the plate is a little outside of the mark.

Here follow some pictures:


VNA plot and plate/load positions on 3551 kHz (good, but a little more 
load capacitance would be needed for a more perfect match)

https://drive.google.com/open?id=1kGCGLOvlJmNzS8llSpXj3e20XMjQt7rI

https://drive.google.com/open?id=1mX6s2aqRd-IwOF4zbZiBGqiNsbmcVfnA


VNA plot and plate/load positions on 7029 kHz (the needed plate 
capacitance is a little more than normal)

https://drive.google.com/open?id=1oYfZnBUO9e37GG5zMfaLBRolA19cB27T

https://drive.google.com/open?id=1USTfnzrN77pYqBisqqxQzTTgZEWWRxhh


VNA plot and plate/load positions on 14020 kHz

https://drive.google.com/open?id=1GEjLFc46T1K8Z8FR5ykHSKU0JYNiaLje

https://drive.google.com/open?id=1A5xPFp88kuWhxO2qn1Izapn6-yydpHnI


Now I'm in a "cul de sac" and I don't understand this behavior and how 
to solve the mystery...

Remember that on 15 and 10m the PA tunes correctly

73 and thanks

Guido, ik2bcp / k2bcp