[Moon-net] W6PO 2m Amp I/P circuit help.

[Hugh Duff]

I'm having an interesting problem with my newly built W6PO 2m 
amplifier;  The amp was built to spec per the Eimac newsletter reprint  
to the best of my knowledge and ability with the exception of a G3SEK 
triode board. 

The input T circuit is comprised of the N connector - a series 1000pF 
doorknob cap - the first coil, shunt tuning cap to ground, the   second 
coil - tube cathode.  The first coil is airwound, not slug tuned but 
extra turns were added and the coil is tuned by expanding/ compressing 
turns (not critical as I understand by reading some archive posts).

I cannot get the input circuit to yield a low SWR to the FT-847 exciter. 
The transmitter is folding back  and I'm only able to see about 500 
watts out of the amp.  With the transmitter set for maximum power out 
(50w) the best I can get is  between 3 and 4:1 SWR and the drive power 
drops to about 25 or 30 watts as seen on the power meter. 500v out seems 
about right  with 25 or 30w drive and 2900V on the plate so the gain of 
the amp is normal. (HVPS uses a pole-pig xfrmer good for well over 1  amp.)

My observations:

- I can get a good SWR reading when driving the amp with very low power 
but once I increase the drive beyond a few watts the SWR  rises to about 
3:1 or 4:1. Any attempts to retune the shunt capacitor for lower SWR are 
futile. I've even tried compressing/expanding the  coils with the input 
compartment cover removed (cover off-adjust-cover on) but can't do it 
actively since the cover needs to be  installed in order for the blower 
air to keep the tube cool. I've wound and rewound both inductors +/- the 
prescribed number of turns  and different I.D. in a variety of 
combinations. I also tried mounting the variable cap in a couple of 
different places for better grounding  but still no joy.

- Tuning of the shunt capacitor for a dip in SWR is extremely sharp, far 
more than I would expect it to be. (A clue !?)  I can almost  blow on 
the tuning control and the SWR will take off.  The fully meshed 
capacitance is 25pF, unmeshed is about 5pF.

- I have two 3cpx1500a7 tubes (assumed to be brand new) and the amp is 
behaving the same regardless of which one is installed.

I prefer not to use an intermediate amplifier between the radio and 
amp.  I want to make the amp work properly.

I know it's hard to troubleshoot from afar but I'm all ears if anyone 
cares to take a stab at helping to locate the problem. 
I'm anxious to see the amp produce QRO power.

Tnx es 73 de Hugh VA3TO