[TMC] TMC PAL-1K Low Output Power Issues

[triodes at optonline.net]

Hello, Fellow TMC Enthusiasts!
  I have been spending a lot of time debugging this TMC PAL-1K amplifier 
subsystem I have here. As some of you know, the PS-4A and the PS-5 units 
were basket cases when I got them, but the final amplifier was in 
surprisingly good shape, and I figured the power amplifier unit would be 
largely plug and play, but that has proven to not be the case. I have 
made a lot of progress, and I am now down to one remaining issue; the 
final amplifier is suffering from very poor efficiency on all bands. 
With 1400 watts DC input, I am only getting out around 100 to 150 watts, 
where it should normally be on the order of 700 to 850 watts CW output.
The fact that this is true on all bands indicates that the coarse 
loading padding capacitors at the output-side of the pi-network are 
good. If they were shorted or open, that would be a hell of a job to get 
in there to remove them. Like many of the components in the RFC-1 or 
RFD-1 1 KW Power Amplifiers, these amplifiers were literally built 
around these parts, with very little consideration given to future 
serviceability, etc.
I think the issue is a weak PL-172A output tube, or the final stage is 
perhaps over-neutralized. PL-172As are becoming unobtainium, and I have 
one (hopefully) good tube on the shelf, but I'm reluctant to install it 
unless I know what is really going on here. Over-neutralization is of 
course easily dealt with in either the RFC-1 or RFD-1 amplifiers; TMC 
made the neutralization process very simple. Remove the hole plug on the 
front panel, and neutralize the final stage per the procedure in the 
tech manual.
I think the PL-172A is weak, as I cannot get the resting cathode current 
above 175 Ma with 3000 VDC on the plate, and the TMC spec is 220 Ma 
idling current with (of course) no signal applied to the grid. That is 
usually indicative of a tube suffering from weak emission.
There can't be too many issues causing this low efficiency in the final 
amplifier stage. The voltages on the plate, screen, grid, and heater are 
all correct, and the tube is getting plenty of drive. Part of the 
problem is I don't have any way of properly testing the PL-172A, and I 
certainly don't want a repeat of what happened the last time, fireworks 
and all, when I installed a PL-172A that came with the RFC-1 marked, 
"Used, Tests Fine". That tube may have tested fine 40 years ago, but it 
most probably became gassy sitting on the shelf unused for all of these 
years.
I know I'll eventually get there, but I figured I would solicit some 
thoughts and suggestions from any of you who have been down this PAL-1K 
(or similar TMC equipment) road before.
73,
Bruce, W2XR