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

[triodes at optonline.net]

HI Gary,

Thanks for your comments!

I can get a reasonably good dip for the cathode current at resonance. 
Swinging the plate tuning capacitor off resonance yields greater output 
power, and a peak in Isg.

The measured parameters for the PL-172A are as follows:

Ip: 400 Ma
Ep: 2850 VDC
Esg: +500 VDC
Isg: 15 Ma
Grid Drive: This is not displayed in MA on the RFC-1 multimeter, but is 
a relative reading instead, as the drive circuit detects the RF drive 
level to the PL-172A grid, and this is a relative DC volts reading. The 
measurement is 14 on the multimeter, and the TMC recommendation is 10 
for maximum RF drive to the PL-172A. So you can see that I am somewhat 
overdriving the PL-172, to get the measured 150 watts RF output.
RF Output: 150 watts, into a calibrated Bird 1000 watt load with 
built-in wattmeter.

73,

Bruce


    ------ Original Message ------
    From: garyschafer at largeriver.net
    To: triodes at optonline.net; TMC at mailman.qth.net
    Sent: Wednesday, January 25, 2023 1:37 PM
    Subject: RE: [TMC] TMC PAL-1K Low Output Power Issues

          Hi Bruce,

  How much of a plate current dip do you have. Tetrodes will have very 
little
  plate current dip when properly loaded.
  With not much of a dip you could also be over coupled.
  What is the grid current and screen current when driven?
  As you know that tube exhibits negative screen current when properly 
loaded.

  All of the PL172A,s that I have had were all gassy and would arc.
  The ceramic version, can't remember the number were much better as 
they
  didn't leak.

  73
  Gary  K4FMX

  > -----Original Message-----
  > From: tmc-bounces at mailman.qth.net
  > [mailto:tmc-bounces at mailman.qth.net] On Behalf Of
  > triodes at optonline.net
  > Sent: Tuesday, January 24, 2023 10:16 PM
  > To: TMC Reflector
  > Subject: [TMC] TMC PAL-1K Low Output Power Issues
  >
  >
  > 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
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