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

[triodes]

Hi John,Thanks very much for the courtesy of your reply.Over the weekend, I terminated the PL-172A plate with an 1800 ohm non-inductive resistor to ground. This was to simulate the PL-172A plate impedance, as it looks into the pi-network.I then fed my MFJ network analyzer into the 50 ohm output port of the RFC-1 Power Amplifier. I did this on all bands, and I was able to get a sharply defined 1.2:1 or better VSWR on all bands, with a minimal reactive component, by adjusting the plate tuning, output loading, and the coarse loading on the RFC-1.These leads me to believe the pi-network and it's associated components, including the plate blocking capacitors (a pair of 500 pF caps in parallel), are good.What I don't understand is with 1400 watts DC input to the PL-172A,  where is all of this RF going, as I'm only seeing around 150 watts RF output on all bands? Are the plate blocking capacitors, if they were bad, not coupling most of the RF, and as a result, the bulk of the RF is being dissipated within the PL-172A? That may be an unlikely scenario,  but who knows?And the PL-172A, if it were indeed bad or weak, and with 1400 watts DC input, be responsible for such a gross reduction in RF output?I'm not that familiar with the possible failure modes of large pentode transmitting tubes.73,Bruce, W2XR Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: "J.F. Samuels - K2CIB" <radiowhiz at gmail.com> Date: 1/25/23  9:30 AM  (GMT-05:00) To: triodes at optonline.net, TMC Reflector <TMC at mailman.qth.net> Subject: Re: [TMC] TMC PAL-1K Low Output Power Issues 
    Hi Bruce,
    If you are getting a good dip without the loading
        at either ends of the cap on all bands, then we can assume that
        the tube impedance is matching to the 50 ohm load (you've
        checked this, right?)  Then, with the idling current and output
        so low and all voltages checking out, it certainly sounds like
        the tube is soft.  There is no indication that you would be
        endangering the shelf tube, although if the tube has an internal
        short, you could endanger the amp :-)
    
    
            
              73, John
            Final Test Tech TMC 1958-1959
                
            
            
            
          
    On 1/24/2023 11:15 PM,
      triodes at optonline.net wrote:
    
    
      
      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
      
      ______________________________________________________________
      
      TMC mailing list
      
      Home: http://mailman.qth.net/mailman/listinfo/tmc
      
      Help: http://mailman.qth.net/mmfaq.htm
      
      Post: mailto:TMC at mailman.qth.net
      
      
      This list hosted by: http://www.qsl.net
      
      Please help support this email list:
      http://www.qsl.net/donate.html