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

[Gary Schafer]

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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