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

[J.F. Samuels - K2CIB]

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
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