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

Wed Jan 25 16:05:54 EST 2023

Hi Bruce,

I have several TMC transmitters which have various versions of this 
power amplifier that I've been working with over the last 40 years.  
They are excellent, stable, trouble-free amplifiers.  From your 
description, and assuming all element voltages are correct at the tube 
socket, I agree that you very likely just have a bad output tube.  If at 
the nominal grid bias of about -110V you cannot obtain the correct idle 
plate current of about 220 mA, the tube probably has low cathode 
emission.   Your amplifier is a perfectly useful tube tester.  A simple 
emission test can be done by adjusting grid bias (no RF drive) and 
checking plate current against a few points on the published 
characteristic curves.  You should be able to adjust up near max 
dissipation if the tube is good.  Do it fairly quickly, and keep screen 
and plate dissipation within limits.

Relatively few original Penta Labs PL-172s (with the glass seals) are 
still good today.  Many have become gassy, others have developed 
internal "barnacling", which can cause arcs when HV or RF drive are 
applied which can damage the amplifier.  The ceramic seal 8295A is much 
less prone to gas problems than the glass seal PL-172 but many, 
especially the oldest, are no longer useable.

Arcing between anode and screen grid may vaporize R222, the 12 ohm 
resistor in the screen grid circuit.  It's very important to make sure 
shunt and multiplier resistors in the metering circuits are OK so the 
meters read accurately.

Before attempting operation with a new tube, I would recommend running 
it overnight on filament and grid bias (with normal cooling air, of 
course), and afterward, do a comprehensive hi-pot test for inter-element 
leakage current, if you can.   Problems not evident with simple ohmeter 
tests often show up this way, and in some cases, the tube can be 
"debarnacled" to the point where it functions properly.  Parallel all 
grids, cathode, and filament terminals, and measure leakage current 
between anode and the paralleled elements.  Test with anode voltage up 
to at least twice the DC plate voltage.  Leakage should be somewhere 
under 50 microamps or so.

Unfortunately, good PL-172s and 8295As are getting harder to find.   
Tubes advertised as NOS frequently are not, and even those that are NOS 
may be defective, so it's all a bit of a crap-shoot.

Correct neutralization is of course important, but it is not at all 
critical on this amplifier.  Even if grossly maladjusted, it will 
absolutely not cause the problems you're having.   If neutralization is 
correct, plate and screen currents should dip at very nearly the same 
plate tuning setting, and there should be little to no interaction 
between the grid tuning and plate tuning adjustments.

Improper tuning and loading of these amplifiers is a very common 
problem.  All you need do is follow the tuning procedure in the manual, 
but here are a few brief pointers.  When tuning up, always keep the 
multimeter switch in the Isg position, and simultaneously monitor both 
plate and screen currents.  Many PL-172/8295A tubes were trashed by 
improper tuning/loading adjustments resulting in excessive screen 
current.  This warps the screen grid, and the old arcing problems 
begin.  Keep screen dissipation within limits (35W) at all times.

Unlike many amateur amplifiers with tetrodes, which are tuned for max 
screen current and min plate current, the PAL-1K is tuned for MINIMUM 
plate and screen currents, and these should nearly coincide.  High (and 
very touchy) screen current indicates an underloaded condition, and will 
result in erratic operation, poor linearity, frequent breaker trips, and 
possibly damage to the tube.  As loading is incrementally increased and 
tuning is re-dipped, screen current will decrease and become less touchy 
as you approach a properly loaded condition.  At proper load, plate and 
screen dips should be smooth and uncritical.  If necessary, it's better 
to run slightly high plate current to get the screen current in-range.  
The PL-172 is a radial beam pentode, and contrary to popular belief, you 
will not see negative screen current on this amplifier.  If the screen 
current meter goes negative, something is seriously wrong.

With proper tune and load procedure and a good tube, you should very 
easily obtain 1 kW CW at plate currents of 400-500 mA, and well under 
100 mW drive, depending on frequency.  Below 14 Mc, you may only need 
20-30 mW drive to get full output.  Screen current should not exceed 
35-40 mA.  For best linearity, you may need heavier plate loading, 
running at the high end of the aforementioned plate current range, but 
it shouldn't exceed around 550 mA.  You must load the amplifier to full 
output on single tone before operating SSB, even if you intend to 
operate at reduced output.  With a two-tone signal at 1 kW PEP 3rd order 
IM should approach -40 dB relative to either tone.

Hope this helps, good luck, and let us know how things go.  It will be 
good to hear another of these transmitters on the air...

73,

John K9WT

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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