[TMC] TMC PAL-1K Low Output Power Issues
w1nzr at cs.com
w1nzr at cs.com
Wed Jan 25 19:21:04 EST 2023
John,
Thank you for reminding us of the finer points of sustaining the venerable but aged PL172 as it relates to these old workhorses that we attempt to keep going.....
Bruce, that is an important piece of radio history that needs to be kept alive ! A WSL artifact.....very impressive......
Regards,
Brown W1NZR
On Wednesday, January 25, 2023 at 05:21:40 PM EST, <triodes at optonline.net> wrote:
HI John,
Wow, your reply is incredibly helpful, and I thank you for taking the
time for providing such a detailed and concise reply. I'm sure that
other users of the RFC-1 and RFD-1 will find your guidance extremely
helpful as well, and are equally appreciative!
It is good to see, based upon your response, that the likely culprit
here is the PL-172A.
Do you think there is also the possibility, as another poster has
suggested, that perhaps the two 500 pF plate blocking capacitors may be
bad? Not shorted, but open? My feeling is this is probably not the case,
as I doubt that the test I had performed earlier, that being terminating
the PL-172A plate to ground with an 1800 ohm resistor, and looking into
the RF output port of the amplifier with my network analyzer, would have
yielded the kind of positive results I was seeing, assuming the blocking
capacitor was bad.
Assuming the PL-172A is indeed weak, where is all of the RF output
going, with roughly 1400 watts of DC input? Will a weak PL-172A result
in such a gross loss of efficiency, that in reality, the tube is only
delivering around 150 watts to the input of the pi-network, with 1400
watts of plate input?
My PAL-1K "clone" consists of the PS-5 HV PSU, the PS-4A intermediate
voltage PSU, and the RFC-1 PA unit. Unlike the "real" PAL-1K, I am using
the RFC-1, instead of the RFD-1. The RFC-1 was given to me a number of
years ago in beautiful cosmetic condition, and it was pulled out of a
TMC GPT-40K at coastal station WSL, here on Long Island. It's been a
long road, but I want to put that RFC-1 back on the air, and actively
use it.
That said, I had to create a custom wiring harness to correctly match
the interfaces on the PS-4A, to that of the RFC-1. Of course when using
the RFD-1, this does not need to be done, and the captive pendant cable
on the RFD-1 connects directly to the mating MS-connector on the PS-4A.
Thanks again, John. Very much appreciated!
73,
Bruce, W2XR
------ Original Message ------
From: jvendely at cfl.rr.com
To: tmc at mailman.qth.net
Sent: Wednesday, January 25, 2023 4:05 PM
Subject: Re: [TMC] TMC PAL-1K Low Output Power Issues
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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