[TMC] TMC PAL-1K Low Output Power Issues
triodes at optonline.net
triodes at optonline.net
Wed Jan 25 20:55:30 EST 2023
Hi John,
Thanks for the reply!
I can't tell you off-hand what the grid bias value is for a
resting/quiescent Ip of 175 Ma, but it is with the grid bias adjustment
pot providing the minimum possible bias voltage.
Here are the operating parameters I have recorded for this amplifier:
Ep: Approx. 3000 VDC
Ip, with Grid Drive: 400 Ma
Isg: 15 ma
Esg: +500 VDC
Grid Drive: 14 on the Multimeter. Recall that the multimeter does not
read the grid drive in Ma, but rather it is a relative reading based
upon the rectified RF drive voltage as applied to the PL-172A grid. Note
that the TMC-recommended reading for drive to the PL-172A is 10, so I am
driving the PL-172A harder than recommended. At a drive level of 10, the
RF output power drops to around 50 watts on all bands.
RF Output Power: Approx. 150 watts, on all bands
These parameters and the RF output power are pretty consistent on all of
the bands within the RFC-1.
That is interesting about the driver stages, and the possibility that
they may be outputting a 2nd harmonic of the excitation frequency. But
would this be true on all of the bands, based upon the consistency of
the relationship of DC input to RF output/poor efficiency on all bands?
I don't think so.
There may be an issue with the driver stage. The 2-4 Mhz band requires
that the 1st Amplifier Tuning cap be fully counter-clockwise (position
0) to obtain drive to the driver stage, when tuning the amplifier on
3850 Khz. This is indicative of the 2-4 Mhz driver band either requiring
realignment, or a defective component, etc. The 4-8 Mhz band within the
driver stage is completely dead, but I can get adequate drive from the
8-16 Mhz band. All of the other driver bands operate correctly. My plan
was to attend to the 2-4 and 4-8 driver band issues once I got the low
RF output issue addressed.
Can you also provide any guidance as to the possibility that the plate
blocking capacitors may be defective, and they could be responsible for
the gross inefficiency of the output stage, etc.?
Any more suggestions and guidance that you can recommend here?
73,
Bruce, W2XR
------ Original Message ------
From: jvendely at cfl.rr.com
To: triodes at optonline.net; tmc at mailman.qth.net
Sent: Wednesday, January 25, 2023 8:04 PM
Subject: Re: [TMC] TMC PAL-1K Low Output Power Issues
Sorry, I completely missed your comment that you're getting
high plate current but little RF output, which puts a somewhat different
twist on things. Still, the fact that you can't get idle current over
175 mA at any reasonable grid bias suggests low emission. -110V Eg1
should produce about 220mA. I would start with a DC emission test as I
described earlier, and go from there. What grid bias produces 175 mA
plate current in your amp? What plate and screen currents do you get
when applying RF and getting the high input power/low output power
condition? Tubes with 10-15% emission drop may still get you close to
full output, though with much higher input drive and poor linearity.
That horribly low plate efficiency sounds more like a grossly mistuned
condition. Have you aligned the driver stages? Don't assume they're
correct, as these amps may be badly screwdrivered if previously owned by
a ham. I've seen some real butchery.
In some cases, it is possible to mistune the output of the driver
section to the second harmonic of the input stages, essentially turning
the driver into a frequency doubler. This will make it impossible to
tune the PA section. This could happen more easily if the driver is
badly out of alignment. If you haven't done so, carefully go through
the driver stages alignment procedure and make sure things are tracking
properly. Before you begin, measure the input resistors in the 1st RF
driver, as these have often been overheated and are out of tolerance.
The RFC amp which serves as the IPA for the GPT-10K and 40K is almost
identical to the RFD amplifier in the PAL-1K, but there are some
differences. The driver stages are somewhat different, and the output
inductor taps are in different locations than on the PAL-1K, as the amp
only tunes to 28 Mc. So, follow driver alignment procedure in the
GPT-10K manual.
73,
John K9WT
On 1/25/2023 5:21 PM, 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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