[TMC] TMC PAL-1K Low Output Power Issues
John Vendely
jvendely at cfl.rr.com
Wed Jan 25 20:04:25 EST 2023
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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