[TMC] TMC PAL-1K Low Output Power Issues
triodes at optonline.net
triodes at optonline.net
Wed Jan 25 14:58:14 EST 2023
Hi Bob,
Thanks for your suggestion.
But if I'm able to measure a 1.2:1 or less VSWR from the RF output port,
to the plate of the tube when the PL-172A plate is terminated in a 1800
ohm resistive load to ground, and the plate blocking caps are of course
in the circuit for this test (between the 1800 ohm resistor and the
input to the pi-net), how could the plate blocking caps be bad? I would
have to assume their failure mode would have to be completely open, and
that does not appear to be the case.
And there are two 500 pF at 5KV in parallel that comprise the plate
blocking capacitor. Both capacitors would have to be bad for this to
occur. They are very robust oil-filled capacitors, and to me at least,
it is somewhat unlikely that both capacitors could have failed at around
the same time, etc.
Any thoughts, etc.?
73,
Bruce, W2XR
------ Original Message ------
From: wa1zru at bellsouth.net
To: triodes at optonline.net
Sent: Wednesday, January 25, 2023 2:26 PM
Subject: RE: [TMC] TMC PAL-1K Low Output Power Issues
Sounds to me that your blocking cap is the problem. If you are
pulling 1400 watts from the supply and only getting 150 watts output
from the tank circuit and output, the tank is not matched to the plate
impedance. Replace the blocking cap with a .002 uf at 10,000 volt and I
think that will be the fix.
Bob
WA1ZRU
-----Original Message-----
From: tmc-bounces at mailman.qth.net <tmc-bounces at mailman.qth.net> On
Behalf Of triodes
Sent: Wednesday, January 25, 2023 12:30 PM
To: J.F. Samuels - K2CIB <radiowhiz at gmail.com>; TMC Reflector
<TMC at mailman.qth.net>
Subject: Re: [TMC] TMC PAL-1K Low Output Power Issues
Hi John,Thanks very much for the courtesy of your reply.Over the
weekend, I terminated the PL-172A plate with an 1800 ohm non-inductive
resistor to ground. This was to simulate the PL-172A plate impedance, as
it looks into the pi-network.I then fed my MFJ network analyzer into the
50 ohm output port of the RFC-1 Power Amplifier. I did this on all
bands, and I was able to get a sharply defined 1.2:1 or better VSWR on
all bands, with a minimal reactive component, by adjusting the plate
tuning, output loading, and the coarse loading on the RFC-1.These leads
me to believe the pi-network and it's associated components, including
the plate blocking capacitors (a pair of 500 pF caps in parallel), are
good.What I don't understand is with 1400 watts DC input to the PL-172A,
where is all of this RF going, as I'm only seeing around 150 watts RF
output on all bands? Are the plate blocking capacitors, if they were
bad, not coupling most of the RF, and as a result, the bulk of the RF is
being dissipated within the PL-172A? That may be an unlikely scenario,
but who knows?And the PL-172A, if it were indeed bad or weak, and with
1400 watts DC input, be responsible for such a gross reduction in RF
output?I'm not that familiar with the possible failure modes of large
pentode transmitting tubes.73,Bruce, W2XR Sent from my Verizon, Samsung
Galaxy smartphone
-------- Original message --------From: "J.F. Samuels - K2CIB"
<radiowhiz at gmail.com> Date: 1/25/23 9:30 AM (GMT-05:00) To:
triodes at optonline.net, TMC Reflector <TMC at mailman.qth.net> Subject: Re:
[TMC] TMC PAL-1K Low Output Power Issues
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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