[TMC] TMC PAL-1K Low Output Power Issues
J.F. Samuels - K2CIB
radiowhiz at gmail.com
Wed Jan 25 13:29:04 EST 2023
Hi Chris - I am sorry I didn't explain, but I was just saying that the
dummy load should be checked to make sure it really is 50 ohms
resistive. I guess that the Watt meter on the output should be
double-checked too...
Low idling current could be caused by the screen inside the tube being
disconnected or the tube pin unsoldered.
Looking, just now, at the diagram, the screen bypass caps could cause
trouble too, but would not be the problem of the low idling current.
Sometimes bypass values are chosen to cause negative feedback/gain loss
to improve linearity, but if not chosen that way, a changed value could
cause inefficiency while the amp was driven. The diagram shows four
caps, and it appears that the screen appears on two tube pins.
Sometimes a strap would be placed between the two pins and the
capacitors would connect to the mid-point of the strap to ground. It
may be that the other two caps are built into the tube base.
*
*/73, John/*
*
On 1/25/2023 11:44 AM, Chris Bolkan wrote:
> Hi John!
>
> I read your reply to Bruce with great interest. Would you mind further
> explaining:
>
> "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?) "
>
> I do not understand exactly what you are saying to do here and I think
> it is important that I do. I know that we want the plate impedance
> matched to the load impedance and that the output tuning network is
> the impedance transformer. But what does the test you are asking if
> Bruce has run look like procedurally?
>
> My apologies for hijacking the question, but I have my PAL-1K up and
> running (seemingly OK) but have not tuned it up or loaded it up with
> RF yet.
>
> Best regards,
>
> Chris Bolkan
> Ki7NXY
>
>
>
>
>
>
>
> On Wed, Jan 25, 2023 at 6:30 AM J.F. Samuels - K2CIB
> <radiowhiz at gmail.com> wrote:
>
> 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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