[TMC] TMC PAL-1K Low Output Power Issues

Wed Jan 25 13:37:27 EST 2023

Thank you John!

I understand now.

Warm regards,

Chris Bolkan

On Wed, Jan 25, 2023 at 10:29 AM J.F. Samuels - K2CIB <radiowhiz at gmail.com>
wrote:

> 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
>> > ______________________________________________________________
>> > TMC mailing list
>> > Home: http://mailman.qth.net/mailman/listinfo/tmc
>> > Help: http://mailman.qth.net/mmfaq.htm
>> > Post: mailto:TMC at mailman.qth.net
>> >
>> > This list hosted by: http://www.qsl.net
>> > Please help support this email list: http://www.qsl.net/donate.html
>> ______________________________________________________________
>> TMC mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/tmc
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:TMC at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>
>