[TMC] Pal 350/500

[triodes]

Hi John & Fellow TMC Enthusiasts!The issue of turning the HV on and off for PTT quick break-in operation should not create any fears, particularly if the primary AC circuit for the HV PSU includes a simple current limiting/soft-start circuit In my homebrew 2x 4-400A modulated by 2x 833As kilowatt-class HF rig, I soft-start the HV with a 10 ohm 100 watt resistor in series with the 240 vac primary circuit to the plate xfmr. After 500 msec, the resistor is shorted out of the circuit. The result is a complete lack of nuisance tripping of any circuit breakers, lights dimming in the house, etc. I built this rig 42 years ago, and I've never had any failures of any of the power supply components, or any of the other components within the rig. And I'm still on the original RCA 833As, and my second set of 4-400As.I also soft-start the 4-400A and 833A filaments, to protect the filaments from  cold inrush current.Soft-start can be easily added to virtually any transmitter or amplifier without drilling any holes, etc., that would otherwise impact the originality of the equipment. A suitable power resistor, contactor, and time delay circuit are all that are required. By the same token, none of the variants of the 1000 watt output GPT-750 used soft-start for the HV on/transmit, and TMC obviously didn't think this protection was necessary,  insofar as protection of the components and long-term reliability was concerned. Operating history in the field has confirmed their thinking was correct, but if you are concerned, add the soft-start capability to your PAL-350 rig. The plate modulated AM variant of the GPT-750, with it's PTT capability,  was probably the most stressed operation of this model transmitter, and they ran for decades with no record of out of the ordinary failures, etc.Just my thoughts....73,Bruce, W2XR Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: John Poulton <jp at cs.unc.edu> Date: 7/21/21  4:38 PM  (GMT-05:00) To: Chris Bolkan <bolkyboats at gmail.com> Cc: tmc collector's group <tmc at mailman.qth.net> Subject: Re: [TMC] Pal 350/500 I have to agree.  Nick's absolutely right.. lots of ham rigs just slammedthe HV on and off by switching the primary voltage on the HV transformer(but obviously not the rectifier filaments, if any)..   That whole ideagives me the willies, though.. mainly because of the inrush current issue.If the transformers hadn't been really inefficient, that problem could havebeen fatal.I confess I've never thought about the keying problem for the TMC amps.  Ijust had another (of many!) looks at the PAL-500 PSP-500 schematic.  Thereis, of course, a relay that controls the HV primary, but it's part of theinterlock system, and I'm virtually certain TMC didn't intend for it to beused for keying.  I'm kinda thinking that the standard way that TMC keyedthese rigs was just to remove the RF excitation from the exciter, leavingthe amp idling.  The only issue with that is the (considerable) noisegenerated by an idling linear amp, but in most installations, the amp, whenin standby, was isolated from the receiver by the antenna relay, so thenoise may not have been a bother.   But, that seems a little iffy to me aswell, in that the amp's antenna output is wide open.  What happens if theRF excitation comes up before the antenna relay has a chance to switch overto transmit?  Maybe it's just not a problem, given the robust design ofthese amps..?In the 'standard' PAL-500, S206 switches the bias from CW to SSB mode, andit seems to be the only circuitry that can mess with the bias.  If you'llhave a look at the schematics for PA and pwr supply, you'll see that thebottom of S206 goes to pin 6 of the power supply connector, and is involvedin CW mode.  It is routed back to a relay in the power supply, K101.  Thisrelay's coil is connected to a couple of terminals labelled "remote PAon/off".  I'm thinking THAT might be the keying system!  I can't quite seehow it works, but.. I think that's it.  A reading of the manual text mayhelp, if it describes what these terminals do.73, John K4OZYOn Wed, Jul 21, 2021 at 4:07 PM Chris Bolkan <bolkyboats at gmail.com> wrote:> I'm not trying to bait anyone with my comment regarding turning on and> off the entire HV power supply just to key the amp. Maybe that is the> way it is supposed to work. I'd be the LAST to know. It just doesn't> seem like the best way to do it to me. I realize the power supply is> choke input which helps a lot, but cycling something (almost anything)> completely on and off every time it is used will shorten its life over> just leaving it on or off.  Inrush current, charging and discharging> those big caps repeatedly, the time it takes to charge and discharge,> completely cycling the power through the ENTIRE circuit every time the> mic button is pushed seems to me like a great way to shorten the> system's life life compared to either leaving it biased on all of the> time it is in use, or simply biasing the final tubes on and off.> That's the way I see it anyway. I am only trying to learn and> understand this stuff. It's the first of anything like this I have> ever owned, and I want to make sure I understand as much about the> intended operation as I can.>> And THANK YOU for having me! This type of discussion is exactly what I> was hoping for!>> Chris>> On Wed, Jul 21, 2021 at 10:49 AM Nick England <navy.radio at gmail.com>> wrote:> >> > OK, I'll bite - what's so bad about keying the AC to the B+> > transformer? If I use PTT on my SBT-1K, that's what happens, Ditto for> > all the 100-1000 watt AM transmitters I've come across. Don't know> > about bigger rigs.> > Granted I'm not experienced with SSB linears and have read you can> > leave B+ on as long as you're not getting noise into your receiver.> > Hoping to learn something here - -> > Nick England K4NYW> > www.navy-radio.com> >> > On Wed, Jul 21, 2021 at 1:21 PM Chris Bolkan <bolkyboats at gmail.com>> wrote:> > >> > > Thank you John!> > >> > > I will look at the system drawings to see if I can figure anything out.> > >> > > I am (relatively) certain that the power supplies were not turned on> > > and off every time a communication took place or data was transmitted.> > > That would be poor implementation and result in shortened lifespan> > > unless transmissions were very long with significant down time between> > > (unlike amateur radio). That leaves having the amplifier keyed on> > > biased to transmit 100% of the time for most of the variants. Some> > > variants did not even have the grids connected to pin 6 which would> > > have made the only viable way to operate them as keyed on 100% of the> > > time. Other variants that do send the grid connection to the power> > > supply terminate it in different ways, some which still result in> > > being keyed on all of the time as the only viable way to operate. I am> > > most curious about the particular PAL 500 variant that controls the> > > grids with the remote relay (as that is the power supply variant I> > > have). Is anyone aware of these amps actually being keyed by changing> > > bias with the remote relay in this manner?> > >> > > Maybe I am thinking about this all wrong since they are designed for> > > 100% duty cycle operation, what difference does it make if the tubes> > > are biased on all of the time other than more current draw? I am just> > > used to seeing final grids biased off during non transmit times, and I> > > ultimately want to use this for amateur radio and that is how I am> > > used to seeing it done. I imagine my Heathkit SB-220 would not live> > > near as long if it were biased on ready to transmit 100% of the time> > > it was powered up, but it wasn't designed to be biased "on" 100% of> > > the time.> > >> > > If keying by bias change was actually practiced on some of these amps,> > > I would like to at least try. I do not want to significantly alter or> > > use these in a way that was not intended. I want to stick as true to> > > original functionality as I can.> > >> > > Chris> > >> > >> > > On Wed, Jul 21, 2021 at 8:47 AM John Poulton <jp at cs.unc.edu> wrote:> > > >> > > > Chris,> > > >> > > > Welcome to the TMC list!  And, congrats on finding all three parts> of a PAL-350/500.. so many of these beautiful amps have either gotten> separated from their power supplies, or are missing their connecting cable,> which is damn' near impossible to reproduce.> > > >> > > > Your keying question is an interesting one.  There are several folks> on this list who operate PAL-350/500 amps, so it's likely one of them could> advise the best way to key the amp.  But, if all else fails, TMC sold these> amplifiers in various systems, like the SYM-1202/1203/1204 or "Shipboard> Station", for example.  You might have a look at the documentation for> these systems at:> > > >> > > > https://tmchistory.org/tmc_manuals/tmc_systems_manual_page.htm> > > >> > > > and perhaps dope out how TMC did the keying "officially".  Let me> know if I can help in any way, such as chasing down TMC drawings..> > > >> > > > Best 73, John K4OZY (TMC "Librarian")> > > >> > > >> > > > On Tue, Jul 20, 2021 at 7:46 PM Chris Bolkan <bolkyboats at gmail.com>> wrote:> > > >>> > > >> First, Thank you Nick England for getting me on this site and being> > > >> able to post!> > > >>> > > >> Hello! I am new to the list but not new to the admiration of TMC> gear.> > > >> I used to stare at the TMC pages in the back of the ARRL handbook> > > >> catalog sections when I was a kid and wish I could have something> like> > > >> that. Everything else in those pages paled in comparison. It was the> > > >> favorite equipment I wished I had and those pictures burned an image> > > >> in my brain that will never go away.> > > >>> > > >> Over the years I have acquired a straight GPR90 and an RXD which I> > > >> have used, held onto and displayed with great pride! Recently I> > > >> acquired a PAL350 amp and PAL500 power supply with interconnecting> > > >> cable. I am in the process of checking everything out, getting> > > >> acquainted, fixing the little things that were not right and> starting> > > >> to power it up.> > > >>> > > >> To that end I have read every post in the archives of this list and> > > >> printed out everything that I think will be helpful in getting it up> > > >> and running properly.> > > >>> > > >> My first question has to do with the operation of these amps. I have> > > >> printed out schematics of the three 350 variants and the two 500> > > >> variants from the History site plus the instructions and addendums.> > > >> Studying these it appears that there are two (maybe 3) ways to "KEY"> > > >> these amplifiers.> > > >> First would be to turn the (HV) power supply on and off with the> > > >> "remote" relay circuit every time I want to key up, but I find that> > > >> idea unacceptable for obvious reasons, so I don't think that is the> > > >> way TMC intended them to be operated.> > > >> The second way I can imagine using it is to key the amp on (output> > > >> tubes biased to transmit) and have it resting in the biased on> > > >> position the whole time it is turned on ready to receive a signal> from> > > >> the exciter to amplify. Is this how these amps were used with TMC> > > >> exciters? (which I do not have)> > > >> The third way to key the amp appears to have actually been an option> > > >> on one of the 500 variants, which was to use a remote keying signal> to> > > >> change the bias of the output tubes from operate to cut off and> back.> > > >> This seems to me to be the most logical way to do it. That way the> amp> > > >> isn't keyed on 100% of the time, more like conventional amps I am> > > >> familiar with are operated.> > > >>> > > >> One concern to me is that the PAL 500 variant that appears to have> > > >> been sold this way seems to have an addenda to have the user reverse> > > >> this functionality bringing it back to the grids always being biased> > > >> on, removing the provision to bias them off from the remote relay in> > > >> the power supply.> > > >>> > > >> Any help and guidance is greatly appreciated.> > > >>> > > >> Thank you so much! I am sure more questions are to come and I hope> you> > > >> all have patience with me.> > > >>> > > >> Chris Bolkan> > > >> KI7NXY> > > >> ______________________________________________________________> > > >> 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>______________________________________________________________TMC mailing listHome: http://mailman.qth.net/mailman/listinfo/tmcHelp: http://mailman.qth.net/mmfaq.htmPost: mailto:TMC at mailman.qth.netThis list hosted by: http://www.qsl.netPlease help support this email list: http://www.qsl.net/donate.html