The AM Forum

I am the proud new owner of a beautiful Technical Materiel Corporation (TMC) GPT-750D-2 transmitter. It will go nicely with my recently obtained GPR-90 and GSB-1 SSB Adaptor, and some other TMC gear I have acquired over the years.

No, this is not the GPT-750 variant with the internal P-P 810 modulator, but rather the more common version with the integral SBE-2A ISB/DSB/SSB exciter. My plan is to build an external P-P 833A modulator for it, similar in design to the P-P 833A modulator for my homebrew plate-modulated 2x 4-400A rig, and retain the SSB exciter that fits within the center drawer of the transmitter, so as to have an all-mode high-power rig that will cover the entire spectrum from 2 to 32 Mhz.

My question is this; I vaguely recall seeing or hearing somewhere that there is a very simple modification available for the GPT-750 that enables the transmitter to cover the full 160 meter band. Is anyone familiar with this modification?

Thanks & 73,

Bruce

Congrats OM!
Please post the photos we can drool over the find....

yeah man, give us some hot tx porno!  :D

It won't cover 160 with the internal VFO, Bruce, but if you use an external VFO it'll do 160 if you cut the strap that shorts the last 2 or so turns of the final tank coil.

Congrats.. Is this the one that was on Ebay?   Last week, I saw one and forgot to bid..  I know a man that Has the matching Reciever if thats something you need, I can get you in contact with him.

Clark

Al, VE3AJM has a GPT-750  and may know about mods for these rigs.  PM or email me  for Al's contact info.

Guys, thanks for all of the nice comments, etc.

No, I did not find it on e-Bay.

John, W3JN: many thanks for the input. You may have been the one who mentioned in passing what was involved in getting this transmitter to go down to 160M, perhaps when I saw your GPT-750 during the visit to your QTH of about 1-1/2 years ago, after the KB3AHE party. At that time, I had no plans of ever obtaining a GPT-750; the one I acquired kind of fell into my lap unexpectedly, all 900 lbs of it. It's a beautifully built rig, like most of the TMC gear I have had the good fortune to be acquainted with.

Did you also convert your GPT-750 for use with an external frequency synthesizer, so as to reduce the very long and slow QSY time of the stock VFO? I seem to remember that too, when you sold off the TMC VOX-5 you had up at Nearfest. If that is the case, it kills two birds with one stone; the reduction of slow QSY, and it allows the rig to operate on 160 meters once the link on the HPA tank coil is removed.

73,

Bruce

Yes, it involves disconnecting a BNC cable inside the RF deck; there's a BNC connector on the back that takes an external VFO input.

I'm using a HP3325A synthesizer for the VFO.  It needs between .5-1V or so from the synth to properly drive it.  Of course all bets are off using the SBE exciter.

It is an absolutely beautiful piece of equipment.  The only thing I would have done differently, as an engineer, was that VOX VFO.  It needs a drive motor!

I strongly suggest you put a NTC thermister in the primary of the plate tranny, as I fried the primary in mine.  The NTC mounts easily between the two halves of the primary windings.

Hope to hear it on the air!

How would a thermistor protect the primary? Inrush?? For that kind of power I would do a step start

Hi John,

Thanks for the info!

With regard to the thermistor at the primary of the plate xfmr; I agree with Frank/GFZ. I don't see how that would protect the transformer. A soft-start relay/resistor circuit would limit the inrush current to the plate xfmr and the rectifiers, and would offer much greater protection to these components. Could you clarify your thermistor rationale for me?

A failure of the GPT-750 plate xfmr would be serious, as the xfmr is of sealed construction and probably oil-filled and therefore somewhat difficult to unseal and repair, etc. Did you find a used replacement for yours, or did you go to the trouble of having it rewound? Just curious.

73,

Bruce

Yeah, I got a replacement from W7FG who had parted out a few.

The NTC is for inrush limiting; I got a box of 2 ohm cold/20 amp jobs.  As good as a step start except when you're in a fast-break in.  The NTC doesn't have time to cool down in those cases.  Still it's a cheap, easy, and quiet way to absorb most of the inrush surges that the transformer is subjected to.

2 ohms sounds a bit low. That is still a lot of current.

It's not clear to me why you would need any in-rush protection on a choke input supply.

Indeed, but it's roughly equivalent to the DC resistance of the primary.  If you happen to key the xmitter at the peak of the AC line voltage, you get a hell of a momentary inrush.  The NTC will absorb some; so will the resistance of the line cord, plug, etc.  I'm not saying it's a panacea, but for a buck, it can't hurt.

For sure. Simple cheap and can't hurt. And I think you have a life time supply of those things!

You can build a step start with a relay and a resistor and get real protection.

Indeed. I'm still at a loss as to why it would be needed on a properly designed choke input supply. Where does the in rush come from, unless you have an unusually large filter cap and an undersized choke.

In my opinion, cz I like to run my yap, is that this rig doesn't need a step start, but it wouldn't hurt anything either. I think if TMC thought it should be in there, the would have put it in.

One thing TMC didn't do was cut corners or underbuild anything. In today's dollarettes (as Don calls them), these rigs were more than $50,000. ($7500ish in 1966)  Compare that to today's top of line military HF gear, and we see how the pricing is similar for that level of engineering. (Forget YaeComWood) So needless to say, design and build budget wasn't an issue and marketing them to hammy hambone wasn't an issue either.

Both the HV and MidV power suckplies already have pretty fast overload protection. If anyone want's to add a little to it, then add a real step start as Frank already said. (C'mon jn, WTF dude? that's not a Junkston Valiant!!! Just kidding.  :P :P )

To dig in a bit further,

Replace K601, the transmitter plates relay with a a pair of nice plug-n-play relays. Unless your a God and can locate a 5 pole double throw with a 115vdc coil. This relay turns the whole shootin' match on and off every time you pull the trigger, so it does wear out after a while. I used 110vac coil relays on my first 750 with a diode in series with the coil. I suggested to use pluggin type common relay's so that when I'm doing my little maintenance dance, I could un plug them and drop them in the garbage and plug in two new one's. When the OEM one went bad, it would drop out and remake like a jackhammer and before I could get my hand over to the toggle to unkey, it smoked one of my 4x4's. (A nice new Eimac too, boy was I pissed)

Ok, next to get the axe is K602. It's a little vacuum tube time delay relay that is SUPPOSED to allow a minute for warmup. It starts to get a mind of it's own when it gets old, and will take forever to kick in, or it will drop out during a transmission which can get aggravating. So,out with the old and in with the jumpers.  ;D The quick and dirty is to take a 2" long piece of solid 18ga and jump the socket pin 8 over to pin 4. Put the tube shield back on and forget about it. Works like a nice key too if you ever want to disable the TX, just pull the jumper.

Shitcan the 872a merc rectifiers if they're still in there and shove in a pair of 4B32's. Won't arc out, reliable, and you can buy ton's of them on ebay for what a pair of big diode's will cost.

Double triple check/replace the OA2's in the PS drawer. There's a string of 4 of them to regulate the screen voltage. (which is had by dividing it off the Hi B+) The screen voltage runs thru one of the Cannon connectors on the PS drawer (The outside one I think). It's already 700 volts I believe, and if it gets funny, it'll arc out that Cannon connector and not mentioning that it's $50 per side, it is a real serious pain in the ass to replace it.

I got tons more mods, but it's chow time. Seeya guys later.

 

   

So this rig keys the whole deal, then no questions I would go with a real step start.
I can just imagine the windings in the transformers dancing. Say the hammer drops at the peak of the waveform and the high voltage cap is at zero. I wonder how much current flows over the next 5ms. Will the inductance of the transformer and choke windings limit the current so the wire doesn't rub the enamal off. Sure it might for 30 or so years but say you add a simple step start with 10 0hms and a relay that hangs open for 10 cycles. 2 ohms is better than nothing are you feeling lucky?

Believe that's what happened to my xformer, Frank; far as I can determine some windings on the primary shorted.  It draws heavy current with no load on the secondary.

Jared, good tip on K602.  Every once in a while mine will drop out and the thermal relay will need a couple minutes to reset.  The only reason it's there is to warm up the 872s, so with 4B32s or SS rectumfriers it's unnecessary.

Tube rectifiers are useless in 2009
they make better lamps

Tube rectifier such as MV are great if they are in a place where they can be seen.

If they cannot be seen, then replace them with solid state. It's just the way I do things as I do it for show  ;D ;D

My Westinghouse MW-2

Yeah, that's a thing of beauty Terry.  In the GPT, however, the 872s are locked away in the back corner of the bottom drawer.  No reason not to solid state 'em, or run 4B32s.

that IS quite a "cathodral".. my term for such a vault of glass-enveloped vapor rectifiers. time to burn some instench.

How much current is that 10H PS choke rated for?  Probably for full plate current plus a bit?

750 mills Steve. The one in my rig is an 8-15Hy swinger. Some models of this rig came with a 10-25hy swinger however. The caps are 3 paralleled 4mf/4000v bricks.

The whole point, which was sorrily missed, about the rectifiers was to just get the 872a power supply killers out of there. Of course I would like to run big diodes in there as well, but the proper devices are hard to find. Surplus thieves, oops, I meant to say "Sales" of Nevada had a run of them a while back and he wanted an arm/leg for them. These are the black stackable diodes shaped like a HV insulator that you can screw together with a base and cap, and drop them into the socket. I don't remember right now what the exact ratings were on them, but I know you had to stack at least 2 of them on each side, (3 would be better) and they were $25 each. (4pcs altogether) Add $50ea for two jumbo 4 pin adapter bases, and $15ea for two caps and you were at $280 with 3 diodes in each rectifier. The hell with that. I can shoot 4b32's out of a potato gun off my back porch all day long and not spend that much. They average $5-$10 each on ePay and they far out-rate the solid state way discussed above. They have a 10kV PIV, forward avg anode cur is 1.25A and peak is 5A. Now solid stating can be had by a group of diodes on a circuit board stuffed down into the socket, sure, but I'm not comfortable with half-assed, js'd hackery like that in a tx'er like this. (Junkston? sure, but not in this rig) Maybe a couple of those bricks sold by that ham who's call I can't remember right now would be cool, then just pull the tubes, sockets and fil tranny all together.

The way it was explained to me, the failure mode on an XG rectifier is that it just stops conducting, vs. the crazy flashover (and associated destruction) of a merc. Is this true, I don't know. I've never killed a 4B32 to find out. I have two of them in a 4x1 slopbucket amp (4200 on the plate) right next to me that get almost daily abuse and they're still plugging along just fine.

I didn't know there was a point missed? Your right about the 872's and ancient  potted transformers. I only had one 4B32 fail. This is my only experience with just one of them. What happened is that the voltage gradually declined in a full wave circuit. Some 4B32 are designed to be able to view the glow and others are not. I have often wondered why they were built that way. 

I guess I didn't really mean that to sound harsh. My point was more about the 4b32's vs solid state blocks. In these big tx'ers, there so much avail room that there's no reason to go out of your way to use ss diodes and alter everything original, vs just plopping a couple of easy to find and cheap 4b32's in the sockets and calling it a day.