GE XT-1A ???

[kc6mcw]

I am in search of my first broadcast transmitter and have my eye on the big GE XT-1A. I have always been fond of the 833 tubes and thought this would be a cool box to have. I am hoping for high fidelity audio with 160, 80, 40 meter band switching after all the necessary mods. I have heard some goods and bads about the circuit design of this transmitter. What am I to expect with this 1947 era technoligy? Can I get 100% modulation out of this thing? And how about the audio bandwidth capabilities? I understand that it has a 2' high modulation transformer...I have included the schematics. I am concerned that they are triodes and might have neutralization issues. Any thoughts and opinions on this possible project is greatly appreciated!

[KA1ZGC]

Never been into one, but it looks like they've wrapped feedback around any triodes in the RF stage. It's not a simple vacuum-variable like you might expect, but adjusting those inductances is probably how the rig is neutralized.

Given that these are designed to be parked on one frequency, you probably can't get to those adjustments from the front panel.

On the other hand, if you're going to mod the thing for 160, 80, and 40 meter operation, you'll need to tear into that anyway, so it'll be a good time to address that issue.

Once neutralized for a given frequency, you should be okay. Just gotta hit it with some drive with the plate volatage off and adjust the neutralizing circuit for minimum plate current. Then fire up the plate supply and you're off. It's just one more step in tuning the rig when you make large frequency excursions or change bands.

[KA1ZGC]

I just looked at the schematic again, and I noticed something else.

Unless there's a label somewhere I can't see, it looks like they may be running the filaments on some of the lower-level audio stages in an unbalanced configuration. That frequently leads to hum issues.

If you do buy it, put that on your list of things to address when it comes time for the mods. If any of the filaments are tied to ground on one side, lift that ground and run that side straight back to the other end of the filament iron (and lift that ground too while you're at it).

Hum issues can appear not only in the tube with the unbalanced filament, but the AC current flowing through the chassis can have an effect on the other tubes as well.

To answer your original question, though: if you're planning on modding the thing anyway, I don't see any reason not to buy it if the price is right.

[Jeff W9GY]

We had one of these when I was part-time ass't CE at WIOU back in the late '70's.  It was in use as a standby, being replaced by a much newer Gates unit (when the station got 5KW daytime authorization) .  It is a big heavy contraption for sure. 

On the plus side, there's plenty of metering and you can tell most problems pretty quickly.

On the negative side, we were never able to get more than 80% positive peak modulation, and I think we even tried fresh modulators and finals at one point. Also, at some point in it's lifetime, the modulation iron developed a short to the case.  So the only way to keep it working was to jack up the transformer and isolate it from the chassis on a couple pieces of wood!

It didn't get a lot of air-time, being a standby.  I'd run it once a week in the evenings (in night power(1KW)/pattern) for a couple of hours. In the 4 years I was there, I don't remember any failure with the unit.

[kc2ifr]

I was given this transmitter for free........just come and get it!
It sat outside for about a year under a tarp and when we picked it up it was full of bees!
Anyway when I got it home it was not worth trying to get it back on the air. It now sits in the warehouse where I work.
According to the timtron, the mod iron is the holy grail of mod iron........very good frequency response etc.

I hope u find one in good shape BUT it is a monster.......   

[kc6mcw]

I appreciate all the input guys. The hum issue is something to consider. I just always liked them 833's and was hoping that it could be a performer after the mods of course. I'm wondering if 100% modulation would be possible if the carrier power was simply lowered to say around 750w or so....

[WA1HZK]

It's a thing of beauty. If you have the room give it a home. Your probably going to change just about everything anyway so don't worry too much about the small stuff right now. If you get it running and get into specific issues I'm sure we can scratch up some old brain cells on this board and put some ideas together. Anyway, don't let it die. Check out my 833 rig posting in the transmitter section for a bunch of ideas. Lotsa guys here have built 833 rigs.
Keith

[KA1ZGC]

Well, the hum may be a non-issue. That's why I suggested it as a "while you're at it" item. In general, it's good engineering practice to provide a floating balanced AC feed to the filaments.

While you're at it (there I go again), running the filament leads in a twisted pair (or at least a bound pair) wouldn't be a bad idea, either. That would cut down on potential issues arising from RF in the filament leads (any stray RF fields would induce their energy to both conductors more equally, and common-mode rejection would be your friend).

I recognize that iron. For the life of me, I don't remember where I recognize it from. I only know it's good iron. Still, there's no reason you can't get 100% and more from a pair of 833s modulated by a pair, which is what you've got here. You may just have to play with the voltages a bit.

They're modulating the unit in a modified-Heising configuration, which is good. You won't have any DC on the iron that way. Nevertheless, following up on what Jeff said about the WIOU transmitter, it would be prudent to insulate the transformer and the Heising reactor from the chassis, just in case either develops a short to the case further down the road; since you now know that has happened at least once to this model.

Unless there's a crappy audio driver xfmr, you shouldn't have to do much of anything to the modulator beyond that (provided it makes at least 100% for you).

One person that just leapt to mind is Robert, W0VMC. I almost think he's done one up before. If so, I'm sure he could tell you all the gotchas behind it, in five-part harmony.

Like Keith said, lots of guys have done up 833 rigs with great success. You won't be alone on this. Give this one a home, and you'll be in good shape by the time you finish up with it.

[kc6mcw]

Ok..After reading this article about a homebrew 833 rig, http://www.criticalradio.com/833%20Rig%20Project/Web%20Pages/Text%20Article.htm ..... I am to assume to that these 833 triode finals need alot of drive, hence they are not tetrodes. It was suggested to use a 4-400 driver to provide this extra drive. Then 100% modulation would be possible. I am missing something here. I have always thought that reducing the carrier power would bring the modulation up higher. BUT this was the case in using an amplifier BEHIND a transmitter of some type. Is it a different case with plate modulation? Would the 833 push pull modulators need more drive? Could I simply raise the 833 modulator plate voltage to achieve higher positive peaks?

[KA1ZGC]

Well, before you go too far down that road, you don't yet know that the unit won't be able to fully modulate. You may be chasing a red herring.

Jeff stated that the WIOU transmitter topped out at about 80%, but in the very same paragraph he also stated the mod iron had a short to the case. That's important.

That being the case (no pun intended), that iron probably also had other issues, like shorted turns. That would easily cause the modulation to drop way off. When the short to the case first developed, it could have done God-knows-what for damage in the power supply or modulator. The lack of modulation Jeff saw may well have been a lingering effect of that failure.

So none of this may actually be applicable to the transmitter in question.

You need to find out the service history of the transmitter you are looking at. Ask the previous owner if they had similar issues, or any issues at all.

The information Jeff gave you is very valuable in preventing a future failure that may be an issue with this model, but not yet an issue with that specific unit.

I can't imagine GE ever putting a transmitter to market that was incapable of 100% modulation when it left the factory. Find out from the current owner what issues it's had, and don't assume you have a given issue until you know you have that issue.

FWIW, the 833s are driven by a single 8005 driver. That's probably giving 50-70 watts. That should be enough to drive the 833s into a reasonable class C; but again, you don't know that you have an underdrive condition yet.

[Jeff W9GY]

Yes, I've got to belive that the WIOU GE unit at one time in the past did deliver 100% modulation.  I can't tell you any more about the circumstances of the mod iron short, other than it happened before my time (prior 1977, if I remember when I stated there)  The old CE told me what he knew at the time. The shorted turns theory is a good one and consistent with what one might expect. 

When we had tours of the station, we would always light up the GE, since it looked so impressive with all those 833's glowing!

[W2XR]

I was given this transmitter for free........just come and get it!
It sat outside for about a year under a tarp and when we picked it up it was full of bees!
Anyway when I got it home it was not worth trying to get it back on the air. It now sits in the warehouse where I work.
According to the timtron, the mod iron is the holy grail of mod iron........very good frequency response etc.

I hope u find one in good shape BUT it is a monster.......   

Did'nt that rig run a UTC LS-series modulation transformer, 20 to 20 Khz, +/-1dB??? Maybe that is why Timmy raved about the iron so much.

73,

Bruce

[WA1HZK]

Re: Drive
I tried every possible ckt. to get the exciter (100) watt DX 100 RF deck into the 833 grids. To get the proper high grid current the easiest way to do that was to drop in a IPA stage. Idle the DX-100 into the 4-400 running the same common power supply as the rest of the transmitter. Doing this easily allows the required 175 ma of grid drive. Save the two years I spent experimenting with this and take the advice that a overpowered IPA stage running a old 4-400 does the trick on the cheap. The proof is in the radio. Also, a lot of commercial rigs ran anything from a pair of 807s (piss weak drive) to a pair of 813's as drivers.

In a class C RF amp that's plate modulated you need to drive the snot out of the tube. How else do you suppose there will still be drive available on a 120% modulation peak when your power output shoots to four times the carrier.
Keith
WA1HZK

[kc6mcw]

As I understand, a 4-400 makes a better driver for the 833 finals than the stock 8005 driver. Question is: If I choose to use the original crystal oscilator and use just one frequency, would the 833's recieve enough drive from all its original components? Or is the 4-400 driver used only if I am to use an external exciter? Is the original 8005 driver simply not good enough for anything? Does this driver tube have something to do with being able to achieve 100+ % modulation? I think I understand that if I use a 4-400 as a driver then I wouldn't have to implement a neutralizing circuit for the driver. Could this be the grand idea?

[KA1ZGC]

Did'nt that rig run a UTC LS-series modulation transformer, 20 to 20 Khz, +/-1dB??? Maybe that is why Timmy raved about the iron so much.

That's it. I checked with Tim on that the other night.

Oddly enough, I've heard of one transmitter with the mod iron shorted to the case, and one with the heising reactor shorted to the case. Concidence?

[WA1HZK]

The original driver is fine. You could drive it's grid with an outside radio. If you can't find the original toob then the dirt cheap replacement 4-400 does the same thing.
Keith

[W2XR]

Did'nt that rig run a UTC LS-series modulation transformer, 20 to 20 Khz, +/-1dB??? Maybe that is why Timmy raved about the iron so much.

That's it. I checked with Tim on that the other night.

Oddly enough, I've heard of one transmitter with the mod iron shorted to the case, and one with the heising reactor shorted to the case. Concidence?

Tom,

Thanks for the confirmation as to what I had suspected; that the mod xfmr and mod reactor in that rig were UTC LS-series components. They were listed in one of the old Harvey Radio catalogs that I have here, from the mid-to-late 1950s. I seem to remember the multi-match mod xfmr was nomenclatured as the LS-692, and was priced around $900.00 in late 1950s dollars. Big bucks back then.

I find it interesting that both the mod xfmr and the mod reactor had failed in GE XT-1As that you were familiar with. It leads me to remember what an old-time radio engineer once told me about UTC and what their initials actually stood for; U Take a Chance.

73,

Bruce

[kc6mcw]

Does the GE XT-1A have mercury vapor rectifiers? Or silicon...

[KA2QFX]

That looks like a sweet project.  Looking at the neutralization I would expect that the tube capacitance and the adjustable neutralization inductor will not track from band to band. Like Keith said you'll be changing a lot anyway so I suppose a conventional adj. cap. neutralizing circuit shouldn't be a big deal.

Good luck on that!  And Bill, why am I not surprised you have one of these sitting around?

Mark

[KA1ZGC]

Does the GE XT-1A have mercury vapor rectifiers? Or silicon...

Accoring to the schematic, they're hollow-state: a 5R4 for the low voltage, 2 866s for the medium voltage, and 2 8008s for the high voltage.

They don't state the rectifiers used in the bias supply, so that may already be solid-state. The other voltages are less of a concern, 15 or 30 volts of an error won't hurt anything if you go solid-state with those supplies.

Which reminds me of another "while you're at it" thing to check when you do the mods: measure the filament voltages at the sockets. This unit was designed in a time of lower AC mains voltages. Hitting these rigs with 240V when they're built for 230V will cause the filament iron to deliver that much more voltage. What should get 6V will actually get 6.2V and shorten the life of the tubes.

You can fix that easily enough if the iron has a multi-tapped primary, otherwise a small resistance in series with the primary does the trick.

It's possible this thing's had at least one good engineer in its long life who took care of that, but it's a good thing to check anyway.

[Jeff W9GY]

Our unit had solid state replacements for the medium and HV rectifiers.  Don't have a clue as to what was done to lower the filament voltage, but  I'm sure it was running on 240+ volt mains.

[kc2ifr]

I have the complete manual for the transmitter. If I can scan it........I will send it to anyone who wants it.

Bill

[kc6mcw]

OK the schematic shows the use of a variable inductor to cancel the tube capacitance in order to neutralize it. So how would a variable cap work in this situation? Do you think it could be band switched? 160, 80, 40...

[KA1ZGC]

You may wind up having to pull all that out.

Typically, all you need for neutralization is a capacitance in parallel with the grid-plate capacitance. It looks like they're using an LC network to counter the grid-plate capacitance, which I suspect won't work well in the higher bands.

This is where my knowledge gets shaky, but the straightforward way to neutralize a tube is to introduce a vacuum variable cap between the plate and grid, and adjust it for minimum output signal with drive applied and the B+ turned off on 40 meters (the highest band in your case). 75 and 160 should both play nicely then without readjustment after that.

Eimac's "Care and Feeding of Power Grid Tubes" has a whole chapter dedicated to the topic of neutralization. There are as many ways to do it as there are tubes to do it to.

That's about all I can tell you.

[w3jn]

Hey Bill, a suggestion - when you scan it, why not send it to K4XL - the owner of BAMA?