The AM Forum

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!

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.

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.

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.

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.......   

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....

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

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.

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?

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.

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!

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

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

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?

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?

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

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

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

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

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.

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.

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

Bill

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...

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.

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

I would love to get a copy of the manual and full schematics. The schematics that I posted is all I have. It doesnt show the power supply or any of the relay architecture. I am very excited about this project and looking forward to the restoration process. Has anyone heard one of these GE XT-1A's on the air? How did it sound and what did you think about it? Here is a pic of its current condition before restoration...

When I get home from work today (at work now) ill find the manual and scan what I can. I think some of the schematics are BIG and Ill have to scan them in sections.
And yes, sending it to BAMA is a great idea.
Ill do my best.

Bill

Stay tooned............ ;)

Sounded gud on the BCB band 'back in the day'.  Lots of MOR and lite rock music went thru it ...  and sounded very nice in the mod. monitor.  It will make you a great rig!  Jeff W9GY

I understand this thing has a full protection relay system. If something isnt right, it shuts it down. Can someone give me more detail on this protection system?

Given my understanding of neutralization:
The variable inductor used here is adjusted to vary the amount of feedbaak from the plate to equal the leakage signal present at the grid from the grid to plate capacitance.  If you were to change bands their reactances would shift in opposition to each other, thereby reducing neg feedback up freq and increasing it down frequency. 
So, the feeedback element needs to be a variable capacitor so as to have the same reactance characteristic as the internal grid to plate capacitance.

Also, you'll note that the feedback signal is applied to the opposite side of a tuned grid input circuit. This is a commonly used method to achieve the required 180 degrees phase shift of the feedback signal to cancel the g-p leakage.

This technigue is pretty common to multi band PA circuits. I don't see why it wouldn't be applicable in modifying this xmtr.

Regards,
Mark

A correction: (my apologies) :-\
That is not a tune grid to which the feedback is applied, it appears to be just an L-R parasitic suppressor.  I would assume in this case that the L in the feedback network supplies sufficient phase shift to cancel the plate leakage.

So, using capacitive feedback would require applying the signal to the opposite side of the driver tank, where driver B+ is applied (where the volt meter is now). This would therefore require an additional plate choke in the B+ lead to the driver stage to prevent the feedback signal being bybassed to ground. 

Sounds a bit convoluted to me. Most neutralization circuits consist of a single variable capacitance directly between the plate and grid. No getting the input network involved.

All you need to do is provide the correct phase shift in a feedback path between the plate and grid circuits. Any reactive component can do that if it's the right value. If the cap is the right size, it need only go between the plate and grid without getting the input tuning network involved.

I am not fully understanding whats exactly happening in a plate modulated transmitter. So in order to modulate the finals to 100 - 120% modulation, you must drive the finals hard to the point where there will be enough carrier power there in order for the modulated signal to shoot up to 4 times the carrier amount. If this is correct thinking then what keeps the carrier at say 1kw and not any higher than that? I would think if you drive the finals harder, you would then just get a higher carrier level while your audio input is zero. I know I'm missing something here! hahaha

Yes, you're forgetting that the tube is in class C. This isn't a re-inserted carrier we're talking about, or a linear amplifier.

In class C, you're already driving the tube to saturation. Only about 20% of the sine wave applied to the grid is even appearing at the plate. The other 80% pushes the grid positive and causes grid current to flow.

Beyond the point of saturation, increases in drive will make almost no difference in the output of the tube. So no, you will not see an increase in carrier level. This is not a linear amplifier, this is a non-linear amplifier.

The only way to change the output signal level of a class C triode is to change the plate voltage. As you increase B+, you increase output, but only as far as you have input power to cover it.

If you drive your class C triode with only twice necessary unmodulated drive, you won't even make 100% modulation. 100% modulation = 4x output power. You won't have enough RF to fill the envelope, and you'll distort on voice peaks.

Hey Tom,
Re neutralization. Pull a few schematics and examine them closely. You'll see that somewhere there will always be something in the path between the adjsutable feedback element and the control grid to achieve a significant, if not 180 degree, phase shift of the signal to achieve cancellation.  Think about it, if the feedback element merely paralleled the G-P capacitance it would simply supply MORE undesirable feedback.  There are naturally occurring phase shifts (<= 90) due to the nature of the feedback reactance.  Were it not for that there would be no need for nuetralization since the plate circuit (at resonance) is already 180 degrees out of phase with the grid. So it turns out, that if the "neg" feedback element has the same reactive sign as the g-p reactance then a significant phase shift ie required for the two to cancel out at the grid. The 180 degrees present across a tuned tank provides this very well.   It gets more involved with real world impedances making nothing 90 or 180 exactly but that's radio.  If you'd like I'll send you a white paper I did on the subject a while ago discussing using broadband transformers to achieve the necessay phase shift when no tuned inputs are available.  

Since I'm typing... regarding the last question.  The excess drive to the final does not produce any greater carrier power because the power of the stages output is determined by the applied plate voltage.  The tube is essentially driven to saturation all the time, otherwise the modulated plate voltage would result in no increase in RF on positive excursions.  or another way of looking at it; as plate voltage rises, plate current must rise in step, so the grid rive must be of sufficient level to enable that peak amount of plate current even when the plate voltage is only at carrier level.

OOOOOOH! I think Im gettin it. So if you were to drive the finals only to the point of saturation and no more, then when you raise the plate voltage, it then pulls the finals out of saturation, but the carrier will not go up (due to underdriving)...Unless there is ALOT of drive, then it keeps them saturated upon the 100% modulation, raising the peak envelope power to 4x the original carrier power...ya?

Essentially, yes. You're getting it now.

...and you need even more snot to break the 100% positive barrier. If you want to hit 120% positive peaks like the broadcasters do, you need to hit the grid even harder than that.

You need enough drive to keep the tube saturated across the entire dynamic range of the modulated B+. It's actually more insidious than that, but that sums it up.

So, given all that: I again find it unlikely that GE would have designed or shipped a BC transmitter that couldn't do all that by design. The existing driver should be sufficient for 100% modulation, maybe higher, provided the tubes are good and the other component values mostly within tolerance.

You may wind up swapping the driver out anyway, because what you've got ain't in every junkpile anymore.

So you won't really know until you fire it up, but you'll definately want to put the mod iron and Heising reactor on some insulation first. Don't worry about bolting them to anything; they're freakin' huge, they're not going anywhere. Jeff has described an XT-1(A) with a mod iron short to the case, and Bill's had the Heising reactor short to the case. That's enough of a trend to warrant preventative measures. You definately do not want to blow that transformer, it's a beauty.

Would the original GL-8005 driver tube be enough to keep the finals saturated for 100% modulation? It looks smaller than a 4-400...

8005's have a plate dissipation of 75 watts CCS.

4-400a's have plate dissipation of 400 watts.

They are not even in the same league.

I would think that 8005's are under kill and 4-400a's are overkill.

Perhaps something in the middle like a 813 (or two).

But you can always use a 4-400a under it's full output...

I suppose it depends on what you have.

4-400a's require a fan -- that's another issue.

Another option, if you want to stick with a triode, might be an 810.

I think it's best to run these ancient tubes well below maximum capacity. Plus 8005s are expensive and hard to find.

Halo

Looking at the schematic diagram.it used a coil and a variable cap in parallel from the plate(after the blocking output Cap) to the grid of the 833 ,

in my opinion it's parallel resonant circuit with the coil  in parallel , with the variable C + the  internal C between plate and grid.(in the tube)

They are resonated to the Frequency of the transmitter ,so there 's a high impedance ( R) between the plate and grid at the operating Frequency ,So it block the  RF feed trough from the grid to the plate of the tube ,via the internal C (C between Grid an Plate in the tube).

So it is used as a neutralizing circuit.

The + and - using this circuit:

It can only be used on a single frequency.

The good point you can used it  to neutralize transmitter that has no neutralizing circuit ,without changing the existing  transmitter circuit,just put the circuit between grid and plate ,and resonate it to the frequency of your transmitter.

Regards

Gito

PS, Sorry since THE L(coil) IS VARIABLE  it can also be resonated with  different Frequencies

That's just it. You can really forget all about the question of a single 8005 being enough to drive a pair of 833s, the real question is "can you access enough 8005s to justify continuing to use one as a driver".

Something along the lines of a 4-125 or 4-250 would be a less expensive, more common, and far more scroteful alternative (a 4-400 just strikes me like too much gunpowder for the bore). Yeah, it's a tetrode; but you could always triode-connect it or feed the screen through a dropping resistor off the B+. The voltages provided to the 8005 are probably in its ballpark anyway.

If you've got a set of 8005s, you're all set. If you don't, you might want to consider changing out the driver just for the sake of your wallet.

You're going to be tearing into this thing for a while, no matter what; so it's probably a good idea to plan on it. You'll thank yourself when a driver goes filaments-up, and you look at the price of 8005s to see what it would have cost!

Halo


I just look at  the TT4 RCA hand book and found this one
using 8005 as a driver we used the data of C.W operation of the tube.(as a driver It's not modulated,so the rating is higher than class telephony)

     GL 8005 
in natural cooling class C.W in C.C.S operation .
used 1250 dc plate v
grid bias -115 dc volt
Power output 170 watt

     833A
in natural cooling class telephony (it was modulated)
used 2500 dc plate voltage
needs 30 watt driving power
output 635 watt
two 833A gives 1270 watt output

assuming the loses in the grid circuit is 100%  so for two 833 you need 120 watt driving power.
The 8005 can deliver 170 watt output

And I think the transmitter is design to give 1000 watt output.
smaller output means smaller driving power.

maybe I'm wrong .

Regards.

Gito

P.S yes the 8005 is rare to find and expensive,
you can use an 805 triode ,without much modifiying your transmitter.it has the same filament /heater voltage(10v, 3.2 a),the 805-bias(-105) is also close to 8005 - bias(-115)
output power with 1250 v plate voltage is 170 watt,with 1500 v plate voltage is 215 watt

the difference is the socket connection, the control grid of 8005 is connected
to pin3, the 805 connection is pin 2.

Thanks, Gito. I was looking at the 8005's Class-C AM specs, not CW specs. I stand corrected.

Given that, if all the tubes are good and surrounding components are within tolerance, you'll likely have enough drive from that 8005 to get a little better than 100% positive modulation with reasonable headroom.

Still, we all agree that the 8005 should be changed out unless you have a stock of them on hand. The 805 is a very good idea too, you'll nearly double your available drive. Problem solved. An 805 would just be loafing along in that role, they're easier to come by, and less expensive.

Thank you guys for the help on this thing. I guess i'll just have to get it running then determine if more drive will be needed. I have not picked it up yet. Anything I should look for before I load it on the trailer? I will be checking:

1. Mod iron short to case. But would like to know what the winding resistances are or should be.

2. Do the 833's come with it? Dont know. But they are still available I noticed.

3. Mod reactor testing? Check for short to case again.

4. HV transformer resistance check.

I guess everything else can be either fixed, replaced, and/or updated.

Well, checking for short to the case is simple. With everything unhooked, you should read an open circuit between the case and each of the contact lugs with an ohmmeter. If any of them show less-than-infinite resistance to the case, you've got a problem.

You don't necessarily need to know the correct resistances of the HV transformer, it's the ratio of resistances that matters.

One thing you could try is backfeeding the HV xfmr secondary with 120 VAC and measure the voltage at the primary to ensure the voltage ratio is what it should be. If it's 2000V iron, you'd probably see something like 5-6 VAC on the primary with 120 VAC backfed. If you don't draw any current, it should handle that okay.

To be honest, I wouldn't worry too much about the PS iron, though. That will probably be okay.

You'll have to pull all the iron to move the transmitter, anyway; so once it's out you can check the mod iron and Heising reactor for shorts to the case. Don't plan on re-installing those two pieces without insulating them, though, even if they aren't shorted.

I know I sound like a broken record, here, but you really will save yourself a metric sh*tload of maintenance headaches if you do. If the cases of the mod iron and Heising reactor are electrically floating there's less likelihood of that short ever happening in the first place. If turns short to a grounded case, the failure can become cancerous in some transmitters. Little bits of your power supply might start dropping off one by one after that issue is fixed.

The history I'm hearing suggests GE was cracking the whip on the iron a bit too hard not to insulate it. I wonder if they did so with later revisions.

What would be the best way to key this thing? I was thinking of using the Heathkit SB221 method. Leave the high voltage on the plates and just apply a positive bias on the filiment centertap point while in standby. Then apply a lower bias to (turn on) the tube when in transmit mode. I have heard of guys putting in a relay at the HV transformer primary and keying it that way but wouldnt there be some delay there on the tubes due to the caps charging up? Just doesnt seem like that would be the way.

I also read the DX100 afterburner article again...thanks for posting that. I will be writing of my experience in this restoration project as well.

The delay wouldn't be all that bad, and keyed HV is probably the way the unit is built already. If the keyup inrush is excessive, you might need a step-start of say 200 milliseconds or so, but the transmitter may already be equipped that way.

I'm not a big fan of the SB221 method. Too many potential failure points are added that can destroy your 833s.

One temptation you absolutely want to avoid is cathode-keying (using a relay at the filament iron center-tap). What happens with that system is that when you unkey, the cathode is set afloat, so the cathode voltage soars almost to anode potential, and the tube will commence to arc, zorch, and bang.

Personally, I'd stick with the keyed HV. Fewest question marks for safety, both yours and the rig's.

Ive been reading about the three diode negative peak limiting circuit. Would this be a good idea to prevent any overmodulation and to better protect the mod transformer? Or does it have no effect because no plate voltage goes thru this mod transformer in the first place...

That may not be a bad idea given the XT-1's appearant tendency to sock some heavy voltages to the iron.

I'm not as familiar with the three-diode limiting scheme as I really should be, but I've heard plenty of rigs that employ it and they sound just fine. By its very nature, it will cause some distortion on heavy peaks, but that's the price you pay.

On the other hand, even without the three-diode limiter, if your xfmr and reactor are insulated from chassis ground, and the arc gaps on the primary (never the secondary!) are set correctly, you shouldn't blow the iron, provided the iron isn't already damaged somehow. That's where a monitor scope is your friend. You'll be able to see when you're hitting 100% negative, and whether your positive peaks start knuckling under at that point. If they don't, you're probably safe.

The big difference a lack of DC on the mod xfmr makes is that the core is that much further away from saturation. That buys you a ton of headroom, and allows you to use a cross-laminated core for more inductance and better low-end response. What kills most mod xfmrs is sharp transients, which is what the three-diode limiter addresses. That can happen with or without DC on the secondary.

So it's your call. If you're going to be directly feeding a mike into the unit, it wouldn't be a bad idea. If you're going to be feeding processed audio to it, it may not be necessary.

Either way, it's an option.

I am still scanning the manual....having a few problems.
I want to make sure the scans are perfect and readable, not like some of the stuff u see on BAMA.

Bill

PS.....this is not a slam on BAMA.....they post what they get.

First time I recall coming across a note regarding having the spark gap on the mod iron primary.  I recall seeing them common on the secondary windings of various plate-modulated rigs.  Does a spark gap (once firing) on the secondary do nasty reflective things back to the primary of the modulator?  I'm too fuzzy after work today to work this one through the logic  :)

Mark K9TR

100 percent correct. 

And on the monitor scope, it makes a neat little valley, RIGHT on the tip of the peak of the envelope. 

Took me long time figguh dissah problem out fo y00.

I increased the drive to the final stage ,and the distortion, valley at the crest, etc. all went away.

Until this explanation, I didn't understand fully why, only that you HAD to saturate the device, or nearly so, to maintain a solid signal...  Tnx!

--Shane

Bill,

In that case, scan the document into PNG format, not JPG.

JPG does a wonderful job with photographs, because that's what it's designed for. It does a lousy job with schematics and similar diagrams. That's why so many BAMA manuals have unreadable schematics, they were scanned in JPG.

PNG does a much better job with text and diagrams, and its compression algorithm works better with those images than JPG. You'll get a much smaller file with much crisper text and diagrams.

PNG is as widely supported as JPG and GIF, so nobody would get left in the dark, either.

Might have been more helpful if I told you that last week, but c'est la vie.

I think this is "coil" or "shunt" neutralization.  It is VERY frequency dependent. A much better approach would be to build a balanced input circuit and use a readily available neutralizing cap.  I've included a scan of what I think is the method of neutralization used in this xmtr.  Again, it is best used on single freq xmtrs.  Otherwise every time you change freq, you would have to adjust the inductance which would, to me, be a PITA.

Al

Scan was taken from an 1945 ARRL Handbook.

Ah, but are you sure those arc gaps were actually on the secondaries; or were you assuming they were? For years, I assumed they were, and I know quite a few seasoned hams who did, too. Nobody had told us otherwise.

Take a look at the XT-1 schematic at the very beginning of this thread. GE put the arc gaps on the primary of the UTC.

Consider what happens when an arc gap fires: you have a sudden conductive path of ionized air shorting the transformer out. There's a tremendous current surge when that happens. On the primary, that's no biggie, because you only need to bleed off the excess potential coming from the tubes, and they can handle that brief current spike. Once the voice peak passes, the voltage shoots back down again and the arc gap stops firing.

If the arc gap is on the secondary, then you're in for a more interesting ride. The arc gap fires, but now instead of just bleeding off the potential from the tubes, you're now bleeding off all the energy stored in the core of the transformer by way of the secondary. The secondary winding will not handle huge current spikes the way the tubes will, and like you said, that will also reflect to the primary.

Because the voltage is that much higher, and there are that many more joules waiting behind the arc point, the arc is more likely to be sustained, and it's up to the transformer to provide that energy.

This can lead to any number of failures, depending on the transformer design. The secondary could open, or short several turns. The sudden drop in secondary voltage when the arc fires could cause a cascasing effect and draw another arc from the primary to the secondary (as the primary will still have max potential on it when the gap fires on the secondary). The way the iron dies depends on the iron itself, but it's a pretty abusive situation no matter what.

That's actually a very simplistic description and probably not totally accurate; but the moral is that an arc gap on the secondary can become a crowbar across the secondary and blow the transformer any number of ways.

Putting the arc gaps on the primary stops voltage spikes from ever making it to the transformer in the first place. You want the arc gaps to draw voltage spikes away from the transformer, not through it.

That's it in a nutshell, but I'm sure there are others who can describe it far better than that.

I've always seen arc gaps in pairs, thus on the primary.

I have the exception to that rule. I only have one gap on the Junkyard Dawg, and it's across the primary. Nothing between the ends and center.

Of course, that VM-4 is cross-laminated and wired as an autotransformer, so it's not like anything normal is being done in my modulator. There's just the one arc gap across the whole primary.

Then again, it is called the "Junkyard Dawg"!  ;D

When I had my 800 pound 1:1 RCA mod iron (now gone)  it had a stock set of gaps on BOTH the pri and sec.

Later on I used other mod iron. When the Tron came over he always told me to put the gaps on the PRIMARY, (two gaps) if I had only one set and one choice.  If the Tron says so, then these are facts!  ;D


T

Well, I suppose if your primary gap was aggressive enough to fire on peaks without the secondary gap firing, a secondary gap could be useful if you somehow experience some kind of back EMF when you unkey. Absent that, I think I'd feel safer sticking to just the primary.

Yep, maybe the gap setting procedure for my RCA iron required just that - setting the primary very agressive and the sec looser for backup.

Joe, I have ARRL handbooks from the late 40s to the late 60s in a book shelf here at the house.  If you need a copy. I would be happy to send you one.  Alot of the books are redundant and include the same information over and over year to year.

Clark

Thats my motto.

Picked up a GE XT-1A today. I will need to find an original mod plate current meter as you will notice its missing. Also, I would like to replace the RF output meter and the PA plate current meter for originals. The main power ON/OFF switch has failed and will need a replacement. Here are some pics of its current condition...

Some more pics...

One insulator is broke on the mod reactor. I think it can be fixed though.

Something I find interesting is their use of a pair of 845's as a cathode follower with a couple of chokes, instead of a conventional triode driver with a transformer.

I always mount my modulation transformers, modulation reactors and power supply chokes on insulation.  No need to put that unnecessary stress on the internal insulation in the transformer by grounding the case.

UTC made a LS-691 at a LS-692 transformer.  The 691 is designed to match a pair of 833A's.  It is a brute for the power level it runs. Mine weighs about 300 lbs.

I haven't had too much of a problem with UTC transformers crapping out, but from my experience they don't always quite match up to the frequency response specs claimed by the manufacturer.

Back when the company was in business, I tried to get some information from them about one of their transformers, but they never bothered to respond to my letter.

Thank you, Don! I really didn't want to be the only one who said that!  ;D

I'd recommend storing that reactor somewhere dry with a fairly stable temperature until you get the insulator fixed. Got a closet with a stack of old magazines in it? That'll do. You just want to keep moisture from settling in and reacting with metal and oxygen any further than it may have already.

Ok, I will bring the reactor into the house. Its still sitting on the trailer with a tarp on it. I was also thinking it was a good idea to isolate the transformers too. Maybe something like a 3/4" piece of plywood cut to the size of the cabinet base then set them all in place. Don, I am still concerned of the mod transformer frequency response. I have heard good things due to its size but Im hoping that it wont be a let down in the audio frequency bandwidth.

I have the mod plate current meter.
Contact me off the board or pm me.

Bill

A pic of the meters in my 1A.