GE XT-1A ???

[kc6mcw]

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...

[kc2ifr]

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............

[Jeff W9GY]

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

[kc6mcw]

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?

[KA2QFX]

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...

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

[KA2QFX]

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. 

[KA1ZGC]

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.

[kc6mcw]

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

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

[KA1ZGC]

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!

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.

[KA2QFX]

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.

[kc6mcw]

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?

[KA1ZGC]

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.

[kc6mcw]

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

[KE6DF]

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.

[Gito]

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

[KA1ZGC]

Plus 8005s are expensive and hard to find.

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!

[Gito]

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.

[KA1ZGC]

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.

[kc6mcw]

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.

[KA1ZGC]

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.

[kc6mcw]

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.

[KA1ZGC]

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.

[kc6mcw]

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...

[KA1ZGC]

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.

[kc2ifr]

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.