GE XT-1A ???

[K9TR]

and the arc gaps on the primary (never the secondary!) are set correctly,

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

[KD6VXI]

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.

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

[KA1ZGC]

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.

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.

[w1vtp]

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.

[KA1ZGC]

and the arc gaps on the primary (never the secondary!) are set correctly,

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

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.

[Steve - WB3HUZ]

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

[KA1ZGC]

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"! 

[K1JJ]

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! 


T

[KA1ZGC]

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.

[K1JJ]

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

[ke7trp]

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

[N3DRB The Derb]

If the Tron says so, then these are facts!

Thats my motto.

[kc6mcw]

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...

[kc6mcw]

Some more pics...

[kc6mcw]

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

[k4kyv]

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.

[KA1ZGC]

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.

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

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.

[kc6mcw]

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.

[kc2ifr]

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

Bill

A pic of the meters in my 1A.