SPARK GAPS FOR YOUR MODULATION TRANSFORMER

[W2PFY]

This is actually a guy on eBay selling them but it's a good idea. The lugs can be purchased in Lowe's Or Home Depot or perhaps your electrical supply store.  The tungsten stock he uses could easily be replace with brazing rods. They could melt under extreme arching.

I'm going to be building a dooms day machine this winter and it will need spark gaps.

What do you use in the absents of sparks gaps already on the transformer?

[N2DTS]

I think only high end transformers have them, not sure why, they cant be expensive.
The RCA transformers I use have them built in, adjustable big thick things.
The cvm5 and other mod transformers did not, even Collins did not use them (on my 30K1 anyway).

I only had one mod transformer fail, in the 30K1, while doing frequency response tests at full power, when I got down to 20 hz the fuse blew. Live and learn, but I only paid $250.00 for the rig anyway.
It sounded better with the big thordarson transformer anyway.

I had my share of 32v3's, dx100, Johnson stuff, multi elmac, gonset, etc.
Most ran more voltage than normal...without any failures.

Brett

[k4kyv]

You only need them on the primary side of the mod xfmr, from the plates of the modulator tubes to the primary midtap.  Putting them on the secondary side will not protect the transformer.  If they arc over on the secondary side, the inductive kick  from the abrupt short circuit reflects back through the transformer and may cause insulation break down. The RCA broadcast transformers have them only on the primary side.  The 5500Z:5500Z 1:1 ratio transformers designed for MCW use on Liberty ships have a set on both the primary and secondary.

I don't think it matters whether the gap material is made of tungsten, copper, nickel plated or bare brass.  Even steel nails would probably do just as well.

[N3DRB The Derb]

he gets a hell of a price for his coils.

[KE6DF]

I can't see why the metal they are made out of would matter.

It's not like they would be arcing all the time hour after hour -- just an occasional spark.

Another issue is how wide do you set the gap?

[W2PFY]

test

[W2PFY]

I can't see why the metal they are made out of would matter

You are correct. I however did have a situation with my Westinghouse with an ark over that occurred on the mod reactor spark gaps. It looked like a mig welder arching away. It got so hot that the Bakelite formed a carbon path which after a fashion made it imposable to transmit. I cleaned the whole mess up and tuned down the gain and I was golden.

[WBear2GCR]

The things the rods are held by in Terry's jpegs are standard connector lugs that can be found in almost any wholesale electrical suppliers bins... they don't even have to be made like those with screws and holes through, the "other style" that is used for heavy wire consists of a similar phosphor bronze lug with a flatted end and a little assembly with a set screw in a box like thing that fits over the end of the lug and squeezes the wire - the advantage is that various size wires can be accomodated...

Tungsten wire can be had from any welding supply in the form of electrodes for TIG welders! 

The gap is based on the breakdown of air vs. distance.

I think a point will initiate breakdown at a slightly lower potential than a flat or round end... fwiw.

          _-_-bear

[N2DTS]

Yes, I have hundreds of those lugs, we throw them away at work.
You can use anything, even just wire under the terminals bent to the correct gap.

The stock setup on the RCA's uses partly threaded rods (to set the gap) with rounded tips.
You could get a fine thread bolt, the kind with the unthreaded part twards the head, and grind the head off to a rounded tip and have the same looking thing.

My guess is the big thing is setting the gap to the point where it protects the iron, but does not arc in normal use.

There are variables, humidity, dust, pointed or rounded tips, etc....

Brett

[k4kyv]

I have always set mine to just beyond the point where they arc on occasional voice peaks.  Just keep increasing the gap slightly until they no longer arc under normal operating conditions.   I'll have to measure the gap distance, but it seems to be somewhere about 1/16".  I  run 2600 volts on the modulator plates.

[Jim, W5JO]

Don, am curious to know what the ends of the gap are like?   Do they have pointy ends. flat ends or rounded.  The only type of spark gaps I have seen have pointy ends at about 1/16 inch, or big round balls that screw on the terminals of high power transmitters.

In the high power stuff they have the balls on both the primary and secondary of the modulation transformer.  You say from the CT of the primary to each side of the winding is all that is needed, correct?

[KE6DF]

" Don, am curious to know what the ends of the gap are like? "

Well, I'm not Don, but I it seems to me that, particularly with high power, an arc might erode away a sharp point after a while leaving the gap farther apart.

A round ball would spread the heat better and would be less likely to erode so the gap would stay the same longer.

A large diameter rod with a blunt, slightly rounded end, like the ones in the picture in the first post on this thread makes sense also.

At the other extreme a really sharp small diameter needle point would be burned away and the gap widened.

[Jim, W5JO]

Well, I'm not Don, but I it seems to me that, particularly with high power, an arc might erode away a sharp point after a while leaving the gap farther apart.

 

Your comment is very interesting.  The transmitters I have worked with that have the pointy ends are up to 1 KW and I have never seen them burn away.  The 10 KW ones have had the balls.  I have also seen the rounded ends like described on the RCA transformers so I guess it depends on the manufacturer.

If you draw an arc big enough and long enough to burn away part of the gaps, you are way over doing things.

[k4kyv]

Don, am curious to know what the ends of the gap are like?   Do they have pointy ends. flat ends or rounded.  The only type of spark gaps I have seen have pointy ends at about 1/16 inch, or big round balls that screw on the terminals of high power transmitters.

In the high power stuff they have the balls on both the primary and secondary of the modulation transformer.  You say from the CT of the primary to each side of the winding is all that is needed, correct?

My RCA broadcast mod xfmr has rounded ends to the gap.  I added a set of gaps to the mod xfmr in my Gates BC1-T, using an identical gap assembly removed from a crapped out 1:1 RCA Liberty Ship mod xfmr.

Another kind of gap is a "horn" gap.  You make it using a couple of pieces of stiff wire.  The wires approach each other at about a 15° angle in an upward direction, and each wire is bent so that they flare away from each other above a certain point.  If an arc does occur, it tends to extinguish itself as it travels up the wires like a Jacob's ladder.