What is the proper method to set modulation transformer and reactor spark gaps?

[K1JJ]

I didn't see much about setting gaps in the archives, so maybe we can start a thread about it.

There are spark gaps on both my modulation transformer primary and secondary, as well as on the Heising reactor.   What is the best way to set them?  Right now they are too wide, as I get no gap arcing even with a flat topped audio overload. Time to tighten them up.

Do we want them to arc occasionally under heavy modulation or only when there is a mistake?

Can we use the HV and modulation transformer voltages as part of a calculation to arrive at an approximate gap setting?  (Like  0.125", etc.)

What is a safe procedure to test and set them without risking damage to the iron?



**  Update - Don / K4KYV  said:


"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."
 


T

[N8ETQ]

   This might get you in the ballpark.

http://wiki.4hv.org/index.php/Paschen%27s_law

  BC-610's had them prior to the "E" model, I recall seeing a procedure
in one of the TM's but can't find it now.

/Dan

[n2bc]

FWIW... From the RCA BTA-1R1 manual (3100V @ 1KW input, 4-400s x 4-400s):

HV Filter Choke (L502) = 1/16" gap
Mod Xfmr (T501) = 3/16" gap from CT to each mode anode lead

Added:  no spark gap on the Mod Reactor

Under final setup: "Adjust the spacing on T501 until the gaps occasionally flash over on 100% modulation peaks. Then increase the spacing slightly beyond this point."

There was a mod that allowed up to 125% positive but no further mention of the spark gap adjustment.

In another FWIW, my BTA-1 mod xfmr spark gap mounting plate was one crispy critter when I got it. As far as I could tell the xfmr was the original with 40 years worth of 'mileage' and no worse for wear(zaps).

73, Bill  N2BC

[WQ9E]

Johnson specifies setting the mod transformer ball gap at .050 using a feeler gauge for the Desk KW.  It runs at 2500 volts DC in the high power (1 KW input) switch setting.  For the Viking 500 the specified gap setting is .018 for the modulation transformer, it has a second gap on the HV line which also serves as the safety shorting switch and .125 is the specified gap for this one.

[K1JJ]

OK, thanks.  Looks like it's a trial and error thang to adjust the gaps until they stop arcing under normal modulation.


I've seen varying opinions on whether to use gaps on BOTH the mod xfmr primary and secondary -  and whether to incluce gaps for the Heising reactor.

I once had some large BC mod iron that included stock spark gaps on all three places.

Any ideas on why or why not to gap all three?

T

[N8ETQ]

  Spacing would vary as the "shape" of the gap elements. "Pointy"
stuff arcs sooner than Balls,   Just sayin'

/Dan

     Your right though, trial and error..  IE: experience.

[K1JJ]

Hi Dan,

Yes, round gaps will last longer and give a more controlled spark.


Today I set all four gaps.   Since the rig is in the cabinet when I test, I can't see the gaps arc. To make it easy to identify the gap that was arcing, I put a small 1/2" piece of Scotch tape on the edge of each gap. When it arcs, it is easy to later see the melted tape.  So, fire up the rig, sock some Yallos, pull the cabinet off and close the gaps of the ones with no melted tape and open the ones with melted tape.

I found that the Heising reactor gap needed t be about twice the width as the gaps across the primary/ CT. This makes sense since we are seeing 1/2 the voltage across the CT to ends of the primary.

In the end, after about eight trials, I can JUST get the gaps to arc when really hammering the audio at full power.

I know the transformer can take it cuz the gaps, when set wider, used to arc like crazy before I did spark suppression using vac relays and power resistors across the reactors.  The mod xfmr and Heising reactor are on Easy Street now.

I ran Fabio II today at full power with the cabinet on with no problems. We are almost there.

Hope to be on with it this weekend for the AM rally.

[w8khk]

I ran Fabio II today at full power with the cabinet on with no problems. We are almost there.

Hope to be on with it this weekend for the AM rally.

Tom, I was listening to you and Frank and the Tron this evening from my Marietta GA QTH.  You all were booming in on 75 meters, very fine sounding signals.  I was getting my rig ready for the rally, the 51J4 had a crapout, so I replaced it with the 75A3.  Will be starting late, because several of us will be going to the Ellijay GA hamfest for most of the day.  Hope to work you in the rally.  I do mostly 75 and 40.

[K1JJ]

Hola Rick -

Thanks for the report.  There was quite the group coming and going tonight on 75M. Hope to catch ya in the rally this weekend.

So far Fabio has been behaving. Only problem was the screen shutdown circuit needed some adjustment... was tripping out too easily.  Ready to give out some rally points Sat.   We also have a flea mkt here -   on Sunday.

T