Re: Beating?

Tesla List wrote:
> >From tesla-at-america-dot-comFri Aug 23 22:11:39 1996
> Date: Fri, 23 Aug 96 09:33 EDT
> From: Bob Schumann <tesla-at-america-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Beating?
> I have read of beating but do not know if this is what
> I am experiencing.First if needed, here are my coil's
> specs:
> 9kv-at-30ma neon transformer.
> secondary: 4" prepared PVC 15" wind length 808 turns #21.
> primary: 9 turns 3/16' copper tubing conical form.
> Richard Quich type cylindrical spark gap
> 4" x 14" o.d. aluminum ducting torroid
> .01uf -at- 15kvdc cap (wimpy studs; probably made to be a
> filter cap - gets hot quick so shorts runs; can't wait until my
> CP caps arrive!)

> The Richard Quick spark gap has 6 gaps (7 tubes)
> If I use only 5 gaps (6 tubes), I get not the longest
> spark but it is smooth and fluidy. When I bring a
> grounded screwdriver to the torroid I get an evenly
> illuminated arc.
> When I employ the last gap (6 gaps), I get sparks
> of considerable longer distance but the sparks look
> and sounds very raspy, not smooth and not fluidy.
> When I bring the grounded screwdriver to the
> torroid now, I get a section in the middle of the
> arc that is much brighter than the majority of the
> arc. The illumination is not evenly dispersed.
> I have tried many tunings and coupling adjustments
> with both setups but the main observation still is
> prevelant. I do not know if this is 'beating' or not.
> I also wonder about my current cap since it has
> small studs as if were designed as a filter cap and
> gets hot quick.
> Does this symptom sound like something others
> have experienced?
> Would appreciate feedback :)
> Thanks all,
> Bob Schumann


The 30 MA transformer/0.01 uF primary cap combination is close to
resonating at 60 Hz. If this is a +/- 10% cap, and its on the low side,
you're just about there. Tests to see if this is occuring include:

1. Measure the current going to primary of your neon with it short
circuited and then under actual coil operation. If the average current
drawn under operation is ever substantially higher than when short
circuited, you're seeing the effects of being near or at 60 Hz
2. With 5 gaps, if the coil begins firing at the 15-25% setting of the
variac, you are probably near resonance. This is usually accompanied by
a significant increase in 60 Hz humming sound coming from the neon. If
you are measuring primary current flow, you will see a sudden "jump"
when this point is reached. 

"Rough running" at 6 gaps may simply mean you are approaching the limits
of capacitor voltage that the 30 MA transformer can deliver, and may not
be consistently firing once each half cycle. If you are encountering
resonance effects AND you open the gaps too wide, you run a risk of
overstressing the neon. There's usually no advance warning - the neon
simply expires. If you've followed Richard's guidelines of 28 mils per
gap, and use his rule of thumb of about 2 kV/gap, you might be running
too great a gap-length for your 9 kV neon. A 12 KV neon will work better
(once you get a higher voltage/lower loss cap). 

The effects you're seeing with the grounded screwdriver, where portions
of the discharge are brighter than others, is not related to neon/tank
cap resonance. This is most likely due to heavy discharge current going
through narrower conduction regions (i.e., a smaller branch of a brush
discharge from a previous gap-firing cycle). For a given discharge
current to the screwdriver, the "thinner" portions of the spark will be
brighter than the thicker portions. However, a full explanation for why
this occurs is not yet fully understood...

Safe coilin' to ya!

-- Bert --