Sync, Jitter, LTR observations

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

All,

A report on recent work follows.

I completed the small 120 bps 1800 rpm sync rotary gap with 
3.25" rotor and tested it on the TT-42 TC.  Results were good.
Next I tested the rotary on the TT-32 but the results were very
poor.  The spark was very short and unsteady with a sputtering
characteristic.  Scope observations suggested the gap has too 
much firing jitter.  It seems the amount of jitter in a sync gap 
tends to increase with smaller rotors and lower rotor speeds.  
This happens mostly because the electrode closing speed is 
slower, making the gap "indecisive" about when to fire, but 
possibly also because it's harder to achieve mechanical 
precision on a small rotor.  At least that's my explanation.

For best spark output, the TT-32 needs to run in an
inefficient sync-phase mode.  This is the mode where the
cap voltage rises, then falls through zero and rises at the
opposite polarity.  This mode gives a poor power factor, 
but gives much stronger sparks from the TT-32 TC.
Normally I don't use this mode for coils.  Actually I didn't 
even realize at first that the TT-32 was running in this mode.
When I first built the TT-32, I simply adjusted the phase for 
maximum spark output.  I noticed the unusual phase setting
during recent scope observations.   It would appear that this
mode requires a rotary with very little jitter.  I say this because
the TT-32 worked fine with the original 120 bps larger sync gap 
which runs at 3600 rpm.  I suspect that this phase-mode is 
critical of jitter because of the fast rising voltage at the point of firing. 


Apparently, the small new sync rotary works well in the 
TT-42 because that coil runs in the normal phase mode, which
is more tolerant of jitter.  Although this phase mode tolerates
the jitter better, I do see more jitter on the scope, and I hear
a slight unsteadiness in the spark, compared to the operation using
the larger original TT-42 rotary gap.  Spark length and appearance
seem the same though.

I don't know why the TT-32 demands to be run in the other phase 
position.  It may have something to do with the amount of LTR-ness.
It's a 9/30 NST, with a 0.0142uF MMC cap.  I may try using 
different sized capacitors to see if this affects the need for this
atypical phase mode.  Spark output is 32" at 120 VAC input. 
The other (normal) phase position gives only about 18" sparks
with either sync rotary.  

Next I replaced the 1800rpm motor with a 3600 rpm motor, but
kept the same small 3.25" rotor.  I removed two spinning electrodes
to keep the BPS unchanged at 120.  This change made a world
of difference.  The spark reached out to it's normal length, and
most of the jitter went away.  The spark looked and sounded
quite steady.  It was a success.

I should mention that I'm not 100% sure that jitter is the
real problem.  After all, it could be some other type of
problem such as poor quenching, power arcing, or refiring.
In any case, small sync rotaries may work slightly better in
general using 3600 rpm motors than using 1800 rpm motors,
even on coils which run in the normal phase mode.  The 
difference is slight, and can only be noticed by direct
comparison.  The TT-32 coil, which demands the atypical
phase setting, demands the use of 3600 rpm for the small 
rotor.  I'm assuming the problem is not mechanical jitter in the
1800 rpm motor.  

A STSG should also eliminate jitter problems, although I wasn't
able to get the STSG to work well on the TT-32.  This may
have been because the lamp dimmer controller did not have
enough range to fire at the required late timing position.  Also
the lamp dimmer design may have too much jitter for the
TT-32 coil.  I may set up the J. Tebbs controller with the other 
TSG, and do a test.  The Tebbs controller has a wider timing range
and less jitter than the lamp dimmer controller. 

For those who have not yet seen the new small sync rotary,
it's at:    http://hometown.aol-dot-com/futuret/page6.html

Cheers,
John