Re: Tube coil success story (and still more puzzles)

["David Sharpe" <sccr4us-at-erols-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Carl

Great work and good to hear you got the unit working!  Several
additional comments to your post...

1.  MOT's have a tendency to run VERY hot.  Suggest putting a
     shaded pole fan from a salvaged MO on the transformer, and on
     next rebuild, use a more beefy transformer.  A 2.4kV PT (20:1)
     rated at 0.75kW will handle 2.5X that in Tesla service, and would
     be excellent in this application..
2.  Some of the loading seen by the MOT  may be caused by the
     shunt capacitor sizes you are using.  0.4uF is a little large for the
     MOT bypass, and a 0.4uF across the LS HV diode shouldn't
     be needed.

But I can't argue with 20" spark lengths  ;^).  Way to go!

Regards

Dave Sharpe, TCBOR
Chesterfield, VA. USA

Tesla list wrote:

> Original poster: cwillis-at-guilford.edu
>
> Greetings tube coilers,
>
> Thanks to the helpful suggestions of several list members, I fearlessly ran
> my 833A coil at full power today for the first time since last year.  I was
> having problems losing the microwave oven diodes I had been using in my
> doubler, but I now have a string of 20 6-amp 1 KPIV diodes (.001 cap and 10
> meg resistor across each for balancing) that shows no signs of being
> blowable.  I introduced about 0.4 microF of capacitance across my MOT, as
> suggested by Dave Sharpe, and kept 0.4 as the main RF bypass across the
> output of the level shifter.  I kept 1.2 microF as the level-shifter
> capacitor.
>
> I also had lots of time to improve other aspects of my coil as well.  It
> has a new secondary, actually wound on dried and sealed PVC and coated
> three times with polyurethane.  I made a little stand so I can easily
> switch position of plate and grid and secondary coils and change the
> coupling between them.  And I replaced my 1900 pF "RF Parts" doorknob with
> an MMC of two strings of 8 .0082 microF, 1600 V caps from digi-key. I am
> about to add a 50 ohm, 100-watt resistor as a plate protection resistor as
> well.
>
> The end result of these improvements, as seen in today's run, are a long
> writhing discharge that can get about 20 inches long from the toroid.  The
> MMC cap gets slightly warm but not nearly as warm as the doorknob, which I
> had thought was doing a pretty good job.  I solved a few problems: first, I
> had put a little plated-steel tab at the bottom of my secondary to connect
> the ground lead to, but the steel was getting smoking hot near the primary
> coils- basically it was being induction-heated.  So I switched to a thin
> copper braid lead and solved the problem.  Another issue was RF in my
> homemade filament transformer, which caused a ruinous arc in the primary
> winding.  I made another transformer for the few days until my commercially
> made one comes.  The final problem (which I have not solved yet) is my MOT-
> it gets very hot, very fast now.  Somehow, in the combination of capacitor
> across it and new diode, it is getting too hot.  I certainly expect it to
> warm up a bit like it used to do, but maybe it resonates with the
> capacitance in its secondary and causes high current in the windings.  And
> we have 20-amp breakers in these dorms, and that (for some reason!) is just
> not enough for the tube coil every now and then.  I have never had this
> issue in the past, but I have it now.  I plan to move the coil to the
> physics building, where breakers of higher current are used.  But somehow I
> cannot believe my coil eats 2400 W when it used to eat a lot less!  Power
> factor correction required maybe!?
>
> Oh- I am going to be recieving a new 833A shortly.  My current one is fine,
> but I think it was a pull and I want to see what the real thing off the
> shelf does here.
>
> -Carl