More tube coil stuff (Carl Willis)...

[cwillis-at-guilford.edu (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Dave, Other tube coilers,

I do think that the capacitors load down the MOT a lot (I have 0.4 microF
shunting it directly; when the level-shifter diode is not conducting, the L.S.
capacitor of 1.3 microF is in series with the main RF bypass of 0.4) all for a
reactance of ~2300 ohms at 60 Hz.  This is really too much since it will draw
high AC current through the MOT coils.  I am going to slim down the transformer
protection cap to 0.0033 microF (no other capacitors available, that's why)
which is too small according to Dave (0.1 to 1 I think you wrote) but has under
150 ohms at my operating frequency. I like the idea of my .4 uF after the
level-shifter because I am using the standard tuned-plate configuration and I
want to minimize RF reactance of this bypass cap. However, in priciple I don't
think I'd lose much by making it ten times smaller, so maybe I can lower that
capacitance too.

I should mention the peculiarities of my MOT.  I am only using about 4/5 the
windings in the primary in order to increase the step-up ratio.  Secondly, I
popped out the shunts (whatever power limiting those little things did anyway,
I don't really know.) In retrospect that was probably uncalled for, but that's
what I did.  These modifications have always been there and are not what is
making it so gosh darn hot right now.

Finally, a word about my spark.  If I put a nail on top of the toroid Freau
Style, I get about 15" brush discharge maximum.  If the point is under the
toroid, facing outwards, I get a small snappy brush until I bring a piece of
metal to about 20", when the brush gets real long and crackly and will
occasionally hit the metal.  If I use no point and approach a metal rod to the
toroid, a moderately thick but still purple arc and some brush as well will
rush out and snake around.  This also gets out about 20".  If there is no point
and no metal rod nearby, the toroid emits a crackling sound and thin breakout
streamers can be seen.  They are very short (only 3 or 4") but there are 10 or
15 at once.  They come in pulses that correspond with my gridleak components'
action.  I don't like to run the coil with no breakout facilities at all like
this because it occasionally causes a "ping" and "flash" in my 833A- the signs
of flashover.  I've got my 50-ohm protector resistor in there now, for what
it's worth.  It gets hot but other than that, causes no effects on the
operation.

And now I have found that I get best results with the plate coil -above- the
grid coil.  I have a flat-spiral secondary in two parts to allow some tuning by
changing the mutual inductance between the "pancakes" and the grid coil is a
similar flat spiral.  Tomorrow I am going to try a variety of magnifier-like
configurations with a spare flat spiral I have hanging around, i. e. tightly
coupling this coil to my primary, resonating it with a doorknob capacitor, and
directly coupling its hot end to the base of my secondary, stuff like that.  I
know Robert Stevens (sp?) made a "magnifier" like this that did pretty well.

Regards,

Carl

-at-
Sent by: "Tesla list" <tesla-at-pupman-dot-com>
08/30/2000 06:31 PM CST

To: tesla-at-pupman-dot-com
cc: 
bcc: 
Subject: Re: Tube coil success story (and still more puzzles)


Original poster: "David Sharpe" <sccr4us-at-erols-dot-com>

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