Re: Overheating MOT - dual 833A VTTC

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

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

In a message dated 5/22/03 11:45:41 PM Eastern Daylight Time, 
tesla-at-pupman-dot-com writes:

Dan,

I've seen this effect at times in my coils.  Sometimes if the variac
level is too high, and depending on the circuit parameters, the grid
voltage may momentarily rise higher than the plate voltage during a
part of the RF cycle.  The grid then behaves like
the plate in a sense, and draws too much
current.  This can destroy the grid, and will reduce the spark output.
It usually creates a very raspy sound when it does this, and the
spark will become "fuzzy" (not sword-like).  In general a raspy sound
is "not good".  This can possibly be detected better in non-staccato
mode.  The raspiness can be reduced by adjusting the grid resistance,
and the amount of grid coil feedback.  Another possibility is that the
tuning is not at it's best, although you probably tried re-tuning.  If the
tuning is not correct, the spark length will peak before full power is 
applied.
The best tuning for short sparks will be somewhat different than the
best tuning spot for long sparks.  Tuning affects the degree of
sword-like-ness too.  VTTC's can be sensitive to the
input current waveform also.  For example when I ran my coil from
a powerful plate transformer, it was difficult to get very straight sparks.
Simply by installing the MOT in it's place, the sparks became sword-like.
I think this is because the poor quality of an MOT tends to distort
the input waveform, and actually improved the spark appearance.
This may be affected by the number of MO caps in the level-shifter
also.  I obtained the best results with only two MO caps.  All the
things I mentioned may interact also, so a combination of adjustments
may be needed.  Sometimes a change in the coupling may give
some benefit also.  The fact that you're getting 28" at only 80% input
voltage also shows that your tank impedance is lower than what I
used in my coil.  This causes the coil to draw a heavier current at
a lower voltage.  This can often make the vacuum tubes get too red
for comfort.  If your's are OK in that regard, then you may be obtaining
a benefit from the somewhat larger primary and secondary that you
are using compared to my coil.  In any case this may cause some
part of the circuit to show some unexpected non-linearity, etc., and
cause the loss of the swordlike appearance, as you go over 80%
input voltage.  These are just some things to consider.  In general,
it seems to get harder to maintain the sword-like spark appearance
as the spark increases in length.

I should probably mention that there are two types of sword-like
sparks.  In one case, there are multiple sparks which are pointy and
straight.  This can be seen at a 60 PPS rate, and at a somewhat
slower rate.  At slower staccato rates however, the spark can
create just a single straight "sword-blade" without any branching.
This blade will waver and sway around some, but will maintain
it's single blade-like appearance.  This type of spark can be seen
at my website.  Photos of VTTC sparks can sometimes show
this appearance when the appearance actually cannot be seen by the
human eye.  In the case of my sword-like photos, it's what the eye
sees.  Just thought I'd mention this because it's not something I've
discussed a lot on the list.

My 36" spark VTTC was never able to give either type of swordlike
spark for some
reason.  It may have had something to do with the larger secondary
wound with many turns of thin wire.  VTTC's seem to like thicker
secondary wire with fewer turns.  In some tests in which I installed
a much shorter secondary with fewer turns of the same wire (for a
lower resistance), the sparks did become more sword-like.  This
shorter secondary was 7" x 9" tall and gave 35" sparks before a
massive flashover destroyed the secondary, and damaged the
primary.

In any case, you're getting nice results overall.

John


>This evening i got my VTTC to consistently hit the 28" mark.  I was really
>pscyched.  This was at 80% input voltage.  Any additional increase in input
>voltage caused the swordlike appearance of the streamers to decrease and
>therefore decreasing overall length.  BTW, I'm currently using (5) MO
>capacitors for a total of about 4uF.  I'll be posting pictures later this
>evening once I download them off my camera.
>
>Dan
>
>