Re: VTTC, sputter or not?

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

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


The "sputter mode" and "staccato modes" are terms I first used about 
11 years ago to describe two different VTTC pulsed modes.  My
reason for using these modes was to reduce the average current
and let the tubes run cooler, yet maintain a long spark length.  An
added benefit was the interesting sounds and spark style that
resulted.  A few other folks have tried these modes since then.  

The sputter mode was the first method I tried, and this involved
using large values for the grid leak resistor and grid leak capacitor,
to create grid blocking.  This worked, but I lost about 1" of spark
out of a 17" spark, and the sparks seemed a little dimmer.  This
mode can also be rough on the tubes, and may cause internal
arcing if the voltage is already over the tubes rated voltage.  The
advantage of this method is that it's very simple.  I didn't spend
of lot of time working with this mode.

As an improvement, I next built a TTL based electronic timer
circuit with which I could accurately set the pulse timing and
phase it precisely with the incoming AC.  I called this method
the "staccato mode" because of the "rat-tat-tat" sound the
coil made during operation.  Since the tube is enabled before
the positive incoming AC half cycle, the oscillator turn-on is smooth
and tube damage is less likely.  

In my latest pulsed coils, I exclusively use the staccato mode
rather than the sputter mode.  Either mode is very useful for
keeping the tube plate cooler, and for reducing the current draw
from the mains.

Another mode I tried back then but abandoned was the "duty
cycle control" mode.  In this mode, I let the coil run on every
positive 60Hz half-cycle, but I shorted the on-time from
about 8mS to about 3mS.  This worked, but gave a raspy
spark, and didn't help the "efficiency" very much.

John


>
> I can't comment at all on sputter mode
> because I have never tried it. All I can say is that John Freau gets
> excellent performance using that method. He uses 833As though, which are
> very tough compared to the 811As.