I'd like to point out that the issue of "feedback" is not trivial.
There
are many ways to obtain a self-oscillating system, but you can end up
operating in subtle different modes, particularly with double-resonant
systems like a VTTC or DRSSTC.
John Gudenas suggests using essentially, primary feedback. This
works fine
for power oscillators, it may or may not deliver power at the best
phase
angle to the secondary coil, however, does make sparks of good size.
Phillip is positioning his feedback coil to where the coupling is much
stronger with the secondary flux than the primary, this difference
seemed to
give his original VTTC quite a performance edge over any other VTTCs
(before
the HV modulated supply was developed). I suspect that the
operating mode
is more efficient in Phillips case than Johns case, in terms of
providing
power at the right phase to the secondary.
Im not claiming to fully understand these modes (yet) and how to
force the
system to operate at each of them, but i would encourage you at
least do a
AC sweep of the system in SPICE or otherwise, and look at where the
poles
and zeroes (peaks and dips) are in the response, and you will see
there are
indeed many frequencies where the system should theoretically be a
good
oscillator and treat the tube kindly, but i think some mode will
have better
performance than others. And its *not* just an issue of tuning, its
an
issue of how the feedback is derived. You can play with turn count,
grid
leak networks, and primary tuning until you've exhausted every
combination,
and you will not see the operation difference that you do by
changing the
feedback coil location.
Steve
On Thu, Jun 23, 2011 at 9:58 AM, Dr. John W. Gudenas <
comsciprof@xxxxxxxxxxxxx> wrote:
Phillip,
as you know I have been following your progress with your coil as
well as
Steve's work.
I solved the grid coil issue a few years ago and there should be some
posting in the archives.
Instead of using the secondary as the major feedback source, I put
a flat
spiral pancake coil under the primary with the inner diameter about
the size
of the primary. It is also covered by a circular piece of phenolic
so no one
can see it.
I have always used a high wattage adjustable grid leak resistor set
for the
sweet spot. This system is sound, never any flash over. I used 14 awg
insulated wire because it was easier to make the pancake.
Now, there is no reason why you can't pick up the current from a
primary
lead with a current transformer, but then you need to get that to
control a
high voltage with proper timing for the grid. I am not really sure
if this
solves problems or adds to them.
But, I guess you will find out. However, if you can, give the
pancake grid
coil a try.
John W. Gudenas, Ph.D.
Professor Emeritus of Computer Science
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