Re: VTTC controller - First Light

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

Original poster: "Sundog by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sundog-at-timeship-dot-net>

Hi Ken,   Comments interspersed

At 09:21 AM 11/12/2002, you wrote:
>Original poster: "K. C. Herrick by way of Terry Fritz 
><twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>
>
>Shad (& all)-
>
>I too look fondly on tubes although it's been upwards of 50 years since
>I've designed anything with them.  But a comment or two:
>
>1.  Consider using an untuned primary and then deriving the FET/tube
>excitation from the secondary's return current, as I do with my s.s.
>coil.  That makes the coil self-tuning.  You feed the current through a
>capacitor whose reactance is small at Fr, to ground.  Take the small
>capacitor voltage, now shifted 90 degrees w/ respect to the current, and
>couple it to a pair of back-to-back diodes to ground via 1K or so.  Then
>amplify the diode voltage & drive the FET(s) with it.  Voila!...a
>feedback oscillator with the secondary's Fr as the sole
>frequency-determining element.  You need enough gain in the amplifier so
>that noise will start the oscillations going.  Or else, configure it as a
>weak internally-fed-back oscillator that will become swamped with the
>secondary-signal as soon as you turn it on.



Worth looking into :)  Twiddling knobs as it's running is tedious at best.


>2.  I'd characterize the FETs more as just switches rather than as part
>of a divider.  Just semantics...



They are acting as switches of a sort, so you're right there.


>3.  Sounds as if your FETs are turning >off< quite rapidly since the
>excursion to -300 takes only ~200 ns.  They don't turn on fast enough.  I
>suppose you have checked the gate waveforms themselves?  You could use
>N-channel FETs, of course, by merely 1:1-transformer-coupling their
>drives.



The fets are turning *on* quite rapidly.  The drop from -300v to 0v is 
taking place in about 1.6uS. Once the FET is off, it slowly climbs back up 
to -300v.
   A harder turn off is necessary to a point.  Where a FET creates a lot of 
heat while switching, a tube only heats up as it's conducting.  Also, the 
voltage on the grid of the tube that was just turned off is already back 
down to -170v or so when the opposite tube switches on, which is a high 
enough voltage on the grid to prevent tube conduction.   I haven't run 
across any N channel fets with a high negative voltage rating, so far 
everything I've found is all P channel.


>4.  If it's the case instead that it's the return to -300 that's slow,
>then they're, of course, turning >on< slowly.  In any case in a push-pull
>situation, I'd think slow-fast, or fast-slow, would be good since that
>would afford a dead-zone between 1 tube turning on & the other off & vice
>versa.  But during that dead zone, the primary is going to ring something
>fierce--if untuned & unless clamped.



I think you are seeing the fets backwards.  The -300v supply runs through a 
3kohm resistor to limit current, and continues on to ground.  The FET is 
between the resistor's output and ground.  When the fet is off, there's 
-300v on the grid, when the fet is on, it's shorting that line directly to 
ground, and the grid voltage drops to zero.  Given the ramp down to -300v, 
there should be minimal dead-time on the tubes between switching, and the 
scope didn't show much feedback into the primary.


>5.  I should think you could use a totem-pole configuration of 500 V
>FETs, driving the grid & connected between 0 and -300 V, to avoid the use
>of the series resistor.

I'm no EE, can you please elaborate on that a little?  I thought a 500v FET 
would smoke if put between a -300v line and ground.  I'm also under the 
assumption that fets are mainly current-carrying devices, as I could get 
very little voltage swing across it without a load to deliver it into.

>6.  You mention using 2 MOTs in series:  I hope they're insulated for it.
So far, I've had no problems.  The first mot's core is grounded, and the HV 
output is connected to the core of the second MOT.  So far, there's been no 
trouble [knock on wood], and my buddy has run MOTs in this way for a long 
time without failure.  The second MOT is mounted on standoffs.

>7.  Fundamentally, you'll have the same problem as I do with my s.s.
>coil:  Not enough power contained  in the first few cycles of excitation
>to generate the longest sparks.



That's an interesting problem.  With my VTTC's, I always saw the best 
output length with a higher tank voltage, vs more current.  The push-pull 
configuration I'm using will look like a ~10kV voltage swing to the 
secondary.  The RS1026's *should* try to pull around 1A plate current with 
zero voltage on the grid, plus whatever grunt the smoothing caps can push 
into it.  It's hard on the tubes, yes, but they're equipped with a graphite 
anode and should be able to withstand a fair amount of abuse, especially 
with a low duty cycle (via an interrupter.)  The current across the primary 
will be determined by the inductance of the primary and the ability of the 
tubes and DC supply to source current.  The tube should try to pull enough 
to easily fry itself, given the chance.  If I get sparks on par with my 
traditional grid-leak controlled push-pull VTTC, I'll be happy.

>8.  I don't see an advantage to tuning the primary; you'll not get any
>more through-put of power thereby.  What's the point, I wonder?



Not so much to get bigger sparks, but to make it easy to use this circuit 
as a drop-in replacement with for the traditional grid-leak network, 
offering much more flexibility in the control of the duty cycle of the 
coil.  Since the tube is normally biased to -300v or so, it's normally off 
and leaves you much more interesting routes to pursue.

Thanks for the feedback!

Shad


>Ken Herrick
>
>
>On Mon, 11 Nov 2002 12:51:35 -0700 "Tesla list" <tesla-at-pupman-dot-com>
>writes:
> > Original poster: "sundog by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> > <sundog-at-timeship-dot-net>
> >



>[bigsnip]



> >
> >
> > -----------------------------------------------------------------------
> > Shad (Sundog)
> > G-5 #1373
> > "Ever stop to think, and forget to start again?"
> > -----------------------------------------------------------------------
> >
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