Super Staccato MkII VTTC Controller

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

Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-uswest-dot-net>" <sccr4us-at-erols-dot-com>

Terry, John, David, Carl, Tesla List

I'm working on a new VTTC electronic controller for medium to high power
VTTC's.  It will incorporate a true "Super-Staccato" controller which will
allow
1,2 or 4 consecutive pulses with adjustable rest periods. It will also
incorporate
forced filament heating time delay before HV application is allowed and
real-time
electronic over current protection of peak plate and grid current.  It is
my hope
and intent to develop a flexible "all in one" PCB solution that can be
built and used for
VTTC systems from 0.5-10kW or more.

There are several issues with the simple circuit we posted in the past.
Carl touched on it previously that the reset of the Schmidt trigger under
certain
situations may result in the cathode modulation pulse not being synchronous
with the
AC utility wave.  After receiving a very powerful PSpice based electronics
simulation
package at work, I began testing of the earlier circuit and this new
proposed solution.
The bottom line is if the AC wave is below 1/3VCC and the reset of the
Schmidt is
low, output is off and everything is synchronized.  However, if the reset
of the Schmidt
goes high with the input trigger voltage below 1/3Vcc will result in a
"runt" SCR trigger
pulse which does not coincide with 0 crossing, but is delayed variably
during the
positive anode conduction wave of the VT.  One simple solution would be a
hardwired switch on the PRF controller timer and fixed RC time constants to
force the
reset timing to be completed while the VT is in the negative utility
alternation under all
conditions.  Another solution which is more complicated but appears to
solve this
situation under all conditions is:
1.  Pass the AC waveform through an improved Schmidt Trigger which is not reset
     controlled.
2.  This square wave but 50/60hz synchronized signal passes through a
differentiator
3.  Another 555 timer is used as a 7.48ms SCR trigger monostable that is in
fact reset
     controlled as the original circuit, the critical difference in this
case is this monostable
     is edge triggered.
4.  Output from circuit 2 passes into a edge triggered timing selected
monostable which
     delays PRF reset control for time to allow 1, 2, or 4 consecutive SCR
output trigger
     pulses.
5.  Output from 4 passes into another differentiator as 2 above.
6.  Output from 5 passes into edge  triggered PRF monostable control timer.
 Once
      timing is started SCR trigger monostable (3) is reset and held reset
until timing
     is completed.  Once output goes low, reset on 3 is allowed to go high
and monostable
     3 will output high with next edge trigger.  IF an edge trigger is
coincident with reset
     there may be some "jitter" but the effective SCR triggering will
remain synchronized
     to within 0.5 ms of AC utility 0 crossing.

Even though this circuit sounds complex, the actual "Super-Staccato"
controls only requires
2 556 timers, 1 2N3904 output transistor, and 1 4N37 optocoupler to isolate
SCR drive
from timers.  I intend to post timing diagrams, schematics, description of
operation  within
next week or two at Terry's (thank you!) graciously provided posting ground.

Since the electronics "infrastructure" has increased, consideration of
monitoring plate, grid
and filament conditions and operating external relays for HV control made
sense.  These
circuits are still very much in development but are for tube protection at
this time.

Basically operating a relay through a 741 comparator if peak plate or
control grid currents
exceed a set point value, and opening a relay if filament current loss is
seen, and guaranteeing
adequate filament heating time before HV is allowed to be turned on. It may
sound like
extra "fluff" for a small coil, but paying hard earned cash to watch a tube
arc over and crash
at $100-$1000 or more a pop versus $20-30 of electronics and relays seems
like a good
investment of time and materials to me.

Comments and input are welcomed.

Regards
Dave Sharpe, TCBOR
Chesterfield, VA. USA