Home Some Samples of Vacuum Tubes Suitable for VTTC Use The History of VTTC Staccato Development   In the early 1990's I was looking for a way to reduce the required input power of VTTCs.  Generally, VTTCs are not "efficient" in the sense that they demand a lot more input power for a given spark length compared to spark gap Tesla coils.  For VTTCs I use the formula:                       spark length (inches) = 0.5*sqrt input power (watts wallplug) The first method I tried (4/11/93) was to use a large grid leak capacitor to cause grid blocking.  This made the VTTC run in a pulsed, unsteady manner.  The method worked and the input power was greatly reduced and the spark lengths stayed about the same.  I called this the "sputter mode".  But the technique tended to destroy vacuum tubes.  This may have been because the tubes were already being over-volted, as is common in VTTC design. Next (5/1/93) I tested a method that I called "duty cycle control".  For this method I built a TTL timer circuit which turned on the VTTC for only a portion of the 60Hz AC half cycle.  I kept the coil running for about 2mS of the 8mS AC half cycle.  This method also kept the spark lengths about the same while reducing the input power, but it didn't really reduce the input power very much.  This was because most of the power was concentrated in the middle 2mS of the half cycle where I was operating the coil.  Also the sparks became quite dim.  It turns out that the weaker portions of the AC half cycle were contributing brightness but not much power to the spark.  This is actually a good thing.  So overall the duty cycle control method turned out to give rather poor results. I then went on (5/15/93) to test a different idea that I called the "staccato mode" of operation.  In this technique, I ran the VTTC for a full AC half cycle, then disabled the VTTC for a selectable number of AC half cycles.  I used the same TTL circuit to control the operation but I made some changes as needed to get the correct pulse durations.  This method worked very well.  The sparks were bright and long, and the input power was greatly reduced to a degree that depended on the ratio of on-time to off-time. At this point I was still supplying a signal to the grid circuit to disable or enable VTTC operation.  At some point Mark Graalman suggested simply using a transistor to lift the tube filament from ground.  I tried this (1/4/97) and found the operation to be more stable and the method was simpler and easier to implement.  I eventually used a triac, and then finally an SCR for better reliability instead of a transistor.  I also experimented with the use of a mercury thyratron to un-ground the tube filaments to disable the VTTC during staccato "off" times. For a change of pace, I built a mechanical asynch staccato interupter using a rotary switch (10/28/95).  This worked well and gave an interesting sound and visual display. I then tried this controller on a DC powered VTTC (11/18/95).  It worked well. After showing this staccato work on my video tapes and demonstrating the coil at a teslathon, Dave Sharpe designed (but did not build) a 556 timer based staccato controller which was based on my TTL design.  I changed the design to use two 555 timers and made some changes to get the circuit working (11/28/97).  This new controller was more resistant to RF interference.  Whereas the TTL board lost staccato control when the sparks exceeded 28", the 555 based controller worked well while producing 40" VTTC sparks. I knew that the triggering portion of the circuit needed improvement and that I should add a zero-cross triggering feature.  I took the zero-cross trigger circuit from John Tebb's circuit for a triggered synch gap for a spark gap TC, and incorporated this into my staccato board (1/24/03).  This greatly enhanced the stability of the circuit.  I mentioned this to Steve Ward at the time, and he offered to post this improved staccato controller circuit at his website because I was having some trouble with my website at the time.  I mailed Steve a hand-drawn schematic and he re-drew it neatly and posted it at his site.  It's still there click here for the schematic. In order to test the effect of various RF envelope waveforms, I built a synchronous mechanical staccato controller operating at 60PPS (1/6/01).  This worked well and showed that a rising waveform produced swordlike sparks, but a declining waveform produced fuzzy sparks. Later there was a discussion on the Pupman TC Mailing list regarding various possible improvements to the staccato circuit.  One of the things I had wanted to try but never got around to was to add the ability to produce trains of pulses within pulses.  Dan McCauley added this feature to the design, and may have made a few other changes and had some circuit boards made up.  He offered the boards for sale and has the silk screen artwork available at his website.  Using these boards is a lot more convenient than hard-wiring them as I used to do. John Freau