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Re: V-27 tube coil: latest results
Hello coilers,
The following describes my work in obtaining 24" tube coil sparks
from a small 3" by 12" secondary, using a lightweight level shifted
power supply for portability. The pulsed staccato feature provides
added variety and interest to the spark output, and permits the coil
to give 24" sparks while running on a 120V, 15amp circuit.
I installed the level shifted PS, (ac doubler circuit from uWave oven),
this allowed me to use a 120V, 1kW, 2500V 32 lb xfrmer, in place of
the 240V, 6350V 94 lb xfrmer I was using. Sparks are still 24" at
2400 W input. Since the level shifter permits both polarities of the
input power to be used, the xfrmer can be smaller.
Next I tried the staccato, but it refused to work properly with the
level shifter. (it's worked well in the past though). I think too much
RF was getting into the TTL IC's. I tried some attempts to filter the
RF out, and moved the board further from the TC, but it was still NG.
Next I installed a 555 timer based staccato board based on Dave
Sharpe's circuit design, and now the staccato worked flawlessly
with the level shifted PS. (Dave Sharpe's schematic can be seen at
David Trimmell's website.) Dave's circuit has the added advantage
of providing a faster max staccato rate of 30 PPS. (I think mine was
running at 20 PPS max).
Finally, I replaced the 1kW plate xfrmer with a 2400 V MOT. I was
still able to get the 24" sparks using this 15 lb xfrmer, but I had to
turn up the variac to 17% overvoltage. To fix this, I removed the shunts
from the MOT, which then gave me 24" sparks at a normal 120 volts
input. The steady (non-staccato) sparks were a little shorter at 22",
but the sparks jumped up to 24" in staccato mode. With this MOT,
I get fewer spark streamers, esp in staccato mode. When the rate
is slowed, one long straight streamer is formed, similar to my earlier
staccato project that gave 17" sparks circa 1993.
The MOT with level shifter is a lightweight method of powering a VTTC
for a 24" spark output, so it's great for a portable coil. In a little larger
coil, it should be possible to get even longer sparks with this PS.
As the staccato rate is slowed using the potentiometer, more and
more firing pulses (positive ac half cycles) are skipped. When every
other pulse is skipped, this gives 30 PPS, when 2 pulses are skipped,
this gives 20 PPS, etc.
PPS input watts comments (sparks are all 24" max)
60 2400 steady running, multiple torchlike sparks
30 1200 staccato, multiple, but fewer steamers
20 800 staccato, multiple , but fewer
15 600 staccato, may be going to one-blade mode
12 480 staccato, gives one undulating sword-spark
10 400 staccato, gives one undulating sword-spark
etc.....spark can be slowed down to give intermittent appearance
Note: PPS rates between those listed above are not available with a
conventional timer circuit.
I applied the output of U1, pin 3, of the staccato board, directly to the
thryratron through a 1k resistor. Then I grounded the positive side of
the board supply so the thyratron would be at minus 12 volts to hold
it "off" during staccato "off" times.
Powerful tube coils tend to use a lot of power and run quite hot (the
tubes, primary, secondary, grid resistors, etc). But by running in the
staccato (pulsed) mode, the coil runs a lot cooler, and is therefore
suited for longer duration runs. Less ozone is produced also. I
think that any tube coils I might build in the future will include the
staccato feature.
Cheers,
John Freau