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sparks from small tube TC (MOT driven)
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From: FutureT-at-aol-dot-com [SMTP:FutureT-at-aol-dot-com]
Sent: Tuesday, February 24, 1998 10:05 AM
To: tesla-at-pupman-dot-com
Subject: 20" sparks from small tube TC (MOT driven)
All,
Here's the specs for a small tube coil that gives 20" sparks when
powered by a microwave oven transformer (MOT). As was pointed
out some time ago by Dave Sharpe, the use of a doubler, or
level shifted approach, is the key to getting long sparks from a MOT
based system. This doubler is the exact same circuit that is used
in the oven, except that the diode anode is grounded. Only two
components are added to the traditional tube coil design; the diode
(300ma, 12kV minimum rated), and the capacitor (.85uF, 2.4kV WV
rated). The cap and diode are contained within any uWave oven (as
far as I know). Ratings of the diode and/or cap may be slightly
different (depending on the oven), than what I've written above.
The capacitor is connected to the HV output of the tranny, the other
lead of the cap connects to both the TC input, and the diode cathode,
anode goes to ground. Metal core of tranny is grounded. For best
results in my setup, I had to use two oven caps in parallel.
Secondary: 3" x 12" styrene form, wound with #28 formvar magnet
wire, (11 1/2" winding length)
Toroid: 1 1/2" x 5" mounted 1 5/8" above sec. winding
Discharge electrode: Sharpened nail protrudes 1" above top of toroid,
at the center of toroid.
Primary: 23 turns #12 pvc ins. wire wound near the bottom
of a 6" dia plastic form.
Grid coil: 20 turns #24 pvc ins. wire, wound onto the primary form
above the primary. Leave 1/2" space between primary and grid coil.
Tank capacitor: .00167uF 10kV mica or poly
Grid leak capacitor: .002uF, 3kV mica
RF bypass capacitor: .001uF 10kV mica
Filament bypass capacitor: .004uF, 1kV mica
Grid leak resistor: 7k ohm, 20 watt wirewound
Vacuum tube: 833A or similar (plate will run orange-red which is normal)
Transformer: MOT, 800 watt, 2400 volt rated
Diode, doubler cap: see above
Filament transformer: 10 volt, 10 amp, (depending on tube)
Misc: tube socket hookup wire, screws, bolts, base, switches,
line cord, fuse, etc.
Variac: 7amp (minimum), 120volt input, 0 - 140 volt output
Power consumption: 15A at 120V, at 20" spark output
For initial tuning, set the nail about 4" above the toroid to facilitate
spark breakout. With the nail at 1" above the toroid, spark breakout
will not occur until the variac is turned up to around 100 volts. If the
tuning is not correct, the 20" spark may be obtained, but the tube
may run too hot. The 20" spark output was measured by placing a
horizontal bare wire 20" above the coil, and letting the sparks strike
the wire. This spark length will be obtained at a variac setting of 124
volts output. Spark output is thick and bright with multiple sprouting
streamers, very nice IMO. The circuit for the tube coil is the common
and traditional one which is so well known, and most commonly used:
Tank cap and primary are wired in parallel, one side goes to the level
shifted power supply HV, the other side goes to the plate of the tube.
One side of the grid coil connects to the grid of the tube, the other
side goes to one side of the grid leak network (grid leak cap and
resistor in parallel). Other side goes to ground. Tube filament leads
connect to the filament tranny, the filament bypass cap is placed
across the filament, and one side of tube filament is grounded. The
bottom of secondary coil is grounded. The other RF bypass cap goes
from the output of the level shifted HV supply to ground. If the coil
fails to oscillate, reverse the grid coil leads. If no spark is obtained,
hold a fluorescent tube or other glow tube near the coil to see if the
coil is oscillating...it may simply be out of tune.
Have fun and be safe,
John Freau