Pulsed Tube coils, Update 1
Sent: Thursday, January 08, 1998 9:31 AM
Subject: Pulsed Tube coils, Update 1
Did a few things with staccato control of a tube TC. First, I
attached a 5kV tranny to my small 4-250A staccato (pulsed)
TC (18" spark),to see how well the control transistor handles the
higher voltage. The transistor handled the voltage fine, but at the
highest voltages, the spark began to split into two at the top, and
this shortened the spark; the one sword-like spark effect was ruined
at highest voltage and power. I don't know if this can be "corrected"
by retuning or some other adjustment.
In general, for pulsed operation, I let
the coil run for one full 60Hz positive half cycle, and keep the TC
shut off for one or more 60Hz cycles. This reduces the total power
draw of the TC, and helps keep the tubes cool. It also gives an
interesting spark appearance and sound.
Next, I re-assembled my 36" spark tube TC using (3) 833A's, but I
only turned it up to give 33" sparks, I don't want to blow the tubes.
Tubes get slightly red at this power level which happens to be
5500kVA at 5500kV. (Steady running on AC, no staccato in use at
this point). Primary is 29 turns of #12 stranded PVC ins. wire.
Sec is 6 1/2" by 22", #28 formvar. Tank cap is .006uF. Frequency
is about 110kHz. I use a corona shield on top of secondary that is
a top cover from a car air-cleaner (10" dia). Grip leak cap is .004, at
5kVAC, grid resistor is 2.8k ohm. Grid coil is two layers, total of 19
turns #18 pvc ins. solid wire on 1 1/4" by 8" dia cylindrical cardboard
form. Grid coil fits at center of inverted cone primary. Bypass cap
is .01uF at 70kV (ac?). Secondary winding is positioned level with
the inner part of the 15 degree inverted cone type primary. Inner dia.
of the primary is 8 3/8"
Next, I removed one 833A tube so I'm using 2 tubes, gives 30" spark
at 4800kVA, tubes get redder, but staccato will take care of this.
Next, removed another 833A so I'm using 1 tube, gives 23" spark
with ACCEPTABE redness on plate in steady running mode. BTW,
this is my record spark length obtained using only (1) 833A in steady
running (staccato control is not yet installed). Variac can be turned
up in short bursts and gives 26" sparks (still one tube!), but tube
gets too red. Staccato will take care of this.
Now I installed the staccato transistor and board. Interestingly,
at slow staccato rates, the spark is shortened. THERE MUST BE
A PERSISTANCE OF IONS EFFECT AT HIGHER POWER LEVELS!
I did not see this shortening using the old small (17" spark staccato
TC), at low rep-rates. Gives about 23" spark using one tube with
staccato, tube plate runs STONE COLD, transistor also runs cold.
Next I added a second tube which gives 26" sparks, tube plate is
still stone cold. I then turned up the power extra high, so sparks
wildly struck the measuring wire, but then the transistor burned out.
This transistor is connected with emitter to ground, and collector to
the tube cathodes. The transistor base is connected to a solid state
TTL staccato control timer circuit board. The transistor is rated at
only 180 volts and probably was the victim of RF or inductive kickback
that occurs when the output sparks hit a measuring wire that I suspend
above the TC to measure the spark length.
I replaced the transistor with a 600 volt, 30 amp triac, now TC seems
reliable, and spark hits 29", and strikes to the measuring wire
do not blow out anything. I use a potentiometer attached to my control
panel to vary the rep-rate of the staccato action which produces
interesting spark appearances and sounds as the control is varied.
Power draw for the 29" sparks is only about 2kW depending on the
staccato rep-rate. I can slow down the pace to about 2 sparks per
second, or increase it to 30 sparks per second.
In staccato mode, spark never had the straight sword-like look, rather
it had a twisting bent appearance, the sound was really great, the
pulses sounded mucho powerful, and bizarre. I figure that in staccato
mode, with the tube plates cold, the voltage can be turned up higher
than if the tube plates were red, since redness promotes arc-over.
Maybe 6kV or more will be acceptable. I'm using about 5.8kVAC now.
Wish I could get the one straight sword-like spark at this power level!
More updates will follow.
I'm producing a 2 hour, VHS videotape of these staccato tube TC
projects, some of my sync-gap disruptive TC work will also be
shown on the tape. This tape will be available for $18, shipping
by priority mail is included.