Vacuum Tube Tesla Coil
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A rough draft of some 3CX2500 stuff. 9/18/2001
All Pics are "Clickable"
Overview of 3cx2500 setup. 1 sec time exposure pic shows 27-28" arcs.
The voltage was about 4500VAC and I don't know plate current b/c it slams my 1A meter. Probably 1.5-2A.
Finally made a power controller. Nothing too fancy, just prevents powering up the plate before the filament and has a remote deadman switch for the High Voltage.
You can clearly see that the tube is only working during the positive half cycle
This is a screen grab of the Tek TDS210 with a 15KV probe on the plate and another 15KV probe on the grid.
Q: What should the grid V look like under ideal circumstances???
A: A reasonable person would assume that the grid V would look just like the Primary V since they are coupled. I found that the unusual shape of the grid voltage resulted from too many turns on the grid leak coil. After I backed down on the number of turns, the waveforn started to look sinusoidal.
Schematic for 3-1000Z Power Oscillator wired for AC Mode Operation
Most Recent Component Values - see txt below for more complete description
|VR1: 7.5K adj, 100W||C2: 5nF, 2500VDC, mica||L2: 3 turns of 1/2" cu strap, 0.5" dia|
|C1: 3.5 nF, 30KV "doorknob"||C(p): 1.2 nF +/- 0.2 nF||R2: Unknown, probably 50 ohm, 5W|
|C3: 9.3nF, 9600VDC, MMC||L(p):|
(Reverse Chronological Order)
All pictures are "clickable" for a larger view
You can see the death (execution)
Note the new primary with multiple taps, new toroid, and primary former that extends the length of the secondary to allow for easy tuning of the grid coil position.
First and Last Run of New system
28" arcs shortly before the hole melted in the plate
Hole in the plate
(1) increased the max input to the plate tranny from 140V to 280V. This will allow me to get up to 5 KVAC out of the plate transformer
(2) replaced the wimpy. burned out 7Kohm grid leak resistor with a 200W, 1000 ohm variable resistor.
(3) Noticed that about 750 ohms of resistance in the grid leak R seems to clamp I(grid) at around 250ma
I managed to get 24" arcs to a ground rod with the increased tube voltage!
Here is a short video of my VTTC making the 24" arcs. Warning: it is about 6MB and will take 20-30 mins to load with a 56k modem.
(1) Removed the 50 ohm resistor that was in line between the grid coil and the grid.
(2) Removed the choke on the filament transformer. Now use CT for gnd. Placed capacitors between each leg and CT.
(3) Mounted my blower to the base. Plumbed it to the tube base with 2" PVC.
(4) Made a cute lexan stand for the plate current and grid current meters.
----- Here are some stats using the 6T grid
coil placed about 3" above the primary -----
grid leak R = 16 ohms
There seems to be a certain grid leak R
where the coil performs OK. Raising R above the point dramatically
reduces arc lenght. Dropping R below this point adds considerably to
the intensity of the arc but dramatically forces I(grid) so high that I
can't continue testing without fear of destroying the tube.
Made some modifications...
The big picture: made 150% longer arcs while keeping all parameters at more reasonable values.
14" arcs, V(p) = 2400 VAC, I(p) = 700 ma, I(g) = 280 ma with new grid coil sep from primarly by 1.75"
I added turn to the primary because the system seemed to perform better with the variable capacitance in the tank circuit at a maximum. This suggests that the VTTC wants the tank to be at a lower freq. This is easily obtained by increasing either C or L and since I'm out of adjustment on C it is time to increase L. I added 6 more turns so now I have a total of 38. I can vary tank C from 1.125 nF to about 1.50 nF (measured values, noise floor on Wavetek 27XT was about 0.06nF). The coil STILL perfroms best with the tank C at the max position so I still need more turns...
Measured data with secondary removed:
turn F(res) with
Cmax F(res) with Cmin
Just for fun, I took some data with
secondary in place, 38T primary, tank C at min
ROSS' IDEA: grid coil was still connected in "run configuration" and the tube was still connected to the tank circuit for ALL measurements shown in this cell of the table. Maybe one of those skewed the above measurement???
I also measured the Q with 38T and the
primary removed and grid coil still attached in the run config
Pic of the coil with the lights on making about 14" arcs. Exposure is 1/15 of a second. This is a good representation of what you see realtime.
This is an interesting pic of the coil with the lights off. As usual, I used my Olympus C3000z digital camera. Cam was in auto mode and chose a shutter speed of 1 sec. Note the pretty halo on the ceiling from the tube.
This picture shows an interesting helical structure of plasma that looks like a corkscrew. You must view this image full size to see the effect and unfortunately the CCD in the camera still doesn't pick up quite as much light as the human eye.
reflections on the 1st test run after getting feedback from fellow tube
1/21/00 Comments in red
(1) I didn't have time to do much
optimization. I need to experiment with different numbers of turns on the
primary. Apparently tube coils like to operate on either the upper or
lower "sideband" instead of the fundamental freq. I may not be able to tune far enough with just the vacuum variable Capacitor.
(2) My RF ground is quite poor. I couldn't
drive an 8' ground rod even though I was using a 10lb sledge the ground
was muddy from several days of rain (there is an evil layer of clay at
about 3' deep). I had to cut the ground rod in half and use (2) 4'
sections placed about 6" apart and bonded together. The heater
element that I use as a load/test instrument generally draws about 6.5A
when connected to the wall. 120VAC/6.5A = 18.5 ohms. It draws about 3-3.5A
when connected to the "hot" from the breaker box and my new RF
ground. 120VAC/3A = 40 ohms. Looks like I have somewhere around 21.5 ohms
of resistance between my house ground and my RF ground using 60 cycle,
120VAC as a test voltage. My DMM gives me erratic readings when I try to
(3) The temperature of the tube's glass
envelope is concerning. Even with a moderate breeze blowing from the
variac controlled shop-vac, it got too warm to comfortably leave a finger
on it for over 1 second. I need more airflow but the scream of the shop
vac is highly objectionable in an otherwise quite environment. I'm
ordering some blowers from C&H Sales tonight.
(4) The grid leak circuit needs to be
optimized. VR1 gets much too hot and presently needs to be at its lowest
setting. Maybe I'll reduce C a bit. I need to consider metering grid
(5) I get conflicting stories from several tube aficionados on how the filament should be grounded (center tap vs. one side). I need to understand this better.
The bottom line is that I am already running 22.5 amps through the filament. My max plate current is around 800 ma. I'm not gonna hurt the filament by the additional 800 ma even if it is unbalanced. I have a bypass C and big RF choke to protect the filament transformer.
(6) I want to check the resonant freq
of the secondary with everything in place. I also want to check the
range of the primary's F(res) by moving the Jennings from one extreme to
another. Finally, I need to verify the TPI on the secondary to attempt
to resolve the large error in "Medhurst & Wheeler" predicted
Managed about 10" arcs at 2200VAC on the plate and a plate current of 600ma. Found it strange that coil performed best with VR1 at lowest setting. VR1 got quite hot! I was also suprised at how hot the tube ran. It was too warn to comfortably touch the tube's glass after a few test runs. Even the filament seems to get it pretty hot. Need to investigate required air flow for proper operation.
I'm terribly uncertain about what is "normal" in this area...
More later... I put all this up in a hurry just to get some feedback
System Overview. Notice black hose from shop vac. My blower caught fire during checkout so I had to substitute the shop vac dimmed down with a variac.
Nothing was permantly mounted to the bottom side of the upper deck since AC mode is temporary. The dangling capacitor is the plate RF bypass. Notice 1.5" PVC routing air to the tube for cooling.
1/4 sec exposure. 10" arcs! This is about how the arc looks real-time. The images are a bit blurry b/c I was holding the digicam in one hand and the deadman in the other.
1/2 sec exposure. 10" arcs.
1/2 sec exposure. 10" arcs.
12/25/00 - 1/13/00
Started working on tube coil in the Austin, TX lab.
I measured my secondary resonant freq to be about 400 Khz today
Parts are gathered. I have decided to get the coil up and running on AC and then refine it to run on DC.
all pics on left are "clickable"
3-1000Z Power Triode ( The following was copied from here: http://hereford.ampr.org/cgi-bin/tube )
Heater or Filament Voltage . 7.5 volts
Heater or Filament Current . 21.3 amperes
EIA Base . . . . . . . . . . F3
Prefered Substitutes . . . . None
Substitutes. . . . . . . . . None
| Capacitances and Design Maximum Values
Cin . . . . . . . . . . . . 17 pf
Cout . . . . . . . . . . . . 0.12 pf
Cgp . . . . . . . . . . . . 6.9 pf
Plate Dissipation . . . . . 1000 watts
Maximum Plate Voltage . . . 3000 volts
Maximum Plate Current . . . 800 ma.
Full Frequency . . . . . . . 110 MHz
Typical Operation as a Transmitting Tube
Class of Service Vp Vg Ip Ipmax Ig Drive Zl PO
GGB 3000 0 180 670 300 65 - 1360
A pdf of the original EIMAC datasheet was found at http://frank.nostalgiaair.org. I put a local copy here for convenience.
I acquired a couple of old tube driven RF Amplifiers that were designed for HAM use. I found them at a "swap meet" from a junk dealer that apparently was cleaning out the estate of an old Silent Key. I think the call listed on the racks was K1AY. One of the amplifiers contains a 3-1000z and the other contains a large ceramic tube, possibly a 3CX1500. I also obtained the high voltage power supply to drive the tubes.
There doesn't seem to be much on the web about VTTC design. I think this stems from the fact that it's hard to make a canned design for VTTCs since tubes vary so widely. Anyway, here are a few sites with VTTC information.
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