Compact 833A VTTC
While my dual 833A rebuild was on hold, I decided I didn't really want such a large and heavy setup. So, I then decided I wanted to make a single 833A VTTC. After a long day of work, this is what I'm left with: a coil with a base size of only 10"x15"x~27"tall. The coil works surprisingly well producing over 24" sparks with the use of staccato.
Specs for this coil:
Plate supply: Large MOT, shunts removed. 2uF cap and MO diode forming a level shifted supply. Put a .002uf 10kv ceramic cap across the diode. Supplied by a 140V 7.5A variac, only run up to 120V so far.
Filament supply: rewound MOT with 12awg stranded wire. I think I used maybe 12 turns? Use the nearest number of turns to get close to 10VAC at the pins of the tube. I'm running the tube a bit hot at about 10.5-11V depending on line fluctuations... I don't recommend this if you plan on the tubes lasting forever.
Filament bypass: .22uf 600V polypropylene capacitor at the tube socket.
Tank capacitor: 2nF 40kv TDK-UHV type doorknob.
Grid leak values: 10k 50w resistor and a 2nf doorknob (same as above, though a disk cap would work as well at 2nf 10kv).
Primary: 27 turns of 12 awg on a 6 5/8 PVC form. Close wound with winding starting at 1" from the base of the form. Tapped at turn 24. I put taps on turns 20-27.
Feedback coil: 20 turns of 24awg magnet wire, .75" above the primary, close wound.
Secondary coil: 3.5"x17" winding of 26awg magnet wire.
Toroid: 6.5"x1.25". Breakout extends roughly 3.5" above the center of the toroid. Toroid mounts level with the top winding of the secondary.
Some thoughts on this coil and its performance. While the spark length is impressive, I was disappointed in the fact that I could not achieve a good spark length (only 18") at 60pps (no-staccato). The plate simply got much too red (actually, more orange). I'm not sure where the losses are, so until them, this coil is strictly staccato controlled. At ~24" I did not want to exceed 15pps for the tubes health. Also, the input voltage is 140VAC at 24". I think part of this coils good performance is due to the wiring. Everything is very tight in the tank circuit so all of the wires are very short. Also, be sure to ground ALL large metal objects near the primary coil. I mention above that I though I heard tube arcs, this is not the case! It was actually my MO caps building a charge, then arcing to the core of the MOT! I guess when this happened, the primary would channel all of its energy to that arc because the output of the coil fell off dramatically! Look at my SSTC page about capacitive coupling, and this makes perfect sense. Thoughts of buying a brand new 833C from penta labs are nagging at me. Using an 833C would probably allow 24" sparks at 60pps which would be amazing given the coils compact size. UPDATE: I now have a PAIR of brand new 833Cs, but with the recent success with the solid state coils I am putting off any big VTTC projects until the winter when I am stuck indoors, but look forward to an 833C project in the fall and winter months.
I also captured a VIDEO (3MB .AVI format) of the coil pegging the 24.5" mark at full tilt.