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Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
There are 6 reasons that I wanted to build a VTTC
1. Tubes are beautiful 2. Fewer noise complaints 3. Simple circuit 4. I can build it in my apartment 5. Steve Ward 6. Dr. Spark
First I read all I could find about VTTCs to unlock the secret of these coils, but to discover that a class c VTTC would change so drastically in characteristics when streamers are flying that all calculations wouldn't be very helpful.
I choosed to use a matched pair of 811a, simply because of the prices on tubes in Denmark.
With the help from Steve's VTTC math and advice I found materials to build a 75mm x 440mm secondary with 1600 turns 0.25mm magnet wire, using a 45mm x 152mm toroid its resonant frequency is at approximately 307kHz.
I have 2x 4nF/20kV or 3x 10nF/20kV doorknob capacitors for the tank. So I can try to series or parallel them and see what is best to find a golden middleway between Q of the primary circuit and tuning the coil with the 18 to 26 winding taps I got.
The filament supply is a 6.3V / 10A transformer from an old transmitter. Plate supply is the biggest MOT I had in my collection, for about 2300V, controlled by a variac, might add a voltage doubler if its turns out the tubes can handle higher voltage.
Chokes are 17W 30ohm resistors wound with 14 turn awg14, grid leak resistor bank is an adjustable 3-10Kohm/51-170W bank, so I can still adjust duty cycle even with no staccato controller.
So thats basically the plan, here is how far in the process I am.
What is next: Final build
Process:
06. aug 2008: All materials are gathered 09. aug 2008: Staccato controller build on breadboard and tested
13. aug 2008: Build staccato controller on veroboard
30. aug 2008: Make secondary coil 31. aug 2008: Varnishing of secondary
05. sep 2008: Test setup to find the optimal primary and feedback coil Primary can be tapped at 18 to 26 windings Feedback can be tapped at 18 to 26 windings
05. sep 2008: 80mm sparks were achieved.
06. sep 2008: 160mm sparks were achieved. 2300V and 1K/1nF grid circuit 24 turn primary 3.33nF tank
Tubes are not matched even though they were bought as such, the weak tube are running with one less winding from feedback coil than the other tube.
20. september 2008: levelshifter / voltage doubler added, 260mm sparks were achieved. 3200V and 3K/1nF grid circuit 24 turn primary 3.33nF tank
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
progress isn't going as fast as I hoped for, my father have gotten his new lathe so I can start making the secondary and I finished the staccato controller, I redraw the schematics just to pass time and fit my own component values and smaller things, also did a pcb layout for veroboard. They will be available at my homepage later if there is interest in those.
staccato is supplied with 14vac in leftside, where 3rd pin is for the SCR gate. right side terminals are for the two potentiometers with a common +.
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
Excellent stuff, especially for work on perfboard. What are those green connectors that you have called? I've been thinking about getting some, due to my ability to blow transistors quite consistently. Its also why I use those PDIP sockets for all my ICs.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Firefox wrote ...
Excellent stuff, especially for work on perfboard. What are those green connectors that you have called? I've been thinking about getting some, due to my ability to blow transistors quite consistently. Its also why I use those PDIP sockets for all my ICs.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Tonight I had first light of this baby!
For now while experimenting, I am not using the staccato controller.
Getting sparks up to 80mm before the plate on one of the tubes starts getting red, as of now the test setup is a horrible mess and not even tuned.
My first problem to fix is one tube being brighter than the other, I suspect voltage drop on the first tube to be the cause, some rewiring tomorrow will show if thats the case. The dim tube is also the one getting red plate first.
Further experimentation with different tank cap values, feedback coil placement / # windings and primary windings will go on over the weekend and properly for a long while :)
Settings for 80mm sparks were
primary tapped at 22 turns primary cap 5nF
feedback tapped at 26 turns feedback resistor 3K feedback cap 2nF
I will have to work on a huge spreadsheet to plot all the different settings with spark length noted, unfortunately I do not have the means to measure plate current.
applied voltage up to 2300V over two tubes, not much difference in performance from 180 to 230 volt input to mot.
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Dual 811a VTTC, the Kaizer VTTC I
