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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hello, it's the same VTTC as previously- 4 parallel MOTs w/ doubler and 2x GU-81 pentodes in parallel (each with 600W max. plate dissipation). Secondary is 12.5 x 46 cm winding, cotton insulated wire (~0.5mm o.d. 0.4mm i.d.), primary is 44 turns of 2mm enameled copper wire. Resonant cap is 1nF 30kV= mmc, tapped at more turns than the plates- it works better this way. Control grids go to feedback winding through a 10k and 1.5nF grid leak circuit, screen grids go to ~800V peak supply in series with a 100W lightbulb.
The problem is that no matter what I do, I can't get over 40-50 cm sparks from this thing. There seem to be 2 modes of operation- one with screen grids unblocked, in this mode the the sparks grow until a certain voltage but when I increase the input voltage further, the sparks do not get any longer and start making a not-so-nice non-harmonic sound. With the screen grids hf blocked to ground with a 100nF cap, the sparks grow longer than in the previous case but at some point they start making a hissing/screeching noise and my overvoltage protection trips.
I have tried playing with the RC grid circuit values but it doesn't help much.
I thought increasing the Q of the primary circuit might help, but the 2mm thick primary wire is already getting hot.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
have you thought about making your primary smaller to couple more energy into the secondary? Also have you had a chance to use those russian doorknobs and do you have a better picture of your aperatus behind the tesla. What is the resonant frequency of your secondary?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi, I have thought about smaller diameter primary but can't see any advantages, I thought that could not increase the Q of the primary. But guess I don't have any other options left so I'll try it if I can find a suitable wire.
The resonant frequency is ~300 kHz but the primary oscillates at 250 kHz, it seems to work better this way.
Here are some more photos, this is the power supply
This is the wiring behind the secondary. On the psu board you can see the doubler diodes with rf block caps and the screen grid power supply. The bulb is for screen current limiting. Resonant cap 15x 15nF 2kV impulse caps in series. The two red resistors with the gray cap right next to the tubes is a "grid stopper" circuit, without it the tubes would go into a parasitic push-pull oscillation. The white caps with blue MOV's is a heater protection circuit.
Here are the caps I tried to use but they are 1nF at just 6kV so I cannot tap them at more turns than the plates. One cap is too little, two caps too much, the primary was getting stinking hot...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Sounds like you now need some screen stopper resistors too. The RF bypass caps on the screens are good, they increase the power output, but they allow the screen circuit to go into parasitic oscillations just like your grid circuit did before.
In RF amps it's not unknown for the tubes to need grid, screen and plate stoppers.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well so I added two 27 ohms resistors in series with the screens and it's much better now. For the plate stoppers, can I use the coil wound around a resistor or just a resistor (which would dissipate more heat)? Thanks again Steve.
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VTTC tuning problems
