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Dr. Dark Current

Sat Feb 05 2011, 10:46AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I am tuning a VTTC with two Russian pentodes in parallel (GU-81). As I do not want to connect all grids together, only the control grids (g1) go to the feedback winding through a 3.3k 25W resistor in parallel to a 2.2nF 2kV cap. The screen grids (g2) go through two 60watt 230V lightbulbs in series to a ~800volt peak supply. The suppresor grids (g3) are connected to cathodes. The anode supply are halfwave doubled MOTs.

The problem is that at a certain input voltage, the sparks start making a nasty screeching/popping sound and when I further increase the input, my HV overvoltage protection circuit trips indicating that the HV supply is spiking, most likely because the oscillations are stopping.

The weird thing is that I have been using a similar circuit before just the layout of the components on the table was different, and that worked well producing some 50cm sparks, now it can't do even 15cm.

I have tried many variations to the circuit and of course tuning the primary but nothing seems to help. I don't want to connect all grids together and burn 200watts of anode power on some grid leak resistors/lightbulbs. I don't have any more ideas I could try and am pretty much lost on this.

Nah

Sat Feb 05 2011, 02:35PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

Hello!

Are you using AC on the screen grids?

Dr. Dark Current

Sat Feb 05 2011, 02:52PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hi, The voltage on the screen grids has a 400 volt(rms) AC component at the 50Hz line frequency, this comes from the 400V two-phase voltage... It oscillates between ca. -300V when the anode voltage is at zero, and +850V when the anode voltage is at peak maximum. The voltage shift is provided from an electrolytic capacitor.

Steve Conner

Sat Feb 05 2011, 03:01PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It's called squegging. A Google search will turn up lots of info, but the quick answer is to reduce drive to G1 and/or play with the time constant of the grid RC network.

Nah

Sat Feb 05 2011, 06:25PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

Ah, but we call it Moterboating. Just unwanted oscillations.

As Mr. Steve says, play with the grid leak resistor and cap. Just for giggles, when you get this going, try to use DC on the screen supply and use a precise voltage and current supply.

Good day!

Dr. Dark Current

Sat Feb 05 2011, 06:37PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Well, this is a mystery... I finally did something I had to do a long time ago - used the exact same circuit and component values as previously and it still does not work properly.

Only 2 things are different since the previous wireup:
-layout of components on the table has changed
-I replaced the tubes with two identical new ones.

I doubt that the component layout change would have such a big effect, I'm beginning to think that one of the tubes is defective or they have different characteristics than the old ones.

Nah

Sat Feb 05 2011, 06:40PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

Well, the coupling of the wires my cause some oscillations and such. Is everything shielded? I don't see the tube being bad, but perhaps it is picking up EMI. Try covering everything in tinfoil.

This my be a little overkill.

Dr. Dark Current

Sat Feb 05 2011, 07:52PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Ok I don't get this. With just 1 tube connected and double the grid resistor values it works all right, it works with either of the tubes. But when I connect both in parallel and use again half of grid resistance, the problem goes back. With only 1 tube I can turn the voltage all the way up and make longer sparks than with 2 tubes before the sparks start to make the crackling sound.

Steve Conner

Sat Feb 05 2011, 08:03PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, in that case, it could be RF parasitic oscillations. When you have two tubes wired in parallel, they can have parasitic oscillations in push-pull. The tank circuit is the stray inductance of the wiring, which is why they are very sensitive to layout.

Try adding grid stopper resistors: a few hundred ohms in series with each tube's grid connection, right at the tube socket. If that doesn't work, do the same for the screen connections and the plate connections. (The plate stopper resistor may need to be smaller, like 10 ohms, because of the high current.) The idea is to spoil the Q of the unwanted tank circuit.

Dr. Dark Current

Sat Feb 05 2011, 08:34PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

So I put two 150 ohm resistor in series with the control grids and it works well now !!! Thank you Mr. Conner...

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