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need help with a tetrode screen supply

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teravolt

Sat Apr 11 2009, 11:06PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

I am looking for a screen supply for my russian 12kw GU-39B-1 tetrode as seen in my avatar. I plan to make a VTTC out of it. The type of supply will be a shunt type regulator type circuit. I have found a bunch of schematics for amplifiers some well done but none for as screen voltage of 1.5kv. I was thinking of using 2 4cx300a for the regulator circuit. do any of you have any ideas for turning a tetrode into a zener diode for the shunt

Zappy

Sun Apr 12 2009, 06:02AM

Registered Member #2054 Joined: Tue Mar 31 2009, 05:24PM
Location: Amsterdam
Posts: 11

You could use a string of 150V zeners, but they may not provide enough power. Otherwise use a ballast tube, like they did in early color TVs (PD500) to stabilize the 25 kV picture tube acceleration voltage. But do you really need a stabilized screen supply? In my own VTTC the screen has a high valued resistor to avoid screen destruction in case of loss of plate voltage (PG2(max)=7W for a PL519).

Edit: The screen dissipation is max 450 W. Those would have to be heavy zeners...

-Zappy

Steve Conner

Sun Apr 12 2009, 10:39AM

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

It all depends what you're running off.

If the plate supply is smooth DC, and you want CW output, then the screen supply should be smooth DC too. It doesn't need to be regulated: a regulated screen supply is only worthwhile for transmitter applications where low distortion and stable output is needed.

If the plate supply is halfwave rectified AC for maximum spark length, then the screen supply should be halfwave rectified AC too. Using DC on the screen with AC on the plate would burn it out.

teravolt

Sun Apr 12 2009, 05:40PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

The plate voltage would be from two transformers about 5kva at 4kv ac I would like to configure them in a dublier output like standard VTTCs. I am hopping to use Steve Ward's staccato controler on the grid insted of the filliment to controle timing with the line.

" If the plate supply is halfwave rectified AC for maximum spark length, then the screen supply should be halfwave rectified AC too. Using DC on the screen with AC on the plate would burn it out."

Steve I have talked to numerous people that have built amps and thay like stable screen supplies including Harry. Because the transmiter is a tuned stable that is that good for them? The tesla is dynamic in power and frequency output and this gets reflectid in to the plate do you think the screen will have to track the plate for tube safty? if the screen is stable maby operating the tube in a SOA would work. what do you think all

Proud Mary

Sun Apr 12 2009, 07:00PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Good Heavens, have we still not sorted out your screen grid supply, in all the time I have been away from the forum with a broken knee after a silly fall from a ladder?

Stick an 813 in there as shunt regulator, with its control grid pinned by the appropriate number of voltage stabilizer gas discharge valves. There are more elegant and less power consuming solutions, but this one will work, and is the kind of circuit you can draw on the back of a beer mat.

teravolt

Mon Apr 13 2009, 12:36AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Hi harry hope your dooing better. I have all the parts now. you are shure if the plate varies that a regulated screen is the way to go. Steve has a good point what do you think

Proud Mary

Mon Apr 13 2009, 02:25AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Well, it seems to me that with a valve of this size, that any tendency towards instability - the outbreak of parasitic oscillation for example - could easily destroy the valve in seconds.

As you know, I favour an engineered approach where all component values are calculated in accordance with data sheets to ensure that the valve works as its designers intended.

Provide stable voltages and currents in accordance with the data sheet and you are less likely to have expensive mistakes. Provide auto-shutdown relays in the screen and kathode circuits that will immediately power down if excess current is drawn. Make sure you have a thermoelectric switch monitoring the valve case temperature, and the air blower, or your seals may go.

This kind of valve must be carefully run-in for several days to ensure annealing of the seals if has not been used before - first with the heaters alone, and then with reduced anode current before full power is applied.

And so on.

Steve Conner

Mon Apr 13 2009, 08:24AM

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

Well, it's a fact that applying screen voltage without plate voltage will destroy a tube. The screen takes the current meant for the plate, and melts.

If you run off unsmoothed halfwave rectified plate voltage, as many VTTCs do, then there is no plate voltage 50% of the time. So if there is a constant screen voltage, I expect the screen will draw much more current than it's supposed to, and overheat.

Generally the screen should do the same as the plate only smaller. So if you make a 5kV microwave oven-style level shifted supply for your plate, you want a 1.5kV level shifted supply for your screen. And make sure you get them in phase with each other! If they're 180' out, nothing will happen except the screen burning out.

Parasitic oscillations have nothing to do with whether supplies are stabilized or not. The stuff about annealing the seals is all true, but if you operated the tube as the designers intended, you wouldn't have a very good VTTC!

Proud Mary

Mon Apr 13 2009, 12:30PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It never even occured to me that someone would run such a valve in self-rectifying mode, or attempt to power it from a MOT!

It looks like the kind of valve that would have spent much of its working life being pursued across the taiga by a generator truck to me, so really everything I have said, based on study of the data sheet should be set to one side, Teravolt, and you would be better asking someone with more recent experience of commissioning microwave power valves than I am able to offer.

I have seen valves of this size and larger destroy not only themselves but everything else in the same rack bin as a result of what may seem only small excursions from their design parameters, faults sometimes cascading from the failure of one seemingly unimportant part.

But in the light of what Steve has said, all this seems more or less irrelevant to your project, and I would suggest a study of the huge tetrode literature as the best way of designing the circuit yourself.

Dr. Drone

Mon Apr 13 2009, 04:48PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

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