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Strange base fed vttc phenomena.

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Arcstarter

Wed Jan 12 2011, 07:30PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

So, i got some GU-50 vacuum tubes, and i set up a simple base fed vttc, or 'plasma tweeter'. I set it up the same as i always set it up with my TV horizontal amplifier tubes, and it always works fine. Here is the basic schematic...

Instead of the voltage divider for the screen, though, i used an external supply.

Anyway, at first i tried that schematic, with a halfwave voltage doubler for the screen, and halfwave rectified B+, from a MOT with 40vac on the primary. The doubler was not working, but i had a steady 110v on the screen. I used capacitive coupling consisting of 3/4 turn of wire around the antenna. The resonator was 5MHz, and the RF inductor (L1) is the same as half of my resonator (IE the same diameter and wire but half the turns). My DC block cap on the grid is 2.5nf and 1600v and the pulldown (or, maybe that is considered pull up...) resistor is 27Kohm.

What happened, is it made about 1 inch sparks, that where very thin, as if it was runnning at around 5% duty. Occasionally, the tube would arc over internally, and the anode pin would arc to a grounded pin! I am obviously getting some terrible voltages somewhere, the pins are around 2cm from each other.

Then, i tried a fullwave voltage doubler, which did double it, so i had 220v on the screen (rated for 250v max). Without turning the plate supply on, i turned the screen supply on (the heater was obviously on). The screen got very red, luckily i turned it off before any damage was done. So, i put a 4.5k resistor in series, and it stopped getting red altogether. Then, i tried it all again, with plate voltage. Now, the plate pin constantly arcs to other pins. When i take the feedback away, it stops arcing, so i must be getting some resonant rise somewhere bad. So, i have feedback...

What could possibly be going on? Im not sure why the anode is seeing such high voltage, im not sure why the screen is trying to melt itself with 30v less than it is rated for, and im not sure why i got such thin sparks with the 110v on the screen. I have no cap on the HV supply for RF bypass, maybe a cap would cancel out any HF resonance the inductor or something may be causing, i will tinker with it.

Thanks for any suggestions, i am so very puzzled.

EDIT: I put a .15uf cap across the halfwave DC supply, and it now works fine :P How'd i know that might be the problem, even though i have never had that problem before?

Proud Mary

Wed Jan 12 2011, 07:49PM

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

By-pass R3 to Earth with 0.1Î¼F, and see if that helps.

With very poor supply line regulation, plus an absence of a by-pass capacitor on the screen grid, almost anything could be happening.

What is the value in mH of the anode RF choke?

Arcstarter

Wed Jan 12 2011, 08:35PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Proud Mary wrote ...

By-pass R3 to Earth with 0.1Î¼F, and see if that helps.

With very poor supply line regulation, plus an absence of a by-pass capacitor on the screen grid, almost anything could be happening.

What is the value in mH of the anode RF choke?

I have no idea what the choke's inductance is. Seeing that i have never owned an L meter, i don't know much about inductance values. I know a typical MOT is about 20H, but i don't know much else about what a given value would look like in reality, or how much assorted cores and air core changes the inductance. Kind of sad, really :P.

Either way, the B+ bypassed fixed all the problems. It makes sense, too, when i think about it... Now im making another circuit board which is 4 strings of 3 1n4007's for the rectifier with a ceramic bypass cap and a varistor, and it will have a filter cap, too, for CW, so everything should be good.

Proud Mary

Wed Jan 12 2011, 09:39PM

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

For operation at 4MHz, a value of 2.2mH would be a good place to start for the anode RF choke - a reactance of 55k.

It would also be a good thing for stability if you decouple the supply going into the RF choke.

Steve Conner

Thu Jan 13 2011, 11:51AM

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

The screen voltage rating only holds when the rated plate voltage is applied at the same time. With no plate voltage, even a very low screen voltage can overheat the screen.

Proud Mary

Thu Jan 13 2011, 01:33PM

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

Steve McConner wrote ...

The screen voltage rating only holds when the rated plate voltage is applied at the same time. With no plate voltage, even a very low screen voltage can overheat the screen.

Oh Goodness Gracious me, yes - excepting some very special cases, the anode voltage of multi-grid thermionic valves must always exceed the screen grid voltage, or the screen will get it into its head that is the real anode, and like many whose aspirations exceed their abilities, quickly burn out!

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