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811A vttc problems.

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Arcstarter

Mon Jul 21 2008, 04:54AM

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

Well now my 811a vttc doesn't wanna work. It will run on 600 volts and it worked fine, so i figured i would put the full 120 volts and it would work good. Well, it had the same spark length as with 600 volts and the plate became red very fast. I was using all of the same components i was last time it was running. I figure it got so hoot because the tank capacitor was not on there. I couldn't put the tank capacitor on because the spark will stop completely. I hooked a wire(don't worry, well insulated) to the tank capacitor and connected it to the primary to see if it was a direct short, which it wasn't. Just a very small spark, but it stopped the output. So idk what is going on here. I am about ready to get it running finally and box it up nicely, and maybe even get a socket and cap for it.

The specs:
Tank cap: 1nf 20kv capacitor, also tried with a homemade 1nf cap that should withsatnd up to 28kv.

Primary: 28 turns.
Tickler: i can't remember, i think it is 20 something though.
Feedback capacitor: 1.8nf and 32kv ceramic cap(4 series 6.8nf 8kv)
Feedback resistor: two 16kohm resistors and 22kohm resistor all in parallel.
Steve wards schematic

I tried to reverse phasing of the tickler and primary with no success.

Proud Mary

Mon Jul 21 2008, 10:36AM

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

I'd start by checking that the anode current , grid voltage and grid current are within the operating areas specified in the valve data sheet for operation as a Class C oscillator.

I would also check that the valve is not suffering from parasitic oscillation at a frequency other than the one desired.

Steve Conner

Mon Jul 21 2008, 12:11PM

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

I'd be surprised if any VTTC builder here knew how to measure grid current, let alone interpret a tube data sheet to figure out what the grid current should be. I'd love to see someone prove me wrong one day.

Mads Barnkob

Mon Jul 21 2008, 03:41PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

In the 811A datasheet grid current is mentioned to be minimum 25mA and maximum 85mA at 200Vdc on the plate and 50Vdc on the grid.

If that was what you were looking for.

Dr. Drone

Mon Jul 21 2008, 04:22PM

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

Arcstarter

Mon Jul 21 2008, 07:18PM

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

What i really wanna know is why the tank cap wont work with it. I have verified it is not a short because when i connect it with the vttc it is a small spark but the spark still stops.

Proud Mary

Mon Jul 21 2008, 07:46PM

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

Arcstarter wrote ...

What i really wanna know is why the tank cap wont work with it. I have verified it is not a short because when i connect it with the vttc it is a small spark but the spark still stops.

Clearly, because the conditions for oscillation are no longer met i.e. that the positive feedback is not sufficient to maintain oscillation at the (presumably) much lower frequency and Q of the tuned circuit.

Dr Conner has mischeviously pointed out that some here may have jumped into the valve technology without a basic grounding in thermionic devices, but there are (old!) books and papers galore of calculations on Class C valve oscillators which enable the conditions of oscillation of a given circuit to be worked out in advance with a fair degree of accuracy, so that the right components are selected for the best sort of oscillator to undertake the job in hand.

A sickly valve oscillator, as yours sounds to be, which is only oscillating rather feebly, will often stop oscillating altogether when connected to a load, and perhaps this is your problem, but without knowing Va, Ia, Vg, and Ig it is just a guess.

And don't forget that an unscreened Class C power oscillator will radiate interference on all the harmonics between here and next week!

Steve Conner

Mon Jul 21 2008, 09:01PM

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

Indeed. The datasheet grid current isn't necessarily the grid current for your setup, though it may be a good starting point. (I'm pretty sure Arcstarter doesn't have 200V on his plate: more like 2000V. He won't be the first 4hv member I've seen vaporise an 811A completely with a MOT.)

And having a grid current meter doesn't mean you can do anything useful with it. (Dr. Spark, what circuit did you use to measure the grid current, and did you actually use that meter to set up your tickler coil and grid leak resistor, or did you just tune for maximum smoke like everyone else?)

Anyway, back to the topic: if the sparks stay the same length as the voltage is increased, and the plate goes red, to me that says the tube isn't oscillating feebly. It's probably oscillating as hard as it can, but the spark length is being limited by the amount of current it can deliver. So it needs either more grid drive (oscillate even harder!) or a higher impedance tank circuit to let it make more use of the high supply voltage.

Since you also found that it runs best with no tank capacitor, that's another vote for the high-Z tank circuit. If you're only using one tube, you should have used twice the turns and one-quarter the tank capacitance that Steve did, anyway. Try like 40 turns on your primary coil instead of 28.

I'm on vacation just now, will be back at work on the third harmonic of last Tuesday

GeordieBoy

Mon Jul 21 2008, 10:32PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> And don't forget that an unscreened Class C power oscillator will radiate interference on all the harmonics between here and next week!

If it's oscillating feebly and unstable, it will probably radiate in the space between those harmonics rather nicely too!

Arcstarter

Mon Jul 21 2008, 11:37PM

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

I still don't understand why it won't work even though i have all of the same components. Yea, i used 2kv, and yea i know i am feeding the tube way to much. But it would at first oscillate with only a little bit red plate for two minutes. The only reason i would have to stop is cause of my secondary melting

. I am going to go tinker with it at 600 volts and see if i can figure something out.

UPDATE: Well i was just messing with it and i figured out that if anything metal such as an insulated screwdriver stopped the streamers completely.

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