811a vttc arcing issues / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

811a vttc arcing issues

1 2 next

Move Thread

LAN_403

plazmatron

Wed Jun 03 2009, 11:06PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

I built a dual 811A vttc, similar to that on Steve Wards site, some time ago. Occasionally there would be flashover between the grid and the heater lead (just as they exit the glass envelope), which destroyed one of the tubes on short order.
I got replacement RCA tubes, and whilst it no longer happens in ordinary use, I have recently been trying to interrupt the VTTC with an SCR, and lo and behold the flashover problem is back, with a vengance, whenever the HT is suddenly turned on. It has even flashed over between the MOT secondary, and core! (quite a long spark before the arc (1 inch))
If the HT is switched on at the same time as the heaters, this doesnt happen, the coil gently springs to life, however if the heaters are already on it does, which seems backwards, so I figured its kickback from the oscillator when it starts. I have tried tying the grid to ground with between 10M and 100k to no avail. I have also tried suppressing whatever it is by sticking filter caps between the HT and ground, and all that happened was it flashed over the caps (mind you at least if its eating filter caps, it isnt eating my tubes!)

So, anyone come across this one? And have a cure?

Thanks
Les

Dr Hankenstein

Thu Jun 04 2009, 12:01AM

Registered Member #1642 Joined: Sun Aug 17 2008, 11:36PM
Location: Black Canyon City
Posts: 96

That sounds rather odd to me. I built this coil many moons ago

and added the "steve ward" stacatto feature a couple years ago with no problem. I admit, I did blow up the coil once years ago (both tubes) when the tickler coil slid down the form and hit the primary plate coil.....1100volts grid to ground = not good! I since made a more robust "open spool" form for the primary plate and grid feedback coil. Open spool incresed performance greatly, as well. As for your problem, sounds like some leads may be too close (?) I think the problem may be one of those "so obvious I didn't see it" kind of things, maybe? Can you perhaps include some photographs?

Hank

Dr. Drone

Thu Jun 04 2009, 12:54AM

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

plazmatron

Thu Jun 04 2009, 01:04AM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

I thought about extra insulation on the tube leads, but after replacing the tubes it just decided to flash over between the MOT secondary and core. Incidentally, this flashover can be induced by allowing the coil to strike to ground, so its looking like kickback right enough.

...

Thu Jun 04 2009, 03:18AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I observed this effect once with my 811a coil, and after properly tuning the coil the problem went away. For the tube arcing over at least, it means you have (way) to much grid voltage, and you need to reduce the amount of juice you are feeding the grid.

Mads Barnkob

Thu Jun 04 2009, 07:38AM

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

Sounds like you have run it at overvoltage and might have been unlucky to get a small carbon trace on the glass in the socket, this happend to me.

What I did was to very carefully drill a hole in the troublesome tubes socket !!not the glass ofc!! and with a bend piece of hard wire I bend the legs that arced to each other abit from each other, cleaned the dirt as good as I could and it now runs fine again without arcing in the socket.

Steve Conner

Thu Jun 04 2009, 10:14AM

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

Do you have RF bypass capacitors on your plate supply, and is the setup properly grounded? (MOT core, hence plate supply negative, should be connected to mains ground, which should also be RF ground for your oscillator circuit)

The tube may also need a RF bypass cap between its two filament terminals, and obviously the filament transformer shouldn't be part of the RF circuit either. Ie, connect one of the filament terminals to RF ground straight at the tube socket. If the filament transformer has a center tap and you want to use that to return the cathode current, then you need two RF bypass caps, one from each filament terminal to RF ground right at the socket. Otherwise the RF component of the cathode current would get forced through the filament tranny's leakage inductance, and that's bad news.

Then again, you probably have too much grid drive like other posters mentioned.

Proud Mary

Thu Jun 04 2009, 12:21PM

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

So far as I can imagine the circuit you describe, it seems to me that the moment your SCR cuts the circuit, then the MOT voltage must at once rise to its full unregulated value, to which inductive kick-back must be added, probably tailing along in a train of damped waves.

So it may well be that momentarily the anode voltage is doubled or tripled.

So I would diagnose poor supply regulation. What is to be done? There are different sorts of over-voltage protection to be chosen from, and you could use two or more together if the problem if especially severe. For example, series plus shunt regulation, over-current trip, and even a fuse or two!

plazmatron

Thu Jun 04 2009, 08:37PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Thanks guys, it was indeed too much grid drive!
Cant believe I overlooked that! I just reduced the feedback coil from 22 to 14 turns, and now it will happily allow ground strikes with no flashovers to be seen in the rest of the circuit. Same goes for interrupting the HT. So I can now get cracking on a little fine tuning, and an interrupter.

Les

Proud Mary

Fri Jun 05 2009, 09:45AM

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

I will stick with my initial diagnsis - since what will be the effect of reducing feedback on Va (and hence the HT line) be?

If feedback were zero, then the HT line would look totally flat. As you increase feedback, then Va will begin to buck and bounce, which will be transmitted into the HT line depending upon the amount of by-passing etc. MOTs have very poor regulation, and if you do a simple experiment by sampling your HT line through a suitable attenuator whilst you increase feedback, then I think you will see that I am right.

By reducing the feedback, you have done no more than reduce the power, and hence reduce the chaotic ripple developing on the supply line.

As you increase feedback, the conduction angle of the valve will pass through Classes A,B, and C until it arrives at D - where part of the cycle is completely cut off - anode current will be drawn only in pulses, which is called "squegging" when it runs out of control.

Of course, it is not really possible to make remote diagnoses in this way, with any certainty, but a few simple measurements of Ia, Va, Vg and Ig will quickly reveal the cause of your problems.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob