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VTTC pointers

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U4R1A

Wed Dec 29 2010, 04:01AM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

I finally have my VTTC lashed together and have it tuned enough to get some arcs. I seem to be getting between 4-5" arcs but the tubes are running pretty hot. I need to drive each each tube separately from the tickler to even them out but by the look of it where do I need to go from here to get the tubes to run cool but still produce some arcs? Here is a vid let me know if you see any thing blatantly wrong with the setup or if you have any suggestions.

ubuntupokemoninc

Wed Dec 29 2010, 04:28AM

Banned on 1/22/2011 for repeated rule violations after multiple warnings.
Registered Member #3299 Joined: Sat Oct 09 2010, 08:11PM
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with the cost of 2 811a's you could buy a gu-81m, thats supposed to be a good tube!

Arcstarter

Wed Dec 29 2010, 05:58AM

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

Or, he could just make his setup more efficient? Don't buy those cheap GU-81Ms, mine was DOA...

I never liked using two tubes, i never got longer sparks, and it is less efficient overall. Im not sure if it is right for you, but you might want to forget about the second tube altogether. My tubes where very happy with my secondary that was 1.4MHz with toplaod, but it hated my 300KHz secondary. Ask Dr Spark, there really ARE sweet spots with tubes!

I would say select an amount of turns on the primary, like say 25 turns, then calculate the capacitance for that to get the resonant frequency on the primary to match your secondary. I used one cap that was just like yours except 20kv instead of 18 for the primary, but i had much higher freq.

Higher Fres really makes things more compact, and at 1.4Mhz i got 11 inches with the tiniest bit of reddening of the plate. With 800KHz i could not get more than 6". Maybe that is a coincidence, but i'd like to believe there is something to this whole sweet spot thing.

Dr. Dark Current

Wed Dec 29 2010, 09:54AM

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

Tuning the VTTC efficiently - you want enough Q in the primary AND the coil runs the best when primary oscillates UNDER the secondary resonant freq. So you select a big enough resonant cap, start from the outer primary tap and decrease number of turns until you are satisfied with the arcs. Primary heating may be your limit, so use a thick wire. Also it seems to work better with a somewhat loose coupling (again more Q in the primary circuit).
The other thing is conduction angle of the tube. Too big angle results in inefficient operation, too small angle and the tube cannot pass enough current in the short time - again it operates inefficiently. This is tuned by the number of feedback turns and the grid resistor for grid current. However the conduction angle is also altered by the grid leak capacitor, I think the 1-2 nF people use is too small - the cap discharges a lot during the cycle, turns the tube ON too soon and again the angle is too large. The larger cap you can use the better, but from some point (especially with a lower Q primary) the circuit can stop oscillating for a moment, the anode voltage goes up because of the MOT's inductance, and your doubler diodes or transformer(s) can be damaged.

ubuntupokemoninc

Wed Dec 29 2010, 03:50PM

Banned on 1/22/2011 for repeated rule violations after multiple warnings.
Registered Member #3299 Joined: Sat Oct 09 2010, 08:11PM
Location: Bantown, USA
Posts: 220

yeah looks like 1 tube is heating more, the chinese invented the "matched pair" consept, there tubes are soo loosly related to the data sheet, the have to find 2 with some what the same resistance, with old tubes they were all virtually the same. as tekwiz (from another electronic forum) would lecture me about is "you need to lower your plate voltage!" at least while you tune

quicksilver

Wed Dec 29 2010, 05:03PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

For what it's worth I had the same situation (w/ uneven heating) & after some time of just working with the tune (alteration of primary input) I finally got an even glow; however the "swords" dropped substantially. Started very similar to yours then dropping to 2".
I'm paying damn close attention to this thread as I took mine down simply because I just tried most everything I could think of (even writing down adjustment parameters, checking them off to as to eliminate repetitive agenda).
Realistically, I am trying to find design issues way outside what I put together because I was over at a member's home and another fellow had a really NICE VTTC but i simply wasn't looking (& remembering) what was there (primary design, construction issues, numbers, etc) but I KNOW that 6"-8" "swords" are possible! I saw them.

Dr. Drone

Wed Dec 29 2010, 06:30PM

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

ubuntupokemoninc

Wed Dec 29 2010, 08:13PM

Banned on 1/22/2011 for repeated rule violations after multiple warnings.
Registered Member #3299 Joined: Sat Oct 09 2010, 08:11PM
Location: Bantown, USA
Posts: 220

you could wire the cucuit for push pull, instead of two tubes in parrelel.

Dr. Drone

Wed Dec 29 2010, 08:40PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Arcstarter

Wed Dec 29 2010, 09:28PM

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

I know that parallel tubes definately allow more average current. But, with a given plate voltage, the spark length does not grow. Well, it may grow some, but not 2x like you may expect.

I used such tiny wire on my primary with that 811a, I cannot believe how well it worked. I guess that is mostly because even though the wire is thin, the diameter is small enough that the overall resistance is low.

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