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Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
it shouldn't squeal. It sounds like you are running off a rectified/filtered mot, which should make the output completly silent. If you run of unfiltered you should get a slight 120hz buzz
4" sparks probably mean the phasing is right, but try reversing it to be sure... They do spark when they are backwards
4" sparks sound about right though, I get 4" off my little coil running off hlafwave rectified 2100vdc witch a single tube, so if you are running cw...
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
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Posts: 408
It arcs inside one tube now almost continuously after lowering grid leak resistance to 5.65K from 13k, but the horrible squealing is gone and the sparks look different. What happened? If the tube arcs internally, I'm guessing it is dead?
EDIT: I changed the resistor back to 13k, and one tube is dead but i changed the feedback leads and it acts a lot more like the VTTCs that I see here. It sounds a lot better too. The sparks are longer now on one tube than they were on two, but I killed a tube in the process, and it could be soo much better I don't know whether to buy another tube or to call it good with 4 inch sword-like sparks. I really dont want to waste money if it is going to arcover inside the tube again. would wrong feedback phasing kill a tube quickly? I also took off the filter capacitance so things should run cooler. I can't believe that it was working so well with the phasing off ..
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Where was it arcing? Them 811a's seem to like to arc between the grid/filament connections on the bottom of the tube (outside of the vacuum) at the metal/glass seal. I lot a tube this way because it melted the grid connection clean off, so I was left with a diode If it was arcing inside the tube then you probably got a bad tube, a good 811a will take over 4kv on the plate (I have run a voltage doubled mot no problem).
Usually a coil that is tuned and working with a single tube will work just as well (but you can turn the voltage up higher with the second tube), if you parallel in a second. I would go for it, with a secondary that big you need some large sparks to make it look right
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
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Posts: 408
What type of tubes are you using? I am using Cetron JAN811A. Would taking off a couple of turns from the grid feedback help stop some of the arcing inside the tube? feedback to plate is usually what the arcing is, right? Heres a picture of it working better:
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I have used just about all of them
The jan's were the ones that I had arc over... But they are all just about the same
Taking turns off the feedback didn't really help, could you try moving the feedback arround on the coil a little? Perhahpse a smaller grid leak resistor (I use 4k)
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
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Posts: 408
using a smaller grid leak resistor is what killed it, but that could be because the phasing was off on the feedback, i guess. Moving the feedback up made little pinkish purple "speckles" on the plate, so i moved it back down, and they all dissapeared. I also removed the filter capacitance and the diode.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
You may have said this already, but were these new tubes or used? IF you saw *any* purple/pink discharge inside the tube, then its gassy and would also easily arc over inside. I wouldnt be surprised it you bought used tubes to find them in this condition.
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
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They are new, bought from k5svc (military/gov't surplus) on ebay for 17.50 each. After one died, I had to give it an all out death, so i put it on my 6kv NST, and that would make a really cool nightlight! The whole thing glws purple I put the anode cap on the wrog direction the first time, and when i pulled it off, the metal part of the cap came off two, I glued it back on, but this was the tube that died, so i bet it broke the seal on the tube just a *little* bit when it came off.
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
Already bought another one, but if these don't work, thats an option. How important are matched pairs for TC duty? I didn't think it was too important, but i may be mistaken. Mine look like the marshall ones, but maybe i should try the taylors instead next time.
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Starting a new (first) VTTC First light!
but i changed the feedback leads and it acts a lot more like the VTTCs that I see here. It sounds a lot better too. The sparks are longer now on one tube than they were on two, but I killed a tube in the process, and it could be soo much better 
I put the anode cap on the wrog direction the first time, and when i pulled it off, the metal part of the cap came off two, I glued it back on, but this was the tube that died, so i bet it broke the seal on the tube just a *little* bit when it came off.
