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Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
I've built a few SGTC's but I'm going to tread into the unknown and build a VTTC. I've found several plans for dual 811a's and I've started gathering materials. Of course being semi frugal/poor I'm trying to use alot of stuff that I have. For example I have ~50 1.6Kv/.1uf poly caps would there be an issue with my tank cap if I used them vice a doorknob. A string of 13 would yield ~20.8Kv/7.69nf which as far as voltage would not be an issue since I'm going to use a doubler on 2Kv MOT but, would that be to high of value for the capacitance? Also for the grid leak cap I would assume that if the cap valve was to large then if would take forever to bleed off through the resistor and would shorten the spark length production. So as far as the the grid leak cap should I break down and buy some thing 1-3nf/ ~7.5KV or can I make this work by dropping the resistance rating on my resistor to bleed off the cap faster? Last question for now does anyone have a good supplier for the filament transformer? All i've been able to find are 6.3v @ 1-2amps but the data sheet says it needs around 4 and most people I've seen have 8-10amp supplys. Any pointers/guidance would be great and feel free to correct me on anything if I've botched up some theory.
Registered Member #3490
Joined: Wed Dec 08 2010, 11:55PM
Location: The Granite State
Posts: 34
As far as filament transformers go,I've found they tend to be pricey.Better to go with a rewound microwave oven transformer.I did just that and its easy to do(removing the HV secondary was a pain though).If I remember correctly I just Google searched MOT rewinding.Pretty self explanatory though took something like 24 turns IIRC.
Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
Well I appreciate that bit of information and you just saved the lives of several poly caps. I wonder how Litz wire would behave on a VTTC ?? It certainly made a difference on my SGTC as far as spark output.
Registered Member #3490
Joined: Wed Dec 08 2010, 11:55PM
Location: The Granite State
Posts: 34
Yeah,the magnetron in in the micowave uses a lower voltage.Was thinking the rewind route because you stated you were frugal/poor.Just trying to save you a couple of bucks.Good luck!
Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
@ Wyatt I haven't found a good source of MOTs as of yet and it seems people around here aren't throwing them out so I'd have to pay a little either way to get the transformer. I'm in the military so as people move and have to dump their stuff for deployments that's when I get the majority of my parts.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Hi there, First, may I suggest you check out my design at
Furthermore, I have a good number of leftover pieces from that project, I should be able to hook you up with most of the pieces you are missing (filament transformers, caps, sockets, etc). Send me a PM if you are interested.
Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
Are the 811a's beefy enough to run a doubler on the MOT or should I just run the straight 2Kv's to it? Has any one done a Quad 811a before and would there be a benefit to leaving my platform open for expansion? I have some 4.5" and 6" PVC would this be adequate for the secondary and Primary forms because I already have a 4.5" with ~1000 turns of 27awg @ 22" or would that secondary perform poorly with the given power being supplied?
Registered Member #2838
Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333
I'd not use more than a single mot without a controller. As for the filament transformer a rewound mot to produce 6-7v seems to work fine. I think the tube use about 8a if I'm not mistaken. Good luck!
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First VTTC Dual 811a
