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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The insulation on the inner end of the secondary winding is probably pretty weak and might not take the 3kV peaks it would see, I don't recommend doing that, though many people do it.
Registered Member #3806
Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259
Steve Conner wrote ...
Well, loud bangs are part of the fun for sure. But the most likely scenario is an arc inside the tube, between the plate and a grid...
Well if somthing wrong happens in the tube, no big bang, no fun More seriously, this glitch resistor is a nice idea, I will add this to my circuit ! I also like your MOT powered VTTC with the voltage quadrupler
Dr. Dark Current wrote ...
The insulation on the inner end of the secondary winding is probably pretty weak and might not take the 3kV peaks it would see, I don't recommend doing that, though many people do it.
I couldn't resist, too curious...
I took away the filament winding and the shunts, then I was able to slide the secondary in order to have the inner HV end 5mm away from the core.
Bad news here : I can see the secondary is wound on the core with little insulation, just two layers of what I think is varnish impregnated cardboard. (I was expecting some kind of plastic form a few mm thick).
But there's good news too : the secondary outer layer is very close to the core too and is not better insulated. Maybe this varnished cardboard has better dielectric properties than I thought.
As a test, after putting the shunts back in, I hooked my MOT to my variac, and I hooked the normal HV output (outer end) to the grounded MOT core through a 10R 1/4W resistor (glitch resistor ^^).
I slowly turned the variac up to 250V expecting a funny big bang, but nothing happened. Well ok the secondary was not under load, but still the inner secondary layer was at max output voltage...
Do you think I can consider this MOT reliable for intended use ? (full wave rectification for base fed HFVTTC).
I could even pour polyurethane varnish or Envirotex Lite epoxy into the heated secondary to improve its insulation a little bit, which one would be better ?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I tested a MOT for such use with 4kV AC between secondary and core, with a NST. Then I would consider it safe. The insulation on the outer end of the secondary is better of course, usually there is a plastic sheet formed into "U" shape around the winding.
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MOT full wave rectification and grounding problems
More seriously, this glitch resistor is a nice idea, I will add this to my circuit ! I also like your MOT powered VTTC with the voltage quadrupler 
