Re: VTTC capacitors
HV Enthusiast, Mon Jul 24 2006, 06:37PM

Sure.

Re: VTTC capacitors
EDY19, Mon Jul 24 2006, 06:58PM

Was the sure to the normal ceramic disk type?

Re: VTTC capacitors
Steve Conner, Mon Jul 24 2006, 09:43PM

Maybe. I used some random 1kV disc ceramics in my tube plasma globe driver but bear in mind it was only about 30kHz and 20 watts. The caps have a harder time at higher frequencies and powers. I heard a 6146 is good for about 90w in Class-C, but knowing you guys, you'll probably end up running it off a MOT and getting about 250w for 10 seconds before it melts. >_<

If you're working at high frequencies, an air variable capacitor from an old table radio or suchlike might be a good alternative. If I was you though, I'd just try whatever caps were lying around the house

Re: VTTC capacitors
Marko, Mon Jul 24 2006, 10:05PM

ahaha steve. Well your avatars really do pwn =)

Most 'normal' ceramic caps won't work good at Mhz range even if they are high voltage and look 'beefy'. (like those legendary 'blue' 1 and 2kV, mostly 10nF caps =) )

They will heat termedously, show drastic capacitance change and may even fail due to heating.

Almost any cramic cap I had around did this at 1,4Mhz in class E.

knobs and real RF-power caps will probably work if you can scrounge something like that up.

For such low supply voltages air variable cap would be practical and nice, and if you need a fixed one use MKP capacitors. You can find 2kV rated MKP's relatively easy and they work much better than ceramics.

Re: VTTC capacitors
EDY19, Tue Jul 25 2006, 04:58AM

Ill chek out some air variables- I think i have a couple laying around, and that would be really handy for tuning- maybe one of those in parallel with a ~200pf capacitor for a slightly higher starting value. Thanks for the idea! I will actually run the tube right on the edge of the specs at about 650V, so no MOT for this coil

Edit: must have thrown it away some time back I'll keep looking for it.

Re: VTTC capacitors
Finn Hammer, Tue Jul 25 2006, 05:53PM

I would go to E-Bay and get some mica caps for real low loss in a CW application.

Cheers, Finn Hammer

Re: VTTC capacitors
EDY19, Tue Jul 25 2006, 06:31PM

Will air variable have a lot of losses? They would be reallly handy for tuning, but if the losses are too great in this small of a system, I don't think I will get breakout.

Re: VTTC capacitors
Marko, Tue Jul 25 2006, 06:37PM

I think it will be pretty efficient actually, if I go it right >-<
I never tried using such capacitors and I guess it needs to be pretty big physically to do the job tough..

Re: VTTC capacitors
EDY19, Thu Jul 27 2006, 04:03PM

Alright, found this on ebay- would it be suitable for my ~600V in a small TC? I plan on putting another capacitor in parallel to increase the range to about 400pf-653pf The only thing I'm not sure on is how high the voltage will reach in the primary circuit, and if it will "jump" the gaps in the capacitor.


Also, does anyone have any guesses on the capacitance of this unit:


The guy emailed me and said the markings on it are: 154-2-98

Re: VTTC capacitors
Vaxian, Wed Apr 11 2007, 08:51AM

I agree with Finn, go for the Mica.

I have subbed in Mica's in place of my Sprague 2nf doorknob in my 813 VTTC with virtually no difference in spark (no noticable difference).