Tuning a VTTC primary - Is a variable capacitor a good idea?

Cool Dude Clem, Tue Feb 10 2015, 03:30AM

Recently I built a new VTTC, I'm also a little new to the whole VTTC thing, I built a few in the past but this is the only one that's given me any good output. I'm using a doubled MOT as the HV supply, a GU-81 pentode, a 60W light bulb with 3nf capacitor across it as my grid leak, I'm not sure how many turns my secondary is (something like 300-500 turns on a paper towel tube (about 30 cm long)), my feedback is about 12 turns of single strand (telephone) wire wound onto the bottom of the secondary, and my primary is about 44 turns of regular insulated wire, wound onto a 10cm form. The trouble is that I'm a little concerned about what I'm using to tune the primary. Since I've had no luck with using a fixed capacitor and a primary with multiple taps, I decided to do something insane and get an air-gap tuning capacitor from an old junked radio, submerge it in oil and use that. It does work and I can tune the primary to where it is needed, and even running off a doubled MOT I haven't had any arcing across the capacitor plates yet, but is this wise? Or should I find the ideal capacitance and invest in some pulse-rated caps if I ever find any (here in the UK they seem impossible to find)?

I'll add some pictures to give you a better idea of the specs and stuff:

The whole thing: (BTW the primary was wound from 2 pieces of wire joined together)





As much output as I dare to tune it (I could tune it a little better to get more output, but I think that might destroy my secondary)





The tuning capacitor:





and finally the feedback:





I would include a schematic, but it's still in my head and I haven't put it down on paper yet. anyway it IS the same standard VTTC schematic we've all seen like 100 times already!