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Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
I was just wondering- In my new VTTC (based on a single 6146) would a normal high voltage ceramic capacitor work for the tank cap, or should I look for a small doorknob (the power supply voltage is <700V) Even Poly caps would be an option- what would be most cost/space effective type?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
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
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
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.
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
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.
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
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.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
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..
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
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
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VTTC capacitors
I'll keep looking for it.
