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Registered Member #4107
Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53
Hi there, I'm a reasonably experienced coiler with a Svetlana 572B single and double tube builds behind me, as well as a DRSSTC, over a period of about ten years.
Hence I'm surprised to be a little 'stuck' on a GU81M build. I had reasonable results with a secondary resonant at about 500KHz, but the brush discharge wasn't that long.
I have redesigned the secondary to be self-resonant at around 1MHz, and it also has a very high Q, with a DC resistance of just 0.7R - I think the Q is 600 or so. This clearly means that tuning will be critical. But my question is quite simple: Can I use a vacuum capacitor in the tank circuit? I have one of 150pF that would theoretically be the right value for the tank circuit. It is pristine, and huge. I think it's NOS ex Soviet military. It has a fixed capacitance and is rated to 15KV.
Judging by its size it was built for peak currents in the hundreds of amps. The envelope is 9cm long and it is 7cm diameter. You could say,why not just try it? However, because of its dimensions, the mechanical construction needed isn't trivial so I'd rather check in here before trying!
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
you will not find a better capacitor than a vacuum one these are favoured for magnetic loop antennas where 100;s of rf amps circulate and multiple kivs across the capacitor
Arc breakout will strongly load the secondary tank and reduce its Q strongly to values typically below 10. Also secondary resonance frequency will drop somewhat due to the capacitance of the arc.
Registered Member #4107
Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53
Uspring wrote ...
Arc breakout will strongly load the secondary tank and reduce its Q strongly to values typically below 10.
Erm, woah! I didn't realise that. With my Svetlana 572B build, years ago, I had around 12 in plasma from a single MOT. The grid leak was poorly designed (by me) and the anode dissipation was quickly exceeded.
In light of your comment on Q, and the theoretical Q dropping from 600 to a practical 10, how can I assess the potential length of plasma breakout with a single GU81M tube and two MOTs in the power supply? This time I've done really careful calcs on the grid leak: the time constant is now based on 10nF and 2.5K in the grid leak, with an Fres for the secondary around 1MHz.
What's the best formula or rule of thumb? I was hoping for 50cm breakout with this configuration.
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Vaccuum capacitor in GU81M single tube VTTC?
