Vaccuum capacitor in GU81M single tube VTTC?
astralhighway, Tue Sept 26 2017, 01:05PMHi 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!
Thank you
Re: Vaccuum capacitor in GU81M single tube VTTC?
johnf, Tue Sept 26 2017, 06:20PM
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
johnf, Tue Sept 26 2017, 06:20PM
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
Re: Vaccuum capacitor in GU81M single tube VTTC?
Uspring, Wed Sept 27 2017, 08:41AM
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.
Uspring, Wed Sept 27 2017, 08:41AM
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.
Re: Vaccuum capacitor in GU81M single tube VTTC?
astralhighway, Wed Sept 27 2017, 11:50AM
That's great news! Thank you, I can on with the build!
Presumably then they're not first choice for most builders on cost grounds?
astralhighway, Wed Sept 27 2017, 11:50AM
johnf wrote ...
you will not find a better capacitor than a vacuum one
you will not find a better capacitor than a vacuum one
That's great news! Thank you, I can on with the build!
Presumably then they're not first choice for most builders on cost grounds?
Re: Vaccuum capacitor in GU81M single tube VTTC?
astralhighway, Wed Sept 27 2017, 11:56AM
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.
astralhighway, Wed Sept 27 2017, 11:56AM
Uspring wrote ...
Arc breakout will strongly load the secondary tank and reduce its Q strongly to values typically below 10.
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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