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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Can a GUM81M etc. handle 7kV dc anode voltage !
With 7kVdc I wouldn't expect more than 14 kV peak on the anode(s) so the 15 kV higher capacitance units look ok.
I would be tempted to go with the higher voltage rating and use switched parallel capacitance for full range. 200 pF plus whatever additional parallel capacitance required should give a reasonable tuning range I think.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi, the peak voltage on the primary (which the capacitor is parallel to) is at most the same as the input voltage. So the 10 kV cap is actually overkill.
Registered Member #3806
Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259
Thanks for all your suggestions about the VVC ! Still hesitating between the 10kv 1000pF and the 25kV 200pF
Sulaiman wrote ...
Can a GUM81M etc. handle 7kV dc anode voltage !
When I first built my 572B VTTC, I didn't use a voltage doubler because the tube datasheet says max plate voltage is 2750V.
Then I saw a few shematics of 811A coils with voltage doubler. I tried adding one to my 572B coil (not really because I needed a higher voltage, but mainly to avoid MOT's core saturation and the huge current draw that comes with it)
This works fine, the tube perfectly stands the 7kV peak, so I guess these tubez voltage can be pushed a lot further than their standard values. Not sure for the GU81M, I have not tried with that tube yet.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Absolutely surely buy the 10 kV one... 1 nF is a hell lot of capacitance to do a lot of things with, and with "normal" VTTCs MOT-powered you have no way of overvolting it.
Registered Member #3806
Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259
Thanks Dr DC for the precise answer, much appreciated ! (I didn't see you previous answer while posting because we posted together)
btw I also got advice from a guy who is using this kind of VVC in almost all his projetcs (you might guess who I'm talking about) to get the 1000pF 10kV one
So I'll go for this one :)
Thanks again !
EDIT:
Just bought it :)
In case someone else would be interested, this guy has really cool stuff: QRO-Stuff
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Need advices before buying an expensive Variable Vacuum Capacitor
