Vacuum Variable Caps - How to spec voltage rating??

[WBear2GCR]

Ok, so the question is how to choose the voltage rating for a vacuum variable cap?

I'm guessing it may also depend on the position in a circuit - as in loading vs. ant tuning?

Anyhow, take a B+ of 2kV on the plate of the output tube, and assume >100% positive modulation.


If there is a good internet discourse on this, please link it up?

                           _-_-

[PD0RTT]

When you use anode modulation with an transformer the voltage rating is the double of the DC voltage, so minimal 4kV.

[N2DTS]

For plate tune, its at LEAST 2X the plate voltage for plate modulation, and its best to have at least 3X.
My pair of 813's runs 2000 volts and has a 5kv vacuum cap gang tuned with the roller inductor, and despite running loads of modulation I have never arc'd it over.
If you have modulation peaks up around 130% (not hard to do) the voltage is going to be more then 2X the plate voltage.
And I think its 4X if the DC is left on the cap.
Bill Orr handbooks have a good chart.

That said, vacuum caps do not seem to arc as easy as air caps, and I do not think they get hurt much if they do arc, no oxygen...or they do not keep arcing.

For loading caps, its 500 or 1000 volts, its 50 ohms or close....


 

[steve_qix]

Assuming there is no DC on the cap, a 5kV vacuum should work very well in your application.

[John K5PRO]

If you are serious about nitty details like an electrical engineer, vacuum variables, like RF ceramic capacitors, eventually have an RF current limit that varies with frequency. This may be the restricting factor before you reach voltage limitation. So if you have a, say, 100 pF, and you have 29 MHz across it as a shunt element, then the capacitive reactance is 54.9 ohms. With 2500 volts peak to peak, there is 45.5 Amps P-P of reactive current in it, and 16.1 Amps RMS. This may indeed be the limiting factor of the construction of the capacitor, and it can overheat and fail. At least check the calcs before sticking an expensive capacitor in a high power circuit.

[WBear2GCR]

Makes good sense, tnx.

So, I guess if you're running something *big* like 4-1000 with something near the full rated plate voltage, then you're going to need a fairly expensive vacuum variables to do the job.

                                               _-_-

[steve_qix]

Well, yeah - a 4-1000 type of vacuum cap will probably cost upwards of $200.00, but it will have a 10kV rating, and be good (at least if it's like the one I have in one of my class E gits) for 50A.

That should probably cover it!

[WBear2GCR]

These days a 1000pf or 1500pf 10kV vacuum variable anywhere *near* $200 bux would be a big bargain!

[steve_qix]

You can definitely get them for less than $200.00.  I have two of them here - 1000pF at 10kV.  One of them in a transmitter and the other as a spare.  They are from Russia (or some ex portion of Russia), and I got them off Ebay. There are caps just like this on Ebay now.   I bought a "two-fer" - two caps at the same time - both discounted, and only one shipping charge.