Vacuum(less) Variable Caps

[Todd, KA1KAQ]

Just an off-the-wall question of no real value. While talking with Johhny Novice a few nights ago, he mentioned a situation where the vacuum variable in a set he was using was turning blue or some weird color inside, the result of the little glass nipple being broken off allowing air inside. It was funnier to hear him describe it.

Anyhow, my question is: even with air inside, shouldn't the cap still work? Does the vacuum serve any great purpose beyond increasing the voltage it can handle and keeping cooties from jamming the bellows? Seems like it could still be useful at reduced voltage, or is it just better to take a hammer to it and move on?

*zorch* 

[WA1GFZ]

I think you need suckage to pull the plunger back in but it should work at reduced voltage rating.

[Don, W2DL]

Using a figure of abt 40KV/inch for breakdown voltage then an air cap of 1/4" spacing would be good for abt 10KV, this is what I have always gone by and at 2500 - 3000V B+ it works OK under modulation conditions. An old vacuum variable (30 - 400 Pf, 20KV rating) I have in my junk box measures about 0.035 " between plates, therefore this won't be good for much B+ if the cap. has air in it. Maybe for loading capacitor in a pi-net, but certainly NG for tuning  anything with much power involved.

Don, W2DL

[Todd, KA1KAQ]

Pretty much what I figured, it just seems a waste to heave such a purty piece of construction. Spacing in a vac variable is extemely close, so very low voltage indeed. Not sure where it would even be useful in its broken state.

[w3jn]

That was a relation of Tom JJ describing a blown vac variable he had.  I think it was a thread on this board somewhere.

[Todd, KA1KAQ]

Well, it was phunny!

[The Slab Bacon]

Also like Frank (gfz) said, you need the "suckage" on the bellows or diaphrizm for the return spring. Otherwise you would have to fit some kind of springy thingy th pull it back closed. One of the ways to ckheck a vacuuuuuum cap on a hamfest table is to see if the bellows is still tight or not. If the plunger or bellows is not pulled up tight, better leave it on the table.

                                                               The Slab Bacon

[n3lrx]

Re-Suck it and fix it with JB Weld.. It'll work! LOL

[WD8BIL]

Re-Suck it and fix it with JB Weld.. It'll work! LOL

Yup..... if you know anyone that works HVAC systems that has a vacuuuum pump you can restore the thing to very workable condition.

I know a guy who used vacuuuuumm off his tractor manifold to restore the suck !!!

[Todd, KA1KAQ]

If you read some of the old (like, really old teens-twenties) radio magazines, there are numerous articles about people making their own tubes. Talk about a handy skill to have! Of course, they didn't have such exacting requirements back then either in design or power handling. But how do you transition from the suck to the seal, without losing the suck, unless you're working with an entirely new glass envelope? JB Weld and duct tape are in the same fix-all-woes category. They deserve awards.

Was good woking you over the weekend, Buddly. I always get envious when you mention your SP receiver. What a nice, old rig that is - and you put it to such good use!