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Tune C, grid-driven Tetrode

Naysayer

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Mar 6, 2020
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The lowest Tune capacitance I can apply with an Air Variable is about 15pF. That minimum Tune C reduces PO by nearly 80%. Yet, with no Tune C connected at all (L network Tank) I see about 1:100 PO w/ reduced plate volts/mA.

The RF Input parallel resonance revisions appear to work fine and I will post Cores, # Turns and awg for each band but for now, 11M working best with 2T, 18awg on a small yellow core. 2nd best for 11M is 2T 24awg same Yellow core. Haven’t tried 20M yet but soon. The 11 position rotary input switch lets me try several combos at a time.

I added RF Input attenuation by shifting 300R of the swamping resistance to a series position in the Grid line (diagram uploaded a few days ago). It’s easier to test when amp not going full-tilt as I cannot drive lower than 5w. That’s with the bottom cover On so I should not make that mistake again. Tidied-up the T-R relay compartment too.

Tune C: I’ve never seen a vacuum variable in real life so I’m uncertain about mounting but I have the room and it looks do-able. I see a Soviet part (eBay: 4-100pF @ 5kv, Approx $150). It’s the cheapest ‘new’ I could find and the specs seem appropriate. No problems with Used but Western V-caps require special brackets, clamps and other hardware. Is there anything I should be aware of when it comes to Soviet V-caps? The vacuum relay I got awhile back works fine despite the 25v coil requirement.

I need to source strap for tank connections too. After the Tune C gets sorted I’ll likely revise the Load C padding. No more vacuum stuff though, wife is starting to notice, lol.

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Looks like it was packaged by Khrushchev himself. The foam inserts rotted and fall apart at the touch. A greasy mess but the cap is in a sealed plastic bag. Did they even have plastic back then?

Package insert has several dates. A rubber stamp of 1991 and other handwritten and pre-printed dates: 1989; 1979; 1974 and 1968. In an 8 page Russian booklet.
 

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Why does a vacuum variable have bellows? If the concentric circles that mesh together and apart control the capacitance, and; There is a vacuum inside the glass tube itself, why is there any need for bellows? Bellows also look like copper.

Having a quality capacitor helped me rank my collection of cheap Capacitance meters. Unlike the Inductance functions, the cheaper capacitance meters were fairly accurate. Had a day's fun with the Vac Variable before I began mounting it, which seems to be the hardest part. It's heavy. I combined 2 insulated shaft couplers to get a 1/4 to 5/16" conversion that fits between the three 2.5" bolts holding the front flange to amps front aluminum plate. Pics when it's done but it's going to take awhile.
neil, NYC
 
Why does a vacuum variable have bellows? If the concentric circles that mesh together and apart control the capacitance, and; There is a vacuum inside the glass tube itself, why is there any need for bellows? Bellows also look like copper.

The concentric circle are the actual capacitor plates that mesh. The cap has bellows because they need to expand and contract. As you tune the capacitor, the concentric plate move in and out. One set of concentric circles or plates is connected to each end of the capacitor, so you need a conductor connected to each set of plates. If one set of plates is stationary and the other moveable, you need bellows. Watch them as you adjust the capacitor. Yes they are copper. The stationary plates are connected to the farend and the variable plates are connected to the end with the shaft.
 
Tricky part is coming up with a turns counter to show where it's set. Instead of the parallel flat plates of an air variable, the concentric circles mesh with a gap between them. The bellows is the flexible pressure seal that keeps out the air while permitting linear movement.

73
 

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