Output Tank Variable Cap?

[KI4YAN]

Ok, so everyone turn to page 237 of the RCA Transmitting Tube Manual, Technical Manual TT-4. You'll find a circuit for a pair of 812A in Push Pull, using split-stator caps in both the grid and plate circuitry. Obviously, a single stator, single rotor cap won't work, due to the rotor being at B+. Is there any safe way around this, given that split stator caps are about double the price of single section caps?

I have  35pF per section split stator, and 60pF per section split stator caps on hand, both have 1/8" plate spacings. obviously, I can not tune the final for 80M or lower, but 40M is possible, and the higher bands could be done.

My next idea is homemade capacitors, I have the equipment, and the know-how, but not the guts. what kinds of common insulating materials could be used to support 1250VDC, plus the approx. 250W of RF carrier? would acrylic plastic work?

[k4kyv]

If you maintain a symmetrical layout, you can use a single section capacitor, and establish the reference ground point with the coil instead of the capacitor.  But the plate spacing will need to be twice that of the split stator one, but the total capacitance needs to be only one half the capacitance of one of the sections on the split stator.

The capacitor will have to be well insulated at RF, including the shaft.  The capacitor goes across the coil, and +HV goes directly to the coil midtap, which should be bypassed to ground at rf with the same capacitor you would use to bypass the frame of the split stator.  With this arrangement, the rf choke in the +HV line is not needed, although it won't hurt anything to use it. If the plate current to the tubes won't balance, you may need to move the midtap connection slightly towards one end of the coil or the other.

[w4bfs]

does enyone know of a supply of 812's ... I thot they were soon unobtainium ...73 ...John

[KI4YAN]

Heh, Shuguang makes them. Yeah, I know, they're chinese, and everyone thinks they are cheap junk, but they do make good 812A's. They also make MOST of the 811A's sold today, including the Taylor brand. Svetlana also makes them.

AES has them for ~16$ each, matching is cheap. 811A's are less, ~15$ each, I think.