No.3
Amateur Radio
SK-3A

The Svetlana SK-3A socket has the cathode effectively tied to the chassis ground (via the socket's mounting plate) and an internal bypass capacitor for the screen grid appears between the screen grid and the cathode. In this configuration, the socket is not suitable for cathode - driven applications, or where it is desired to provide some degenerative feedback, or to provide all or part of the operating bias by operating the cathode at a potential other than ground.

The following procedure can be used to modify the socket for such applications, however. To complete the conversion, you will need 4 Teflon or fiberglass (insulating) shoulder washers and 4 similar flat washers, all of which will clear a #4 machine screw.

1. Drill out the four rivets holding the screen ring to the screen contactors at the very top of the socket.

2. Now working at the bottom of the socket, remove the four nuts from the machine screws holding the socket assembly together.

3. Disassemble the socket;

a) first remove the cathode contact ring, marking its position relative to the underlying bakelite layer

b) remove the bakelite socket layer which has the factory markings and serial number, marking its position relative to the socket mounting plate. (This is the 0.060" [1.5mm] silver-plated brass plate.)

c) carefully remove the screen contactor assembly, freeing the contactor "ears" by springing them outward (Don't drop the capacitor! It is the ceramic annulus with the silver plating on each side. It is very brittle!)

d) remove the spring plate, the capacitor, and the other spring plate if they didn't come with the screen contactor assembly in "c" above

e) remove the mounting plate assembly, marking its position relative to the remaining socket assembly

4. Drill out the four holes in the mounting plate assembly using a #14 drill (0.180"). These are the second set of holes in from the outer edge, through which the socket assembly screws pass. (The screws should still be in the top layer of the socket, with heater, grid, and cathode contactors.)

5. Put the Teflon shoulder washers on the screws, and stick the flat washers shims on the flat surface of each cathode contactor strip with rubber cement or other (non-conductive) adhesive. The shims need to be held in place only during reassembly of the socket. Now, when the socket is reassembled, the cathode will be isolated from the main mounting plate and the screen bypass capacitor.

6. Replace the capacitor assembly in the order: spring, capacitor, and spring. Now replace the screen contactor assembly and the bakelite bottom section, taking care to align this section with your previous mark, and carefully guide the socket solder tabs through the bakelite bottom without bending them.

7. Cut the outer tabs off the cathode ring contact. After all of this work, you don't want this ring (the cathode) to be grounded when you mount the socket in the chassis!

8. Replace the washers and nuts on the socket assembly machine screws and tighten each a little at a time, until the assembly is snug.

This completes the socket conversion. Characteristics are now as follows:

a) The screen ring on the 4CX1600B is contacted exactly as before. The upper ring of the screen contact assembly can be re-riveted, but this requires some very small rivets. This metal ring can also be soldered to the contactor where the rivet joint previously was. The internal screen bypass capacitor still appears between the screen grid and ground (via the socket mounting plate.)

b) The cathode ring on the 4CX1600B is contacted exactly as before, but the electrical connection for the cathode is now isolated from the chassis. The cathode contact on the socket is now through the thin cathode ring on the bottom of the socket.

c) The heater, control grid, and screen tabs function exactly as before.

**The information provided in this application note is intended for general design guidance only. The user assumes all responsibility for correct and safe usage of this information. Svetlana Electron Devices does not guarantee the usefulness or marketability of products based on this material.

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