I recently discovered the 6FJ7 tubes which are a 12pin Compactron base equivalent of the 6DN7. What initially caught my interest was almost all of the tubes i found were black plate Sylvanias, my favorite, and also happened to be really cheap as i suspect nobody wants them. Were talking $3-4 per tube, most tube vendors have them but never list them so ask and you will receive.
Anytime i heard of people using converters in the past it was always a complaint about noise, but i am please to say i haven't encountered any such issues, the tubes i have tried were all quite right out of the box. Here is how i did it if anybody wants to eXperiment.
Parts needed are two 8pin Octal bases, and two Compactron 12pin sockets, both readily available on eBay. For the wiring i used the teflon coated wire supplied with the C4S and PJCCS boards (somehow i still have some left over), two home made plastic spacers, and a hot glue gun to stick them together.
1: Download the datasheet for both tubes and mark the numbers on each pin for the pin they correspond to on the other tube. (you will be looking at this alot)
2: Solder short lengths of wire to the terminals being used on the 12pin socket. I used colour coding to help, red for plate, white for grid, and black for cathode, then add a piece of heatshrink over them to prevent internal shorts.
3: Make up a plastic spacer, or devise some other means to support the socket inside the base so it cant be pressed in fully and short out the pins. Remember the tubes will take a bit of force to install into the sockets. I used a length of clear plastic pipe that was the perfect diameter cut into short lengths.
4: Drop the spacer into the base and position each wire from the 12pin socket into the correct pin on the octal base. With the Octal base key at 12 o'clock the key for the Compactron socket should line up at 3 o'clock. You will find most of the pins line up, its really only the 2nd cathode connection that end up being dog legged. Press the socket into the base and then from the bottom gently pull the cables through the pins and strip the insulation flush with the end of the pin.
5: Remove the socket from the base and strip the insulation on each wire another 5-7mm, of ~ half the length of the pins.
6: Reinsert the socket again checking each wire is in the correct pin and press the unit together. At this stage get the multimeter out and buzz out each connection from the octal socket to the 12 pin socket confirming each is wired correctly, and also check all the other pins to confirm nothing is shorting. Remember the diagrams in the datasheet are viewed from the bottom of the tube/socket. It can be confusing at first, you just have to remember to look from the bottom whenever working out pin positions.
7: Once you have confirmed each pin is correct lay a bead of hot glue around the outside edge of the socket and quickly press it into the base while confirming the 12 o'clock, 3 o'clock orientation of the two pieces. Wipe off the excess, or cut off with a knife after it has cooled.
8: Crank your soldering iron upto 11 and position the tip so it is heating the pin and the wire while leaving an open gap to feed solder into. The solder should wick its way down inside the pin as it heats up giving a good solid connection. Because of the heat involved i recommend using only teflon coated wire so the insulation dosent get burnt off.
9: Snip off the excess wire from each pin and give them a good clean with a toothbrush and IPA to remove any flux residue, you dont want that stuff inside your tube sockets.
10: Plug in a Compactron tube and enjoy!
Final pic shows a Sylvania Coin base 6DN7 vs a Sylvania 6FJ7
