[Boatanchors] Query BC-683 Tank Receiver

[w7prv at teleport.com]

Digging through old nearly forgotten boatanchors, I found a real one. This is the receiver used in tanks and is built like one. BC-683. O.K. Let's fire it up. Applying 24 volts, the dynamotor hums away but silence from the speaker. Opening it up, I see an empty tube socket. That might be a problem. With the radio on the bench with the multi-pin power and other wires socket at the lower right corner, the missing tube socket is right inside with a 6H6 tube there in the next socket to the left. What is the missing tube? We used a lot of that radio and the BC-603 when setting up a C.D. station years ago. There was a transmitter that went with the set-up and both operated with pushbutton frequency selection. Help would be appreciated!  Thanks, Fred W7PRV