[Bradley Rush]

I have a heathkit sg-8 that some one has modified
They put a 8 pin socket on the right upper side.

Pin 5  goes to pin 1 of the 12AU7
Pin 6  goes to RF out jack
Pin 7  goes to ground 
Pin 8  goes to 6.3 Vac 

I ask to find out if any knows of this mod?
 If so what tube should I get?
My guess is this tube is amplifying the RF.

[Bradley Rush]

This is the front of the SG-8

[Bradley Rush]

This is the back

[Radioroslyn]

I doubt it's a socket for an additional tube, probably for some sort of accessory. Maybe an 100kc calibrator.

[Bradley Rush]

With the signal gen  hooked up you could monitor rf plate voltage on the 12AU7 and filaments for stability?

[Bradley Rush]

HEATHKIT SG-8 now up and going