Equivalent to X78 Valve?

[boater sam]

Yes, it's a horrid idea really! Would have been better to have the heater separate, and free-up another pin by taking the signal grid to a top cap.

(Typing that makes me think, are there any B7G's with top caps? There certainly are B9A's!)

  Good point, I have never seen a top cap on B7G based valve.  Are there any?  Octals with 2 top caps are rare,  B9A are not common, B8A ?

It is a horrible thing to do, how you design with a cathode joined to a heater is problematical. I guess the cathode has to be grounded which with an AC connection to the heater must make it more difficult.