[Arran]

6C6 interchangeable with 6D6? A sharp cutoff pentode with a remote cutoff pentode?

My comments in post #2 also apply to trying to use a 6D6 in place of a 78. Where Philco calls for 77 and 78 tubes, that is what should be used. Not 6C6 or 6D6. Again, my opinion, YMMV.

Oh, I looked on my favorite online tube site - nj7p.org - and it lists 77 as a preferred sub for a 6C6; 78 as a preferred sub for a 6D6; 6C6 for 77 and 6D6 for 78. But again...in my experience, 6C6 for 77 and 6D6 for 78 in Philco sets will work poorly or not at all. Arran, you raise a good point here.

Ron;
  It was kind of late when I posted that, I was looking at interelectrode input and output capacities and not the biasing of the tube. None of these tubes are particularly hard to find, I looked up #77 tube on Duck Duck Go and a fleabay page full of them came up, everything from overpriced single NOS ones to very cheap used but good testing ones sold by the dozen. I haven't run into very many #77s in the stuff I own, mostly #78s and #58s. Another thing that may cause problems is that the spec sheets mention a 6C6 having an internal shield connected to the cathode, which the #77 does not. I don't know the reason for the substitution, perhaps tube shortages during war time. However IF amplifier tubes seem to last for a long time, many of the sets I run into still have the originals in place, RF amplifier tubes are somewhat shorter lived but they last a long while as well.
Regards
Arran