[Ron Ramirez]

 The 6A7 was the predecessor to the 6A8, 6A8G, or 6A8GT, electrically the tubes itself has the same specs except for one detail, the inter-electrode capacity is definitely higher then that of a 6A8 or 6A8GT, perhaps even a 6A8G as well.

Yes, but as you said, in this case it does not really matter.

I don't know is these sets were all equipped with 6A8Gs from the factory or whether some were built with 6A7s, Philco was notorious for changing tube layouts in their sets if they got a deal on one type of tube verses another...

As far as I know, they all had 6A8G tubes from the factory. Philco didn't do that det-osc tube switching until the following (1940) season where some radio models use a 7J7 and others of the same model use a 6J8G.

The addition of a 76 tube for AVC seems rather pointless, I think the sales and marketing department had the engineering department add it just so make the 39-30 chassis add up to six tubes, rather then five, to claim it was a better set then a 39-25.

Agreed. There really is no difference, electrically, between the 39-25, 39-30 and 39-31, save for the fact the latter two have an extra tube handling the 2nd detector function and AVC. In the 39-25, the 75 tube (being a dual diode, triode) handles 2nd detector, AVC and 1st audio functions. In the 39-30, the 2nd detector and AVC are handled by a 37 tube connected as two diodes while the 75 is used only as an audio amp.

Philco did this again in the 1940 and 1941 seasons on a few of their models. Nothing like padding the tube count.

Well, anyway...

Philconerd, the 39-30 is still a very nice radio and a surprisingly good performer. I own the Canadian version, the 39-330AT, which is very similar to the USA 39-30T. The main difference is six pushbuttons instead of eight. Being Canadian, it also has cloth-covered wiring instead of rubber, although the IF transformer leads did have that awful rubber stuff which I replaced. I think you are going to like this one.