[Ron Ramirez]

Type 77 is a sharp-cutoff pentode, ideally suited for RF amplifier applications where no AVC voltage is present. The autodyne mixer-oscillator circuits Philco designed around the Type 36, and later Type 77, aren't the greatest in the world when it comes to reliability, especially long-term; Type 6A7 is MUCH better as a mixer-oscillator; it was designed specifically for this purpose.

I don't understand why Philco never improved Model 89 by changing that squirrelly autodyne circuit to one using the 6A7 when these tubes became available...but they did not. Actually, I think I do know why...Skinner was a notorious penny-pincher...and so the extra cost was probably not justified at the time.