[Music in a bottle]

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I've seen the glowing screen grid in many 70 sets (with speaker connected, that is), and here is my opinion on why this happens.

Philco ran a higher B+ on the screen grid than on the plate of the pentode audio output tubes of most of its radios in the 1930s. You take that, combined with the fact that the B+ applied to those 47 tubes was already at design maximum level, and then add in today's higher line voltages, and voila - orange screen grid because the voltage is now too high!

I've started dropping the screen voltage on these sets by adding a 5.1K resistor in series with the 47 screen grid. This lowers the screen voltage to a level below design maximum. It makes the tube much happier. Yes, it's a mod, but it is completely and easily reversible.

Ron, would it be advisable to run the model 70 sets with a Variac to get the correct operating line voltage? What line voltage were these sets designed around?