07-24-2009, 09:13 AM
http://cgi.ebay.com/370232120717
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.
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 Ramirez
Ferdinand IN
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