07-03-2008, 09:51 AM
Many 1937 Philco models used a similar circuit to distribute B+ to the 6A8 oscillator "plate" as well as screen grids of the RF, IF and 6A8 tubes. Only instead of a choke, they used a resistor!
The 37-620 is a good example. A 9000 ohm resistor is used , being fed raw B+ from the junction of the 5Y4G filament, input filter (62), 12 uF, and the input of the field coil. The other end of this 9000 ohm resistor is connected to the positive end of a 16 uF electrolytic ( 18 ) and to a voltage divider circuit, feeding the points mentioned above.
Older Philcos, such as Model 60, did not use this separate feed for the detector-oscillator tube's oscillator "plate" and these work well, without modulation hum. Yet Model 66, which is similar to Model 60, DID use the extra choke to feed the 6A7.
I tried an experiment once, many years ago...I hooked up a Model 66 so that its B+ distribution was identical to Model 60, eliminating the choke. It had unacceptable hum. I then put the circuit back to its original design, and the hum disappeared.
The 37-620 is a good example. A 9000 ohm resistor is used , being fed raw B+ from the junction of the 5Y4G filament, input filter (62), 12 uF, and the input of the field coil. The other end of this 9000 ohm resistor is connected to the positive end of a 16 uF electrolytic ( 18 ) and to a voltage divider circuit, feeding the points mentioned above.
Older Philcos, such as Model 60, did not use this separate feed for the detector-oscillator tube's oscillator "plate" and these work well, without modulation hum. Yet Model 66, which is similar to Model 60, DID use the extra choke to feed the 6A7.
I tried an experiment once, many years ago...I hooked up a Model 66 so that its B+ distribution was identical to Model 60, eliminating the choke. It had unacceptable hum. I then put the circuit back to its original design, and the hum disappeared.
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Ron Ramirez
Ferdinand IN
