11-21-2021, 08:30 PM
The inter-electrode capacitance of the oscillator "plate" grid (G2) and the oscillator grid (G1) are very, very close between the 6A7 and the 12A8GT.
And MrFixr, after thinking, I realized that if the 12A8GT needed more cathode bias, I don't think it would affect the oscillator frequency that much - the entire tube would have poor performance.
Anyway...
I had baked the oscillator coil and did some work to the antenna coil (i.e. replacing old mica caps and an original, hidden, 4.7K resistor).
I studied several Philco schematics. I focused in on model 602 since it is the most similar to the 54 of all the 1930s five tube AC/DC Philco sets.
https://philcoradio.com/library/download...l.%201.pdf
Notice how it has the oscillator coil winding with the highest resistance (7.5 ohms) connected between G1, the oscillator grid, of the 6A7 and B-. The other winding, 2.4 ohms, is connected between the oscillator plate (G2) of the 6A7 and B+.
Just the opposite of the 54.
Just for laughs, I connected the oscillator coil into my hot-rod 54 in a similar manner to the 602.
Boom, the entire AM band came to life. Wow, is this radio sensitive! It even picks up stations without an antenna!
Now, if the trimmers will spread the band out properly, this set will be on its way to completion. It is receiving the entire AM band now, but it is all compressed into the center of the tuning condenser's travel. 700 kc (WLW) is coming in approximately above 80 on the dial; 1500 kc is coming in around 130 on the dial. (These are approximate since the dial is on the cabinet in this model.)
But things are much, much better than they were now.
And MrFixr, after thinking, I realized that if the 12A8GT needed more cathode bias, I don't think it would affect the oscillator frequency that much - the entire tube would have poor performance.
Anyway...
I had baked the oscillator coil and did some work to the antenna coil (i.e. replacing old mica caps and an original, hidden, 4.7K resistor).
I studied several Philco schematics. I focused in on model 602 since it is the most similar to the 54 of all the 1930s five tube AC/DC Philco sets.
https://philcoradio.com/library/download...l.%201.pdf
Notice how it has the oscillator coil winding with the highest resistance (7.5 ohms) connected between G1, the oscillator grid, of the 6A7 and B-. The other winding, 2.4 ohms, is connected between the oscillator plate (G2) of the 6A7 and B+.
Just the opposite of the 54.
Just for laughs, I connected the oscillator coil into my hot-rod 54 in a similar manner to the 602.
Boom, the entire AM band came to life. Wow, is this radio sensitive! It even picks up stations without an antenna!
Now, if the trimmers will spread the band out properly, this set will be on its way to completion. It is receiving the entire AM band now, but it is all compressed into the center of the tuning condenser's travel. 700 kc (WLW) is coming in approximately above 80 on the dial; 1500 kc is coming in around 130 on the dial. (These are approximate since the dial is on the cabinet in this model.)
But things are much, much better than they were now.
--
Ron Ramirez
Ferdinand IN
