07-20-2016, 07:17 AM
In order to eliminate the issue of the RF/IF screen voltage being lower than called for in the tube manual, I used a resistance substitution box in parallel with one of the resistors supplying screen voltage to the RF amp, converter, and IF amps. I raised the voltage to 100 volts (from 87). There was a noticeable increase in volume. The circuit in my radio, while original (based on solder joints and components used), does not match the published schematic. But there must have been a reason Philco changed the circuit. I left my radio as it was (only replacing out of tolerance resistors). There are several other places where my radio does not match the published schematic.
My other concern was the low AVC voltage, which varied from -2.7 to -6.5 volts depending on signal strength. Somewhere I recently read that AVC voltage usually max out at -20 volts in radios using variable MU tubes in the RF and IF sections. In radios using sharp cutoff tubes, the AVC is usually limited to -5 volts, else the controlled tubes may be cut off. The Philco 46-888 uses 7C7 tubes for the RF and one IF amplifier stage. The second IF amplifier stage is a 78, which is variable MU. I'm not sure of the characteristics of the 7J7 heptode section. And one end of the AVC line is connected to a negative voltage supply, which would tend to limit the range of the AVC. So bottom line is that the AVC may be working correctly.
So I am wrapping up this restoration. The radio is back in its cabinet and works great! I am just disappointed with reception performance, especially on short wave. But I have only listened during the day, and with a crappy antenna.
My other concern was the low AVC voltage, which varied from -2.7 to -6.5 volts depending on signal strength. Somewhere I recently read that AVC voltage usually max out at -20 volts in radios using variable MU tubes in the RF and IF sections. In radios using sharp cutoff tubes, the AVC is usually limited to -5 volts, else the controlled tubes may be cut off. The Philco 46-888 uses 7C7 tubes for the RF and one IF amplifier stage. The second IF amplifier stage is a 78, which is variable MU. I'm not sure of the characteristics of the 7J7 heptode section. And one end of the AVC line is connected to a negative voltage supply, which would tend to limit the range of the AVC. So bottom line is that the AVC may be working correctly.
So I am wrapping up this restoration. The radio is back in its cabinet and works great! I am just disappointed with reception performance, especially on short wave. But I have only listened during the day, and with a crappy antenna.
