03-24-2008, 10:06 PM
At the request of Steve in Missouri, here is how I modified the front end of my Philco 71.
As I mentioned in a previous post, the 71 chassis I used was a rust bucket. I stripped everything (including all of the tube sockets) from it; the only parts that remained on the chassis were the aluminum coil and IF shields which were spun on.
The chassis was then cleaned and painted flat gray.
I then reassembled the radio, using all but one of the original tube sockets.
But here is the interesting part.
The detector-oscillator tube socket was switched to a 7 pin standard base socket to accommodate a 6A7 tube.
I also rewound the oscillator coil to match the oscillator coil of a Philco Model 18. Like Model 71, Model 18 has an IF of 260 kc, making this an easy and ideal mod for this radio.
The detector-oscillator stage in my 71 mod is based on the detector-oscillator stage of a Philco Model 18; the only difference being the B+ feed to the oscillator "plate" coming through a single 32K resistor.
See the schematic below.
[Image: http://www.philcoradio.com/images/phorum/osc_010.jpg]
Winding the oscillator coil is not difficult; I will provide details in a later post.
You will have to change the RF coil secondary so that it connects to the AVC line instead of ground. This involves lifting the wire from the bracket which holds the coil in place (and grounds this wire), and making a new terminal of some sort to which you can connect the AVC line. A simple way would be to carefully fasten a long solder lug to one side of the RF coil, and attach the wire formerly connected to ground, to the new lug.
You will also need to remove the common connection between the cathodes of the RF and IF tubes, and provide each with separate resistor/capacitor networks, as shown. Note that the IF cathode resistor is 1000 ohms; this is critical. A lower resistance will allow the IF stage to oscillate. 1000 ohms reduced the gain of the IF stage in my 71 just enough to eliminate oscillation, while still allowing superior performance.
I stripped and painted a Model 89 chassis at the same time I did the 71; however, I have yet to build the 89 with the 6A7 tube. I did the 71 just to see if the old, inefficient autodyne mixer circuit could be improved upon. The superior performance of my 71 mod proves that it can be.
I intend to give complete details on all of the mods I made to my Super 71 (which includes an AVC mod, a volume control with a tap for bass compensation, and a tone control potentiometer) later on, on my website.
As I mentioned in a previous post, the 71 chassis I used was a rust bucket. I stripped everything (including all of the tube sockets) from it; the only parts that remained on the chassis were the aluminum coil and IF shields which were spun on.
The chassis was then cleaned and painted flat gray.
I then reassembled the radio, using all but one of the original tube sockets.
But here is the interesting part.
The detector-oscillator tube socket was switched to a 7 pin standard base socket to accommodate a 6A7 tube.
I also rewound the oscillator coil to match the oscillator coil of a Philco Model 18. Like Model 71, Model 18 has an IF of 260 kc, making this an easy and ideal mod for this radio.
The detector-oscillator stage in my 71 mod is based on the detector-oscillator stage of a Philco Model 18; the only difference being the B+ feed to the oscillator "plate" coming through a single 32K resistor.
See the schematic below.
[Image: http://www.philcoradio.com/images/phorum/osc_010.jpg]
Winding the oscillator coil is not difficult; I will provide details in a later post.
You will have to change the RF coil secondary so that it connects to the AVC line instead of ground. This involves lifting the wire from the bracket which holds the coil in place (and grounds this wire), and making a new terminal of some sort to which you can connect the AVC line. A simple way would be to carefully fasten a long solder lug to one side of the RF coil, and attach the wire formerly connected to ground, to the new lug.
You will also need to remove the common connection between the cathodes of the RF and IF tubes, and provide each with separate resistor/capacitor networks, as shown. Note that the IF cathode resistor is 1000 ohms; this is critical. A lower resistance will allow the IF stage to oscillate. 1000 ohms reduced the gain of the IF stage in my 71 just enough to eliminate oscillation, while still allowing superior performance.
I stripped and painted a Model 89 chassis at the same time I did the 71; however, I have yet to build the 89 with the 6A7 tube. I did the 71 just to see if the old, inefficient autodyne mixer circuit could be improved upon. The superior performance of my 71 mod proves that it can be.
I intend to give complete details on all of the mods I made to my Super 71 (which includes an AVC mod, a volume control with a tap for bass compensation, and a tone control potentiometer) later on, on my website.
--
Ron Ramirez
Ferdinand IN
