06-12-2012, 09:02 AM
A few months ago I showed photos of this super hot-rodded 37-620 but didn’t have a schematic to go with it; this is the schematic and description. It began as a junker model 37-620 and slowly evolved into what can be seen on the schematic today. Since I'm not a pro at drawing and laying out schematics, you will have to accept the somewhat unusual convention.
There are no major changes in the 6K7 RF amplifier and 6A8 Oscillator/Mixer stages with the exception of a BNC oscillator output connector on the rear skirt; that signal goes to an external freq counter module which gives direct reading of the dial freq (these counter modules are available on Ebay for around $50). There is a high Q tank circuit in series with the antenna input designed to kill the local 1250 KHz station; it is not on the schematic; think of it as being an optional circuit.
Beyond the 6K7 and 6A8 circuits there is very little that remains original. The first IF 6K7 is now a much higher gain 6SG7; the 6Q7 was replaced with a high output low distortion 6J7 detector and is located in the center of the RF deck in the place formerly of an electrolytic cap; the AVC detector is now solid state; the detected audio from the 6J7 is fed to a 6K6 audio pre-amplifier; the pre-amp feeds the 6L6 output stage; and the 6L6 drives a 60 watt broadband 8 ohm PM speaker. Great attention was paid toward audio fidelity and bandwidth at all levels of output, the maximum rated output being 11 watts. Distortion is extremely low; the sound quality is extraordinary. Except for the 5Y3 and 6L6, all tubes are metal; that required tube socket changes.
The power supply uses an 8 Hy choke mounted on the speaker. Tube voltages throughout the radio have been adjusted for improved linearity; all voltages are within the proper tube parameters.
The next time I feel like taking out the RF deck, the 6K7 (and socket) will be replaced with a 6SG7. A variable bandwidth IF is under design using high Q ferrites; this circuit was installed earlier but the Q was too high and I hadn’t thought about making the Q or bandwidth variable. (The Q of a Philco IF is usually around 8-10; the ferrite Q was 280). In all of the re-design, the LED replacement for the shadow meter was left out but will be re-installed during the next round of changes.
Gas tube regulators were integrated in a previous version; they didn’t really serve any necessary function so they were removed.
There is still room for two more tubes; three if I get rid of the 5Y3. The only future limitations are from drawing too much from the power transformer; at that point the new tubes would need to be subminiatures. Maybe I'd better quit on it now- nah!
Pete AI2V
There are no major changes in the 6K7 RF amplifier and 6A8 Oscillator/Mixer stages with the exception of a BNC oscillator output connector on the rear skirt; that signal goes to an external freq counter module which gives direct reading of the dial freq (these counter modules are available on Ebay for around $50). There is a high Q tank circuit in series with the antenna input designed to kill the local 1250 KHz station; it is not on the schematic; think of it as being an optional circuit.
Beyond the 6K7 and 6A8 circuits there is very little that remains original. The first IF 6K7 is now a much higher gain 6SG7; the 6Q7 was replaced with a high output low distortion 6J7 detector and is located in the center of the RF deck in the place formerly of an electrolytic cap; the AVC detector is now solid state; the detected audio from the 6J7 is fed to a 6K6 audio pre-amplifier; the pre-amp feeds the 6L6 output stage; and the 6L6 drives a 60 watt broadband 8 ohm PM speaker. Great attention was paid toward audio fidelity and bandwidth at all levels of output, the maximum rated output being 11 watts. Distortion is extremely low; the sound quality is extraordinary. Except for the 5Y3 and 6L6, all tubes are metal; that required tube socket changes.
The power supply uses an 8 Hy choke mounted on the speaker. Tube voltages throughout the radio have been adjusted for improved linearity; all voltages are within the proper tube parameters.
The next time I feel like taking out the RF deck, the 6K7 (and socket) will be replaced with a 6SG7. A variable bandwidth IF is under design using high Q ferrites; this circuit was installed earlier but the Q was too high and I hadn’t thought about making the Q or bandwidth variable. (The Q of a Philco IF is usually around 8-10; the ferrite Q was 280). In all of the re-design, the LED replacement for the shadow meter was left out but will be re-installed during the next round of changes.
Gas tube regulators were integrated in a previous version; they didn’t really serve any necessary function so they were removed.
There is still room for two more tubes; three if I get rid of the 5Y3. The only future limitations are from drawing too much from the power transformer; at that point the new tubes would need to be subminiatures. Maybe I'd better quit on it now- nah!
Pete AI2V
Pete AI2V
