04-02-2011, 09:36 AM
Almost everyone dislikes Frankenradios but when a real junker arrives and it's not a rare model, it's fun to experiment and see where things will end up. This is a description of one of those junkers and three years of playing with it; sorry if I offend any purists:
The project is a 37-620 tombstone. I got it several years ago when it needed a dial, speaker, and power and audio transformers; the finish was long gone too. Replacing the power transformer required moving the rectifier to the chassis and mounting a 5Y3; it got a Rock Sea dial, one of those horrible ivory colored jobs that I had to overspray with clear orange for the right Philco color; the speaker became an 8" 20w full fidelity PM type with a choke and new audio transformer mounted to it. The 6F6 audio tube was rewired for a stronger 6L6; with a higher B+ from the new power transformer, two 0D3 regulator tubes were added in the former filter cap chassis holes to drop it to a regulated 300V. After this the AGC and audio detectors were solid-stated. Shielded cable replaced and RF and IF grid leads; metal tubes are used when possible.
Not content with the distortion in the audio stage, I added an additional tube, a 6J7 sharp cut-off pentode, into the hole formerly occupied by a large cap in the center of the RF deck. A socket fit perfectly in the hole. The new 6J7 is now a detector/audio amp with very low distortion and very high output and eliminated the need for the 6Q7. Fixed bias for this 6J7 was taken from the lowest tap on the B- resistor. The very high output from the 6J7 is perfect for driving the 6L6 and yields a nearly distortion-free full 11 watts audio output. (Even with my bad hearing it is never turned up very high)
The 6Q7 socket, no longer being used, was rewired for a twin-triode 6N7 which serves as a low and high freq emphasis amplifier; it is fed by filtered audio detected from the IF and re-inserted at the grid of the new 6J7 through a resistor network. This is a marginally effective circuit which may be eliminated and made into something else eventually.
Also while playing with the circuitry, I moved my four component LED tuning indicator to a position just above the tuning knobs- a terrific improvement which replaces shadow meters. (eventually I'll get around to posting the very simple and effective circuit)
The cabinet was refinished in walnut stain with a nice looking very dark sprayed highlighted edges. A large escutcheon from a floor model replaced the smaller original with a bit of wood cutting. Except for the tuning knobs, I elected to use a set of non-Philco knobs; after all, this radio isn't really a Philco anymore.
I recorded none of the schematic changes; the new circuits and applications are wired according to the data in any tube manual.
That's it. I'm ready for the volley of criticism for destroying a Philco. Fire away.....
The project is a 37-620 tombstone. I got it several years ago when it needed a dial, speaker, and power and audio transformers; the finish was long gone too. Replacing the power transformer required moving the rectifier to the chassis and mounting a 5Y3; it got a Rock Sea dial, one of those horrible ivory colored jobs that I had to overspray with clear orange for the right Philco color; the speaker became an 8" 20w full fidelity PM type with a choke and new audio transformer mounted to it. The 6F6 audio tube was rewired for a stronger 6L6; with a higher B+ from the new power transformer, two 0D3 regulator tubes were added in the former filter cap chassis holes to drop it to a regulated 300V. After this the AGC and audio detectors were solid-stated. Shielded cable replaced and RF and IF grid leads; metal tubes are used when possible.
Not content with the distortion in the audio stage, I added an additional tube, a 6J7 sharp cut-off pentode, into the hole formerly occupied by a large cap in the center of the RF deck. A socket fit perfectly in the hole. The new 6J7 is now a detector/audio amp with very low distortion and very high output and eliminated the need for the 6Q7. Fixed bias for this 6J7 was taken from the lowest tap on the B- resistor. The very high output from the 6J7 is perfect for driving the 6L6 and yields a nearly distortion-free full 11 watts audio output. (Even with my bad hearing it is never turned up very high)
The 6Q7 socket, no longer being used, was rewired for a twin-triode 6N7 which serves as a low and high freq emphasis amplifier; it is fed by filtered audio detected from the IF and re-inserted at the grid of the new 6J7 through a resistor network. This is a marginally effective circuit which may be eliminated and made into something else eventually.
Also while playing with the circuitry, I moved my four component LED tuning indicator to a position just above the tuning knobs- a terrific improvement which replaces shadow meters. (eventually I'll get around to posting the very simple and effective circuit)
The cabinet was refinished in walnut stain with a nice looking very dark sprayed highlighted edges. A large escutcheon from a floor model replaced the smaller original with a bit of wood cutting. Except for the tuning knobs, I elected to use a set of non-Philco knobs; after all, this radio isn't really a Philco anymore.
I recorded none of the schematic changes; the new circuits and applications are wired according to the data in any tube manual.
That's it. I'm ready for the volley of criticism for destroying a Philco. Fire away.....
Pete AI2V
