12-22-2008, 03:50 PM
You've got a handful of mods in your 37-610.
The VR-90 is a gas voltage regulator, 90 volts; I have no idea why you would need a regulated 90v in that radio.
The 6SF5 is another mystery but it most likely is used as a first audio amplifier after the detector; normally the 6Q7 performs this task as well as AM detection and AGC. In your case I suspect that someone has installed two solid-state detector diodes ( a simple mod) and rewired it for the 6SF5 which doesn't have any detector/AGC plates.
With your ohmmeter in diode resistance mode (and the radio unplugged!), measure from the junction of R35/R36 to ground; reverse the test leads and measure again. If there's a major difference in resistance you have a solid state detector diode connected to that junction, and probably another diode connected to C37. These diodes will look like small resistors with one band or clear glass with one band. I must admit that I have done this modification before when I didn't have a replacement 6Q7 tube; it can improve sensitivity and audio fidelity to some degree. If your chassis has been solid-stated it might be wise to just leave it alone. Otherwise you will have to rewire the 6Q7 circuit by schematic.
Another comment on the VR90: it's probably a 0B3 and you should probably just take it out and see what happens. Somewhere in the B+ line you will probably find a resistor in series with pin 5 (if it's an octal tube). The purpose is to give a regulated 90 volts somewhere or to limit a voltage to 90v across two points. It's unusual to use these tubes in domestic applications- they are mostly used industrially or for special applications. They can also be used in series (usually a VR90 in series with a VR150) to limit the B+ in the case of a replaced power transformer to lower the maximum voltage of the power supply B+. In custom applications (like my 37-620) I use two VR150's but this is NOT a normal thing to do in a Philco and I don't recommend it for a variety of reasons.
The VR-90 is a gas voltage regulator, 90 volts; I have no idea why you would need a regulated 90v in that radio.
The 6SF5 is another mystery but it most likely is used as a first audio amplifier after the detector; normally the 6Q7 performs this task as well as AM detection and AGC. In your case I suspect that someone has installed two solid-state detector diodes ( a simple mod) and rewired it for the 6SF5 which doesn't have any detector/AGC plates.
With your ohmmeter in diode resistance mode (and the radio unplugged!), measure from the junction of R35/R36 to ground; reverse the test leads and measure again. If there's a major difference in resistance you have a solid state detector diode connected to that junction, and probably another diode connected to C37. These diodes will look like small resistors with one band or clear glass with one band. I must admit that I have done this modification before when I didn't have a replacement 6Q7 tube; it can improve sensitivity and audio fidelity to some degree. If your chassis has been solid-stated it might be wise to just leave it alone. Otherwise you will have to rewire the 6Q7 circuit by schematic.
Another comment on the VR90: it's probably a 0B3 and you should probably just take it out and see what happens. Somewhere in the B+ line you will probably find a resistor in series with pin 5 (if it's an octal tube). The purpose is to give a regulated 90 volts somewhere or to limit a voltage to 90v across two points. It's unusual to use these tubes in domestic applications- they are mostly used industrially or for special applications. They can also be used in series (usually a VR90 in series with a VR150) to limit the B+ in the case of a replaced power transformer to lower the maximum voltage of the power supply B+. In custom applications (like my 37-620) I use two VR150's but this is NOT a normal thing to do in a Philco and I don't recommend it for a variety of reasons.
Pete AI2V
