03-05-2008, 12:10 PM
I am restoring a Philco 37-624 6 volt DC radio. I wish to keep the underchassis appearance as original as possible. There are several flexible resistors used in the circuit for regulating the filament voltages. Several of these in the radio are OPEN, and I also have to change the value of one in order to replace a pilot lamp that is no longer available.
The values I need are 8.33 ohms, and 33.3 ohms. The resistors are currently inside red spaghetti tubing. The inside diameter of the tubing will just accomodate a 1/2 watt resistor.
The 8.33 ohm must carry 240ma (.48 watts). The 33.3 ohm resistors must carry 60ma (.12 watts).
I looked into using 2 volt Zeners, but cannot find any axial types at that voltage. I also considered using 1N400x diodes, which drop about 0.8 volts each and will carry one amp. But I am concerned about temperature stability! A short test indicated that forward voltage changed as they heat up while passing rated current (240 ma).
Can I use carbon comp resistors? For example, I picked three 2.7 ohm 1/2 watt resistors in series which adds up to 8.3 ohms, and will fit inside the original spaghetti tubing. Each 1/2 watt resistor must dissipate only .16 watts. But I have concerns about stability when carrying any significant current for any length of time.
For the 33.3 ohm resistors, I planned to use two or three 1/2 watt resistors in series. Same concerns.
Any suggestions for replacing these flexible resistors and keeping an original appearance? I don't know of any way to wind a replacement, even if the suitable resistance wire were available.
I suppose the only alternative is to try and find suitable small wire wound resistors and forego the spaghetti tubing.
The values I need are 8.33 ohms, and 33.3 ohms. The resistors are currently inside red spaghetti tubing. The inside diameter of the tubing will just accomodate a 1/2 watt resistor.
The 8.33 ohm must carry 240ma (.48 watts). The 33.3 ohm resistors must carry 60ma (.12 watts).
I looked into using 2 volt Zeners, but cannot find any axial types at that voltage. I also considered using 1N400x diodes, which drop about 0.8 volts each and will carry one amp. But I am concerned about temperature stability! A short test indicated that forward voltage changed as they heat up while passing rated current (240 ma).
Can I use carbon comp resistors? For example, I picked three 2.7 ohm 1/2 watt resistors in series which adds up to 8.3 ohms, and will fit inside the original spaghetti tubing. Each 1/2 watt resistor must dissipate only .16 watts. But I have concerns about stability when carrying any significant current for any length of time.
For the 33.3 ohm resistors, I planned to use two or three 1/2 watt resistors in series. Same concerns.
Any suggestions for replacing these flexible resistors and keeping an original appearance? I don't know of any way to wind a replacement, even if the suitable resistance wire were available.
I suppose the only alternative is to try and find suitable small wire wound resistors and forego the spaghetti tubing.
