New jack in an old radio

Two things about old tube radios. The phono input is of a different impedance value than what works for a guitar input jack to the active components. You may need to add a 68K ohm resistor from the positive jack lead heading into the circuit before it gets to the pre-amp for the phono input. Another reason why the jack is "dead" is a worn out tube, faulty or dried out capacitors/resistors in the signal path, etc.

Second, and most important, is that older tube radios can have lethal voltages running to the chassis.The super hetradyne circuits are much different than modern circuits.They have no transformer, so all that 120v ac power is going straight to the rectifier tube.

Even if you find a schematic online for the radio, there is a good chance the actual radio you have was not built exactly as the schematic. Parts during that era were scarce due to the war effort. Domestic manufacturers had to make due with what was available.

First, before trying anything, take a 100K ohm resistor and bleed all the capacitors to ground to remove any lethal voltage stored in the capacitors. Then replace the 2 prong plug and lead with a 3 prong. Connect the black + and white - to the original spots taken by the 2 prong setup. Then attach the green ground wire to the chassis. This way, if there are any lethal voltages leaking to the chassis, you will be able to tell us if the rest of the project was a success (meaning, you're alive to tell the tale).

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Scott aka Farmer Ted March 30, 2015 at 6:30pm

Two things about old tube radios. The phono input is of a different impedance value than what works for a guitar input jack to the active components. You may need to add a 68K ohm resistor from the positive jack lead heading into the circuit before it gets to the pre-amp for the phono input. Another reason why the jack is "dead" is a worn out tube, faulty or dried out capacitors/resistors in the signal path, etc.

Second, and most important, is that older tube radios can have lethal voltages running to the chassis.The super hetradyne circuits are much different than modern circuits.They have no transformer, so all that 120v ac power is going straight to the rectifier tube.

Even if you find a schematic online for the radio, there is a good chance the actual radio you have was not built exactly as the schematic. Parts during that era were scarce due to the war effort. Domestic manufacturers had to make due with what was available.

First, before trying anything, take a 100K ohm resistor and bleed all the capacitors to ground to remove any lethal voltage stored in the capacitors. Then replace the 2 prong plug and lead with a 3 prong. Connect the black + and white - to the original spots taken by the 2 prong setup. Then attach the green ground wire to the chassis. This way, if there are any lethal voltages leaking to the chassis, you will be able to tell us if the rest of the project was a success (meaning, you're alive to tell the tale).
Blind Lemon Cello March 30, 2015 at 6:28pm

Is the switch part of the circuit?

Maybe a bad switch?
- Scott aka Farmer Ted > Blind Lemon Cello March 30, 2015 at 6:31pm
  
  yup, good point

This reply was deleted.

Radios turned Guitar Amps

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