> I have a Swan 500C HF radio that doesn't have a mic attached.
 > The connector looks like it is a stereo 1/4 inch plug,
 > that's my guess.
quite accurate ;-)
 > Does anyone know the wiring for this radio?
As I recall,
 ==========|===|===\
 ==========|===|===/
    |        |   |-------- Tip
    |        |------------ Ring
    |--------------------- Ground
The Tip is "normally" where the audio is carried.
The Ring is "normally" where the PTT line connects,
and both of those circuits share a common ground.
Choosing the microphone you will be using for that Swan 500c Tube Type rig is 
important - you first need to know what impedance the mic is.  (The input Z 
of the audio of the Swan, is around 50kOhms.)
For SSB, I'd suggest a good High-Z dynamic Mic.  A D-104 would work, but a 
crystal mic normally has a VERY hi "Z" (a few Meg-ohms) and will sound kinda 
"scratchy" or "tinny".
Best of Luck!
73 = Best Regards            -Jeff KA5THB
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