When is Wire not Suitable for use as Wiring?

Sounds like a contradiction, doesn't it?

Well, wiring has an insulation breakdown voltage rating, and temperature ratings too.

I would suggest never using solid state or old computer wiring for use in tube radios. Why? Much of todays wiring as a breakdown voltage from 50V to 200V, depending on which company made it and what the equipment mfg. thought was adequate. It is often of a smaller gauge also, so will handle less current.

 Tube radios can carry voltages upwards of 450V or more. Why take a chance of arcover, short, or a fire in the equipment?

I never use wire saved from solid state radios and such for use in tube equipment. The breakdown voltage is way too low.
 Now wire from old tube type CRT type sets is generally useable as it is rated at 600V
.
 If you can find an early B&W TV or color TV using a CRT and has tubes in it, that wire has worked for me with no problems at all. That wiring was made to handle high voltages up to, and in some cases in excess of 600V

Also NEVER use wire from any consumer electronics to replace the asbestos covered wire in ovens, clothes dryers, and neon signs. You are dealing with major heat issues  and HV issues in the neon signs.

 Note: I have scrapped out very old 1900's to 1930's vintage equipment where the builder did use asbestos insulated wire, likely because it was all he had it available. Money was often very tight back then. (I disposed of it safely at the local Hazohouse.)

 Also many appliances up into the 60's or later used asbestos in the power cords. This included toasters, electric waffle irons, coffee pots, clothes irons, deep fryers, electric frying pans, and the list goes on from there. 

   In addition, the old radios that used the 3 wire power cords AKA "Curtain Burners" used asbestos insulation to wrap the dropping resistor in the power cord on many models. If undisturbed, the hazardous material folks say asbestos is no problem, only when it is cut, stripped, etc.  then it is possible asbestos can be released into the air.