Wow a 12AP4 Thats a pre war picture tube. I it was used in the 1939 sets RCA made. If you could find a original that was good you should expect to pay $2400 to $4000 but it is doubtful you will find a original! Here is a artical from the ETF That explains a substitute. http://www.earlytelevision.org/12ap4_substitute.html John T