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They may be interchangeable in the socket, but there's a world of difference beyond that. Just start with the power supply required for the filaments. Both require 5.0 volts. The 3-500 filaments require 14.6 amperes. The 3CX1500D7 takes 30 amperes.
Plate voltage? The 3-500 is rated around 2800-3000 volts. Compare that with 6000 for the D7.
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