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Hi Can anyone tell me if the 4-400 on my vacuum tube means 4 volts? & how do I tell what voltage the large RCA tube is as there are only RCA 105 written on it. I'm making them into lights & want to make them glow. I understand that I connect the heater pin to the Ac/Dc adapter of the correct voltage. Thanks Craig

  • \$\begingroup\$ 5v+-.25v. At 14.5 Amps says the data sheet tubecollectors.org/eimac/archives/4400a(48).pdf perhaps more heat than light. \$\endgroup\$
    – glen_geek
    Mar 4, 2017 at 22:36
  • \$\begingroup\$ Can you find a datasheet for the tube? 3-6V covers most tube heater voltages, but big ones can need many tens of amps )not your average plug pack). If you've got access to a variable power supply (either a DC one with lots of amps, or a little AC variac feeding a beefy step-down transformer), try slowly cranking up the voltage until you get that nice yellow glow (unless the thing's an "indirectly heated cathode" in which case, you might not get quite so much light out of it as the filament may be hidden inside the cathode block) \$\endgroup\$
    – Sam
    Mar 4, 2017 at 22:39
  • \$\begingroup\$ The filament voltage and current are hardly important enough parameters to command centre stage on the envelope. \$\endgroup\$
    – Chu
    Mar 5, 2017 at 0:17

2 Answers 2


A quick search for "Eimac 4-400A" found a datasheet which says that the filament is 5 volts, 14.5 amps

A search for "RCA 105 Datasheet" shows the 105 is a mercury-vapour thyatron with a 5 volt, 10 amp heater.


Same here

  • 4A is the continuous Amp rating
  • 400A is the max surge Amp rating ......................


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