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Ballast tube is a resistor mounted within an glass or metal envelop and connected to a tube base. This tube plugs into a regular octal or four-prong (UX) type socket. Ballast tube, besides dissipating the excess voltage, also compensates for voltage variations of the line, making it constant across the load. If voltage increases, ballast resistor gets hotter, its resistance goes up, and the voltage drop increases. If voltage decreases, ballast resistor gets colder, its resistance drops, and the voltage drop decreases. Some ballast tubes also have a voltage divider to supply current to the pilot lamps. The RMA code for ballast resistors consists of a prefix of one or two letters, a central number, and a sufix of one or two letters. The number gives the total voltage drop, including that across the pilot lamp, if it is the case.

Ballast Resistance Code Table

Prefix Indication
B Ballast action on the pilot light. It may be ommited
Type of pilot lamp - 6.3 volts, 150 ma (No. 40 or 47)
Type of pilot lamp - 6.3 volts, 250 ma (No. 44 or 46)
Type of pilot lamp - 6.3 volts, 200 ma (No. 50 or 51)
X Ballast tube has a four-prong base (UX type). It may be ommited
Number Indication
Any Total voltage drop across the resistance, including the pilot lamp(s)
Suffix Indication
First letter Circuit arrangement of the ballast tube. See basing diagrams below.

Last modified: Feb 15, 2016 20:14
Cape Old Radio - Since Nov, 1996
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