BALLAST TUBE
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 |
K
L
M
|
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. |
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