Roy's antique radio and electronics
5E1 ballast tube

A bit about Ballast Tubes

Ballast, or resistance tubes come in two main types
  1. Tubes used to maintain a constant filament current in battery operated radios. These contain a special resistance wire designed to maintain a constant current over a particular voltage range.
  2. Tubes used to drop the supply voltage in ac/dc or 32v radios. These tubes may provide some regulation, but not as much as type 1. Some types may be a simple resistance wire in a glass envelope.

The tubes can also be used in series with the primary of power transformers to maintain a nearly constant secondary voltage with variations in line voltage.
The Sentinal 80B farm radio uses a 5E1 ballast tube to regulate the total filament current supplied to the other four tubes. It is required because the tubes have 2v filaments, but one option is to use a 3v dry cell battery for the "A" filament supply. Using this 3v supply the filaments would draw too much current and have a short life, so current regulation is needed.

The effect of the 5E1 ballast tube is to maintain a nearly constant 500mA filament supply over an "A" battery voltage range of about 3.4 to 2.2v

The tubes are designed to draw the following filament current at 2v, making a total of 500mA
  • 34, 60mA
  • 33, 260mA
  • 1C6, 120mA
  • 1F6, 60mA
So how does it work? The answer is that the resistance wire in the tube is made of an iron alloy which has the property of increasing resistance rapidly at a certain current, this is when the wire is just below red heat. This rapid increase in resistance will prevent the current through the circuit rising as battery voltage is increased. The length of the resistance wire, and presumably the alloy, are selected to give the required resistance at a particular load current. The tube can therefore only be used in a very specific situation, in this case it is designed to give a nominal voltage drop of 1v at 500mA. The tube is also filled with hydrogen gas which conducts heat away from the wire to the envelope so that it will respond more rapidly to changes in applied voltage.

So the tube is a current regulator, but of course you could say that in this particular circuit it regulates the filament voltage at the tube sockets to 2v
A Sylvania manual says that the tube will maintain a socket voltage between 2.2 and 1.8v while the battery supply voltage varies from 3.4 to 2.2v

I was interested to see how well the tube worked, so I set up a little test using a four ohm resistor to simulate the 500mA filament load. The results are shown in the chart below. You can see that "filament" voltage varied from about 1.75 to 2.0v as the supply voltage was varied from 2.2 to 3.4v. Current was therefore kept at about 500mA. So it does work!

5E1 test results
My home page