©2011 Bob Putnak.
I am frequently asked how to test voltage regulator (VR) tubes, which are sometimes called “glow regulator” or “glow discharge” tubes.
Few tube testers test VR tubes, and most models that claim to test VR tubes do a worthless job at this task.
- They use AC voltages instead of DC
- the AC voltages are beyond spec
- they offer no ability to monitor or control the operating current
- they provide no ability to test at the minimum and maximum operating range of the tube
- they provide no voltmeter to show the exact voltage drop across the tube.
(Hickok 123A cardmatic, Hickok 752, and Precision 10-40 would be notable exceptions.)
Since the overwhelming majority of tube testers will not test a VR tube properly, we need an answer. As a general observation, if a VR tube lights up, it is probably acceptable. But, that’s not good enough, so how to test a VR tube?
1. The best way to test a VR tube is to try it in the actual equipment, and measure the voltage drop across the VR tube’s anode and cathode.
2. If the first option is not available, you need to create a real circuit and measure the voltage drop across the VR tube at the minimum and maximum operating current as documented in the datasheet.
The datasheet specifications that are most important for VR tubes are:
- “dc operating voltage” or “average anode drop” — this is the voltage drop across the VR tube
- dc operating current range — the minimum and maximum operating current for the VR tube to regulate properly
- average DC starting voltage
Datasheet specifications for VR tubes are not identical for every manufacturer, but all datasheets are close enough to work from, so use whatever receiving tube technical manual that you have available, such as RCA RC-30, GE Essential Characteristics tube manual, or a Sylvania Technical tubes manual.