Most tube testers were not designed to evaluate balanced plate sections when testing a filament-type rectifier tube.
©2010, Bob Putnak, TubeSound.
When testing rectifiers such as 5U4, 5Y3, 5Z3 …have you ever wondered why one plate section tests stronger on your tube tester? Are all of your rectifier tubes really unbalanced?
I was recently sent an email asking for help to understand this topic. The answer is that most tube testers were not designed to offset the difference in potential between each plate section. For purposes of this discussion, assume that you are testing a rectifier tube with balanced plate sections.
1. This discussion is only relevant to filament-type rectifier tubes. While most people use the words “filament” and “heater” interchangeably, they are not identical.
A filament is a directly-heated cathode; the filament is the cathode and emits the electrons. A heater is an indirectly heated cathode; it heats a separate cathode element. Common examples of filament-type rectifier tubes would be 5W4, 5Y3, 5U4, 5R4, 80, 83, etc. Examples of heater-type rectifiers would be 5AR4, 6CA4, 6X4, 6X5, etc. Therefore, a 5AR4 would not be relevant to this discussion. Most tube testers can evaluate balanced plate sections in a heater-type rectifier tube.
2. The purpose of the article is to explain to end-users the problems they will encounter when trying to analyze a filament-type rectifier tube for balanced plate sections using their tube tester.
Why would anyone want to do that? Historically, old-school technicians did not put much thought into needing a “balanced” rectifier tube. Similarly, I am not aware of any vintage tube tester manufacturer that instructed the user to test a rectifier tube for balanced sections and to reject those that were unbalanced. Nonetheless, some modern tube buyers have been told that they need to buy rectifier tubes with “balanced plate sections”.
Most vintage tube testers were not designed to test for balanced plate sections. It was simply not considered important. As a result, most tube testers will test one plate section stronger than the other, leading the modern user to a false conclusion that the tube is unbalanced. In all cases that I am familiar with, the plate at the higher-numbered pin will always test higher, due to conventional filament wiring.