Bench testing power tubes

Here I am testing a pair of 4-65A's that I bought off ebay. One was good, one the vacuum leaked out, or the air leaked in depending on your point of view. Below is the good one running at full 65 watt input, 650V @100ma. I like this picture so much I am going to use it as the title picture for my blog.

Here is a shot of the test setup. The Heathkit power supply is for screen and bias. The plate supply is to the left and can supply up to 1KV, controlled by the variac. This same setup was used to thest the big RCA 7213 shown further down this blog.

Here is the bad one. The filament was about half brighteness due to air in the tube conducting the heat away. Unfortunately it burned out before I could get a good shot of it. The milky look is smoke inside the tube. (Click the picture for a larger view.) You can see it swirling around near the bottom of the plate.

All glass power tubes without a flashed getter are prone to gas when not used for long periods of time. In this type tube the plate is the getter. High temperature operation of the Tantalum plate cleans up any gas

in the tube by chemical reaction. It is important to rotate your spares to keep them cleaned up and gas free.