top of the 12au7 tube doesn't glow

[agent10508]

During the glow test of the tubes, only the bottom of the 12au7 glowed while the top remained unlit. The stock tubes provided are labelled Beckman which I believe are rebranded Sylvanias. The main power tube glowed excellently, so should I be worried or is this just a quirk of the tubes provided?

 

[Paul Birkeland]

Whether the top and/or bottom glows depends on the position of the heater element inside the cathode sleeve, so if you don't see a glowing dot on the top of one side but you do see it on the bottom of that side, all is well and there's nothing to be concerned about.