Sometime I’m curious how hot your tube is? Well, this short experiment will give you some idea. Different tube may have different thermal characteristic (simple logic: the higher current on the filament, expect higher temperature).

Off course we can use probe to measure the surface temperature. But I’m thinking another approach. So I get myself an infrared thermometer to examine the temperature in more detail. As example, I use Russia 5U4 Rectifier tube, rated at around 2A filament.

Below you can see the temperature on different position on the glass after around 30 minutes of usage. It looks like the top side of the glass accumulate highest temperature, over 100 degrees of Celcius. Meanwhile the tube base maintains around 48 degrees. Not too hot, but quite warm at long period of time (which could break the glue and make your tube “loose” from its base).