DIYbanter - View Single Post - Another old-timer tube/valve question

Ralph Mowery

In article ,
says...

Vacuum Tubes (Valves) work by heating the cathode to "boil off"
electrons. The 5Y3 has a 5v filament that draws 2 Amps. So that's 10
watts even without any current flowing in the plate circuit. So, yes,
I would expect it to get hot quickly.

That is not even counting on power flowing in the tube as you mentioned.

With out looking it up, I would expect the tube to drop around 20 or
more volts. So at 100 ma and 20 volts, that is another 2 watts to add
to the 10 watts of just the filament.

Even some of the smaller glas tubes can get hot enough you can not pull
them out after short time unless you have asbestos fingers.