I saw a tube amplifier circuit working and I was surprised by the heating of the tubes that can not even be touched, because they burn the fingers. Is this normal?
Yes, the principle of tube operation requires that the cathode be set at a high temperature to emit electrons. This means a heating that reaches hundreds of degrees, which is achieved with a filament that even emits the red light characteristic of the tubes. In some cases, in high power circuits, the anode (plate) of the tubes becomes a reddish color (stays hot) with the heating. This heat is transmitted to the valve shell (glass) so that it operates very hot and should not be touched.
