How hot do the power tubes get?

[armandodiaz]

I've owned tube amps, since the 80's, but I've never paid much attention until I started building my own.
So far I've built a JCM800 and an 18W and they both sound amazing.
However, since I have played them both with and without an enclosure, I've noticed just how hot these power tubes get.
When I mean hot, I mean you can warm your hand on a cold day, kind of hot.

I've double checked the voltages and everything is within range, even a small bit lower than spec.
I have the bias set at about 70% on the 800. The 18w has no adjustment.
So my question is, is this right?
I would hate to start blowing tubes.

Thanks,
Armando

 

[Mentalo]

„When I mean hot, I mean you can warm your hand on a cold day, kind of hot“
is a bit of an understatement imho, the power tubes on my amps are more in the „you can fry bacon on them“ territory. I never measured them directly, but I guess they will exceed 150°C easily

 

[PelliX]

They get hot. Too hot to touch without instant burns hot. It's simply how valves work, as a given amount of heat is required for the cathode to start boiling off electrons inside the vacuum. This is why they have heater filaments inside which - as you can see - are glowing hot when operating.

In a head style design that's not so bad. In a combo design (where the valves reside below the rest of the amp, typically) you may want to install a little extractor fan or at least ensure some airflow. The valves themselves are meant to take it, the capacitors, circuit boards, etc less so.

 

[armandodiaz]

Thanks to all for the replies. I feel better now.

 

[Pokey]

They get hot, if it's biased more like as class A they will run hotter.

 

[PelliX]

All kinds of things will influence the temperature. Plate voltage, the power they're actually dissipating which is also based on the bias. Cathode biasing is also often harsh temperature wise in practice. Airflow or the lack thereof. Fact of the matter is that much like a proper lightbulb they will get hot by design. The outside is of course a lot cooler than the filament inside, but enough to give you a good burn or even help heat a small room...