Any max temp specs for common tubes?

[6v6heaven]

There's much debate on optimum bias setting for common tubes. Are there any max temp specs available that can be read with an IR gun?

[MWJB]

For that to work you would need a database of readings from various amps, fitted with various brands of tubes (for which data sheets are often unavailable or erroneous/assumed). Other factors would come into play like ventilation - it seems that it would add to confusion rather than prevent it. It's no good knowing that your power tubes will take so much heat safely, if that is still enough current draw to blow rectifiers & burn out OT primaries.

There's not really that much debate about tube bias, whatever sounds best & isn't damaging the tubes, caps, or OT/amp is what's important. "Sounds best" is subjective & a IR gun isn't going to determine that. There is empirical data around to draw from.

Most amps don't need to run extreme bias settings to work properly, some single ended amps like SF Champs run their tubes way past max plate dissipation, but are reliable.

[Steve Conner]

Most tube datasheets (NOS ones "back in the day") quoted a maximum bulb temperature of 250'C.

The temperature of the tube base is also important, because the bakelite-type material used for octal tube bases can start to leak current at high temperatures. So, if the tubes are mounted upside down, the bases will get hotter, and they may go into thermal runaway from the leakage current before reaching the 250 degree maximum bulb temp.

I lick a finger and quickly dab it on the tube. If it sizzles, I start to worry. If I smell burning flesh, that's probably bad too.

Also, most tubes have the plate nearly red hot when at their maximum rating. If you can see any red in a darkened room, you're running them a bit too hot. The cheap cameras on many cellphones, low-end webcams, etc. can see the "red" at lower temperatures, while it's still infrared. They usually render it as a bluish gray. If your phone camera can see anything, you know you're getting on for the maximum rating.

An IR thermometer may read the bulb temperature, or it may read near IR coming straight through the bulb from the plate. It all depends what wavelengths it uses.