Vacuum tubes found in common guitar amplifiers and most audio equipment can range from cool to the touch to several hundred degrees Fahrenheit. A typical 12AX7/ECC83 in a normal (low current) gain stage can usually be grasped with bare hands even after running for several hours. In some applications 12AX7’s can become too hot to comfortably handle with bare hands. Higher current preamp tubes such as 12AT7/ECC81’s, 12AU7/ECC82’s, etc. can run from warm to the touch to “hot enough to scold you in short order”.
Power and rectifier tubes generally run much warmer than preamp tubes and in some cases can exceed surface temperatures of 350F.
My amp feels like it’s going to burn up! We get a lot of inquiries and comments from customers that have guitar amplifiers where the tubes are suspended underneath the chassis. This would include most Fender amps, and in general most combo’s.
The comments usually go along the lines of:
“By mid band practice my amp’s chassis is too hot touch!! Something must be wrong?!?!”
Our response is generally:
“This is totally normal and a byproduct of the heat from the tubes rising right up into the chassis.”
However, what we’d like to say is: “To be totally frank something is wrong! Your amp was built/designed in such a way that the heat from the tubes rises right up into the chassis and all the corresponding circuitry. This “heat-soaks” the amp in short order. This generally leads to reduced life of most PCB based amps and is more stressful on internal components and wiring.” We usually keep that comment to ourselves… With that said, some clever PCB design and properly spec’d parts can greatly mitigate the heat/convection problem. Of course using a head/cab setup with the tubes mounted above the chassis will always yield the best tube/component life.