Valve amps generally require very little maintenance. 

Sometimes, if it’s been turned off for a long time, and dust has fallen on the valves, when you power it up and the valves heat up, you can smell the dust cooking off the glass envelopes. This is harmless, but if you’re worried about it, you can give it a light touch with a feather duster from time to time when things are switched off and cool.

In Fixed-Bias design amps, every few months the output valves will require a little tune-up. Each valve usually has a recessed adjuster and test point somewhere, and it’s a simple process to adjust it until the right number shows on a meter.

Also after around 8000 to 10000 hours’ operation, output valves uusally need replacing.

Other than that, there are no specific maintenance requirements.