Fuse burn in question

Your question came up some time ago and if you look here, you'll see a YouTube video for a DIY fuse burn in device.

All the best,
Nonoise

Regarding tubes, the components only need to be fully powered-up for burn-in. 
You can do this overnight,  but to be safe, don't leave tube amps on when away from home.

The current being conducted by an AC mains fuse in a preamp (or other line-level component) will be essentially the same all the time, as long as the unit is powered up and not in a standby mode. Playing music will make no difference in any burn-in effects that may occur.

That would not be true, however, in the case of power amplifiers or integrated amplifiers that operate in class AB or class D, as those amplifiers draw AC currents that fluctuate widely as a function of the volume of the music.

Regards,
-- Al