• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

12ax7 12at7 pin diameter

They should be ~1mm or ~40 thou as stated.
Most old stock valves will be the correct size, some of the new Russian made valves can be very slightly smaller, but should still be gripped by the socket pins if they have not been damaged.

You can test the grip with a 1mm drill. (Make sure the amp is switched off, un-plugged and the caps have had time to discharge first though!!)

A bigger problem with new Russian and other eastern manufacture valves, is the pins are more pointed, not rounded as before. This is a problem where the forked type of socket pin is used (a lot in the UK). It is easy to push the 'point' down the side of the socket pin rather than into the centre of the fork. Especially if the socket is worn or the valve pins not straight and parallel... This can damage or snap the fork.