Testing - Continuity / Non-continuity

There are some vital tests that you can do with your multimeter on the continuity / diode check setting.  It helps if it has an audible beep to indicate continuity.

This is fairly crucial:

  

If you don't have continuity between the earth prong of the plug and the chassis (and also the shield connection of a guitar lead plugged into any of the jacks) stop now and fix it!

It's also wise to test all adjacent turrets and adjacent valve pin connections for continuity, just in case of "whiskers" shorting out the connections (you shouldn't get these if you've properly tinned your wires, but it's worth checking).  It's worth disconnecting the positive valve heater wire from the transformer for this test otherwise the valve heater pins on the power valves will show as shorted, due to the low impedance of this wind of the power transformer.