Faultfinding.

[pre65]

Never discount anything.

After replacing a valve base on my Pre3 the other channel stopped working.

I checked things through and could not find the fault. To cut a long story short it was one end of an interconnect had a broken wire.

[Firebottle]

In my experience the great majority of faults in electronic equipment are mechanical in nature.
Broken wire, dirty or loose contact, dry joint etc.

Alan

[Dave the bass]

Yep, I find that too. 30+ yrs in the Service side of Electronics I've found faults often occur where there's a Human interface with the machine, say like a knob/button/lever which'll either go noisy, fall off or get damaged physically. Next in line is the device had been dropped, repeatedly sometimes. Then comes fuses/leads and interconnects of all sorts, then finally comes an electronics fault, usually where either High Current or High Voltage is in use.

The remaining ~ 12.6543654321% are proper trixy

DTB

[rowuk]

I have had my share of aggrevation with all sockets except octal. even althour measuring with an ohmmeter was fine, the tubes act funny when the pin doesn't contact the "right" part of the socket.

Serious noise and distortion but everything measures well. Now I have a small wire brush to clean pins and sockets for every change of valves.
http://www.millrose.com/micro_twisted_wire_brushes.php

The cheap UX-4 and UX5 ceramic sockets also don't seem to like tube rolling.......