A customer bought this valve amp to me, asking for the volume control to be replaced because it was very scratchy when turned.

Volume pot and it’s PCB

No problem, I can do that…
The volume ‘pot’ (potentiometer) is soldered into a small PCB at the front of the amp, and the cost of a new part is £3. I fact I couldn’t economically source an exact part, but one with a longer shaft will do, we’ll just nip the top off. The new part does however have a locating pin (arrowed in red) which helps to keep it in place when it’s mounted.

Old and new volume pots, 100k logarithmic taper

Having got everything back together the scratching/static noise is still there, so I investigate further to find that the valve plate (anode) power supply isn’t at the right voltage, and the cause is a blown fuse. There appears to be no reason for the fuse to blow, so I replace it.
All working again (and the VU meter, based on the 6E2 valve, works too which I should have noticed in the first place)

Diagnosis on the bench. Testing different volume pots (bottom right) just in case the replacement one was faulty. Outboard fuse (the REAL problem) is at bottom