I recently replaced the input in my marshall vs100 and now after playing for a couple of minutes the sound level drops.

However when I hit the guitar really hard the sound returns back to normal volume for the duration of the hit.

I don't know much about amps but I'm thinking it's either a dodgy volume pot, a dodgy valve or my dodgy soldering.

Any help would be appreciated.

I'd like to know what I'm doing before I commit to taking the entire thing apart again!

Won't be a valve, the only thing in there is a pre-amp tube, which controls the gain section. But I'm not sure what the tube does on a VS.

If it's volume it might be a pot.

But I'll let someone who knows this contribute as I'm not sure myself.
Artist with Boult Guitars 2017-
Galaxy Collection X4

Ashdown CTM 100
Ampeg SVT
try a diffrent guitar cable, yours might be dodgy
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate