rob aylestone
Moderator
Lots of new members join to sort a single problem, and this guy hasn’t been around since 2019, so sadly wont read the reply. It’s easy to miss the fact you’re replying to a long dead topic. Welcome, by the way!
Personally I take these "night and day" claims for differences between valves of the same type number with a huge pinch of salt. Valves are made to the same precision as watches because they are expected to work in equipment just a well as a replacement. Yes, critical stages, a field OP valve say, some adjustment to the amplitude (height) and linearity would be needed but that is because the old valve had lost emission and the pots tweaked during the life of the telly.When you go back through this topic, it is actually useful stuff - because as each year goes on, valves get more sort of 'magic' and mysterious. Nowadays, we're looking at swapping identical valves to find the 'best' sound, while years back, you swapped them to find a working one. I remember an early job where the youngsters got the valve testing job - huge box on one side, three on the other labelled Good, OK and U/S, which then we had to stop using to describe unserviceable items - perhaps the first example of a word becoming 'offensive'. One American engineer took great exception to the term, and in fairness, we kind of got it.
I suppose the point here is that swapping valves of the same class does give you differences. Maybe we did lose something with absolute identical performance between solid state components. Maybe this drove the folk who have moved the valve 'magic' to things like capacitors?