[turretslug]

I agree with Sideband- of all the valves in a set, the rectifier is usually the one with highest heater current and a prime case for proneness to contact problems in ageing holders and something with a 4V, 2.5A filament/heater needs as much good contact as it can get! A directly-heated valve heats up and cools down very quickly, so producing dips, bumps and splats on the HT if there is iffy contact and the valve is moved around, and consequent changes in transformer input power and limiter brightness. It sounds as if a good re-seating of the valve has solved the problem for now, but I've sometimes ended up either changing the socket for a good quality, tightly-gripping type, or (particularly hollow-pin octal types) having to clean and thoroughly resolder the bulb-to-pin connections.