This is a bit strange, never noticed it before.
The Mesa Boogie Mk4 has a switch for "Tweed", or "Full Power". Basically a PSU voltage drop for reduced power output.
I had one in that just would not perform in Tweed mode, but was fine in Full Power mode.
I discovered that all 4 of the Electro Harmonix ECC83 pre-amp tubes would not work with the reduced voltage, but substituting 4 new JJTesla ECC83 solved the problem. The EH tubes were actually branded "Watford Valves Cryo" - Watford Valves put these things into liquid nitrogen to make them work better. Apparently. Cough.
Anyway, after a bit of investigation I discovered that this Tweed switch is actually on the mains transformer Primary, and switches in some extra windings. The result of which is all the secondary HT voltages are reduced by around
20%, BUT, so is the heater supply voltage. In this case dropping from 6.1v to 5.1v.I'm no expert in valve amp design, but this seems to be rather a strange design choice.
But then Mesa Boogie design their amps to work with their own branded valves, so they presumably know the reduced voltages, including heater, will mean their tubes will still carry on working.
Interestingly, I had another (non Cryo) Electro Harmonix ECC83 which didn't bug out at the reduced voltage, so perhaps this was a dodgy quad of ECC83, of perhaps something to do with the Cryogenic process?
Gareth.