According to an online reference some equivalents are 6DJ8, CV5358 & M3624.
If sticking to ECC88s you can roll valves from various makes until you find a flavour you prefer (or not). I don't know which brand MF used but popping the lid will soon tell you.
I would of have hazard a guess at Philips/JAN but without markings you could have anything in there. MF do have a reputation of "using what's to hand" during production, assuming they are the original valves and if they are they'll be very over due for replacing as their specs would have drifted way off acceptable parameters by now.
I know the Mullard brand is one of the favourite replacements in other MF gear.
Comments
According to an online reference some equivalents are 6DJ8, CV5358 & M3624.
If sticking to ECC88s you can roll valves from various makes until you find a flavour you prefer (or not). I don't know which brand MF used but popping the lid will soon tell you.
I know the Mullard brand is one of the favourite replacements in other MF gear.