Hi Liam,
I found you while googling for vital Marshall JMP amp info. I want to convert a JMP2204 from 6550's to EL 34's. My tech seems to think it is a matter of just re-biasing the amp, but I think I've read online there is more to this conversion than that.
Please email me details for this conversion at: MEDPAD@YAHOO.COM
I have some 1970's xf2 Mullards I want to put into this amp to get me back to that creamy rich Mullard sound. It is a US released amp made for 6550's, but, someone just dropped Groove Tube EL34's into it and I suspect they biased it way cold cuz distortion is fizzy farty at low volume levels.
THANKS!
Mark in Colorado