people might remember an old post i made complaining that i wanted more controlled bass in my marshall jmp50 (btw plate voltage = 420v). i made a little experiment, and thought some might be interested in the result.
in fact i mostly solved this with new power valves, which i didn't expect as it seems only recently i changed them (in fact i worked it out and its been more than a year after all)
anyway, meanwhile i invested in ebay on two used mullard ecc83 made in great britain from back in the day. they cost twenty pounds each, which i dont think was too bad seeing as they look as if they've hardly been touched.
its definately true (as people noticed here) that putting one of these great old tubes in V1 gives the best improvement... but i must say the JJ ecc83s don't embarass themselves at all, if anything i'm more impressed with them now - they are certainally great tubes for the price.
i'd say the JJs are a bit noisier, but less microphonic.
bass response is obviously more controlled with the mullard, which is what i wanted. the mullard has a little touch more of the 'valve magic' too, and its noticeably smoother... though i quite like the comparative grit of the JJ. gain-wise, i'm not sure which is higher. my initial feeling was the mullard has more gain, but after playing the amp for a bit, i'm not so sure anymore... actually i think the mullard has less gain, but sounds richer.
i tried the last mullard in V2 and V3. was actually a bit dissapointed with the mullard in V3, JJ sounded a lot better there - not sure why. so the last mullard ended up in V2, and it had a positive effect there, again more controlled low end - although the other benefits I saw with V1 are not really that noticeable.
finally i swapped the mullards in V1 and V2 - and there was a difference in sound. the valves looked the same - but didn't sound the same. swapping the JJs around never had any affect when i tried it before.
i need to get used to the amp again now, it definately handles alot differently. i'm happy with the low end now, although i miss some gain. about the sound i dont know - but its definately more sparkly.
so to agree with other posts on here suggesting a nice old tube in V1 - good idea experimenting to see if its worth it for you. differences in the other positions are very slight - and wouldn't be noticed by many cranked at a gig, or miced up.
