harry

Hi,

Recently came across a JMP 50 bass head (May 1970) in a mates garage, having acquired it for £200 and took it to a local electronics buff who said the mains transformer was gone and god knows what else, he advised sending it to Marshall for a quote and indeed the mains transformer was gone, Marshall also advised changing the main switches, valves and pots and one of the fat caps. Service done and sent back, I also told them to send me back the bits that were removed old transformer switches valves etc. I havent got a cab yet to try the amp but I did try the old Brimar and Mullard original valves in a valve hifi system which seemed to be working fine. Does Marshall change valves in older amps as a matter of standard practice when servicing? I know this may seem silly but would it be ok to replace the old valves to keep the amp as original as possible and does it need rebaising? Is there much point in getting the old transformer rewound? I would have done this originally had I known.

Your knowledge is appreciated

Harry

Frankie

I would put the Brimar and Mullards in beyond the shadow of a doubt if they work. If you can put the power valves back in and bias them to the correct working range, then they work. It's standard practice for all techs to replace tubes when an amp is suspect. You've got a hell of an amp there, sir. The mains transformer is no big deal, all the magic is in the output transformer. Pair that with old valves and I bet that's quite a tone machine. Good score!

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harry

Hi,

just waiting for delivery of a Marshall 2061 cab 2x12, i'll put in the old pre-amp valves, I think the power tubes have a variable resistor across 2 pins, which I think is a self biasing thing, cant wait to give it a go.

Cheers for the advice.

Harry

Rockbert

No... a 50W Marshall is not self biased. The bias needs to be manually adjusted every time you change the tubes. If Marshall sent you the amp with tubes installed, presumably they already set the bias correctly. But if you put different tubes in there, it needs to be adjusted again! Otherwise you could possibly damage the amp or the tubes, or both.

janarn

Hmmm. Old Marshall amps didn't have adjustable bias.
Some change their amp so they can adjust the bias,
while others (like me) like to keep them original,
and that works fine, and have done that since the 60's.