1974 Marshall JMP 50W MkII (model 1987). These came with 6550 tubes in the US, and EL34 everywhere else. I’ve had to talk a number of people out of having me convert them to EL34. People get it in their head (or get convinced by the web forum “experts”) that the 6550 doesn’t sound as good. But in my experience, any time the 6550 didn’t sound “right” it was because somebody didn’t bias the amp correctly. I’ve never had anybody regret keeping the 6550 after I set-up their amp.
This amp is here because the original Daly filter caps, and Erie bias caps have finally reached their end. The amp is hissy and noisy. It is getting F&T filter caps, and Sprague for the bias. I also found that one of the Cliff input jacks wasn’t closing the contacts after unplugging the cord, so that is getting fixed.
After changing the caps, cleaning all the pots, jacks, and sockets, and setting the bias correctly, it is back to sounding great. And it looks great, too!