112RD 50 distortion in clean channel

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Author Message   johnmusic Posted on Monday, January 05, 2004 - 03:15 am:    I'm hearing a distant distortion when I play darker jazz settings. It is subtle, but in a recent recording session, the engineer remarked about it. I tried new tubes and hooking up a different speaker. It didn't help. Is this the nature of this amp, or is there something that can be repaired?   johnmusic Posted on Tuesday, January 06, 2004 - 04:20 am:    Hello? Is anybody here?   Terry Posted on Tuesday, January 06, 2004 - 09:03 am:    I would say the amp may need a cap job after 20 years.   johnmusic Posted on Thursday, January 15, 2004 - 02:44 am:    Thanks Terry, Can you explain what a "cap job" would entail? Which capacitors should be replaced? Any help is greatly appreciated. I'm amazed that this 20 year old amp still plays.   TERRY Posted on Saturday, January 17, 2004 - 09:48 pm:    HI JOHN, TO DO A CAP JOB, I WOULD REPLACE ALL THE BLUE MALLORY CAPS AND SOME OF THE DISC CAPACITORS THAT ARE STARTING TO GET STICKY. I WOULD ALSO REPLACE THE ONES ON TOP OF THE POWER TUBES. THIS IS BASICALLY WHAT A CAP JOB WOULD BE. THE TECH WOULD ALSO LOOK AT SOME OF THE OTHER YELLOW CAPACITORS. BUT THESE ARE USUALLY OK. TERRY   johnmusic Posted on Monday, January 26, 2004 - 03:05 am:    Thanks Terry, I'll give it a go.   Anonymous Posted on Saturday, August 28, 2004 - 10:36 am:    puo   Chris Metcalfe (chris_m) Username: chris_m Registered: 10-2006 Posted on Saturday, November 04, 2006 - 05:35 am:    I had the same problem - RD50 distorting on clean channel. I did full electrolytic cap job, which helped a lot, changed TL072s, changed tubes,cleaned everything - but what finally fixed it was altering the bias point to pass about 10ma idle current ( see my recent post on what worked for me in altering the bias point in these amps - I thinks it's in 'circuit modifications').Before that, there was annoying crossover distortion when the amp was idling at about 3/ 4 ma.per tube. Perhaps the original design was set at 6 or so ma. per tube by the MM engineers, but certainly mine had drifted lower over time. Also, my guess is that the amp was originally calibrated for use with older 6l6s, whereas newer 6l6s such as sed and JJ need a different idle point - that's certainly true in all tube amps such as fender, and seems to be true here. MM amps idle much closer to class B than to class A, so bias seems to be pretty important, both in terms of not too much idle current ( lots of posts on that subject here) and also not too little. At least, that's been my experience. Chris