Recently acquired a late '83 RD50 112 Mid Shift (Rev E) model that was basically DOA. Very distorted at all volume levels, and a slight burning smell. Sure enough, R62 (470 ohm resistor) was toast, and a failed D9 (zener diode) was the culprit. Apparently this is a common problem with these amps, I've seen it twice in the six or so RD50's I've worked on. Replacing these two components brought the amp back to life.
Preparing to give the amp a full cap job (replacing all 12 electrolytic caps), I noticed that five of them - C17, C25, C26, C40, C41 - are 10mfd/25v. Every RD50 I've seen and all schematics show 20mfd/25v caps in these positions. Three of the caps are blue Mallorys that look and date to the amp's manufacture, two are smaller black NTEs which I believe are replacements.
Has anyone else experienced this? Should I replace with 10mfd as exist, or the "correct" 20mfd caps I would normally use?
Thanks.
Update: Another oddity, when I compare this amp's Distortion circuit to other Rev E models, caps C10 and C48 off the12AX7 cathodes are 20mfd/25v, and not 2mfd/25v as on the schematic. And the caps look to be factory Sprague silver cans. Hmmm, wonder if this has anything to do with the 10mfd/25v mentioned above...Amp sounds fine since the recap (with 20 mfds replacing the 10 mfds), but I'm tempted to try 2 mfds on the cathodes and see if that makes any difference.
I guess the question is
I guess the question is whether or not it worked well that way. If it was working and the owner was happy, leave it alone and replace with what was in it. It may have HIS tone and you might upset the balance of the universe if you change them! If it is yours, change them but keep the others and see what it sounds like. I have personally NOT worked on any 50's so I am just an opinion in the dark! Mike.
Basically, the amp is mine...
...to do whatever I want with. The person I got it from doesn't play, he got it from a relative who did, but doesn't know how long it sat unused. Based on the few changed caps and the metal oxide plate resistors, looks like it was (partially) serviced at some point. Should mention that one power tube was missing its guide pin post and installed incorrectly when I pulled it. I'm guessing that could have fried the zener diode if powered up, which in turn cooked the 470 resistor. As the broken piece was nowhere to be found, I'm betting the tube's been in that way for a while.
Even without a recap, the amp sounds great, like a healthy RD50 should. The cleans are big and punchy, and the Distortion mode is quite usable (being the latest revision circuit). In fact I will likely upgrade the Distortion circuit in my early RD50 110 to match this 112s.
I actually always wanted one
I actually always wanted one of those but never got around to getting one and now I just don't need it. Cool amps.
IMHO...
...the RD50 may be the finest small/light/loud/clean amp ever made. Factor in what they go for $-wise and their Made in USA heritage, and I can't think of another amp that comes close.
I have now seen three RD 50
I have now seen three RD 50 112 amps, all built 11/83 - 12/83 (two by the same tech) that had the two 20mfd caps on the 12AX7 vs the schematic indicated 2mfd caps. One is a blonde pinwale/wheat grillcloth, one is a black tolex/silver grillcloth, and the last is a black tolex /black grillcloth model. All have the Mid Shift control, and all sound great after a recap, etc.