Grateful Dead Music Forum

A place to talk about the music of the Grateful Dead 

 #105822  by rugger
 Wed Nov 30, 2011 5:41 pm
Hey All,

I'm about to take my mc250 into my amp tech. Any particular suggestions/mods to help achieve "the tone?" Or is this just a standard amp service job?

Thanks in advance.

John in San Diego
 #105832  by waldo041
 Thu Dec 01, 2011 4:37 am
if your in need of a new multi cap can, i have gotten a couple from this place. the can itself is a little different looking then the original, but it is not all that expensive and does the job.

 #105847  by rugger
 Thu Dec 01, 2011 9:39 am
Thanks for the heads up Waldo, if we need specialized replacement parts I will look there.

Before we get there though, am I just telling him to do a cap job? Are there any parts value changes I should be looking at? Any specific mods? I poured through the first three pages of the Jerry Tone section and didn't find anything (search hasn't been my friend either), I'm looking for specific do's and don'ts--basically, I want my hand held (sorry, I'm insecure). :roll:

Thanks again, :hail:

John in San Diego
 #105848  by rugger
 Thu Dec 01, 2011 10:13 am
OK, I finally found the best example of what I was looking for--what's another hour of scrolling through threads on this site? :P

If anyone still wants to add something to this thread, please feel free.

Brad from last year:
I replaced all the electrolytics except for the two large GE cans. Those things seem to hold up pretty well after all these years. Also, they're real pricey to replace, at least $20 each at the bare minimum. Not real pricey I guess, but way more than all the other cap's put together.

What I did was leave the two silver smaller can's in place, but inside the amp I disconnected them. The replacement cap's of the same values are SO much smaller than they used to be, so they all fit very nicely inside there. The only trick was the silver can closest to the front of the amp is actually a single and is for the negative supply. That cap's polarity was a bit confusing at first when looking at the amp because the + side goes to ground. Electrolytic polarity is CRITICAL because if you reverse them, they are like a dead short and things can fry real fast. Plus they can pop or at least smoke and ooze some nasty liquid.

Also, the 2 10uF cap's on the input board are for coupling audio, so I used poly film types instead of electrolytic since film is a far superior audio coupler. I didn't quite go to 10uF, but went 2.2uF instead. Still plenty of bandwidth there, and for Jerry style guitar, not an issue at all. I believe that some of the Mac's even used to use 2.2uF there instead of 10uF.

At first Tracy's Mc250 bench tested about 63 watts on one channel and 68 watts on the other. Kind of mushy and sluggish compared to what it should sound like. After this re-cap job, both channels were delivering a solid 82 watts per channel right at clipping. This particular Mc250 of Tracy's was by far the cleanest circuit I've seen in one yet. Outside had some metal pitting, but inside, I swear this amp had never been abused or even opened up before he got his hands on it. Every component was stock, zero charring around the Sentry Monitor resistors as we see in most used Mc250's. This baby came right back to life with tons of clean power. I love these amps.

John in San Diego
 #106271  by TRG
 Mon Dec 12, 2011 6:00 am
Long time lurker that has learned and applied a vast amount of knowledge from this forum over the years,
so thanks to all that contribute and share their indepth knowledge! I just picked up an MC 250, but
unfortunately don't know how to analyze/work on it. I know what I want to hear from the amp, but don't know how to
get there mechanically.

The first thing I did when I got the mac was took a road trip to an authrorized mac service shop with the
hopes of leaving it and having it "refreshed" and brought up to spec. The tech was nice enough to place
it on the bench for me while I was there and said that it tested at 65 watts on each channel.
He said that's exactly what I want to see and that there's nothing that should be done to it at this point...
that it should be "left alone". I must add that he was horrified that I was using it as a guitar amp and told
me that these are not made for musical instruments and will not sound good. I told him it was by design :-)
Anyway, after reading through this forum, it sounds like I should be getting atleast 70 watts/channel and at the
very least should have it re-capped.

Since then I've called a different mac authrorized service shop and was told that it was incorrect to think that
I will get anything other than 50 watts/channel on the MC 250. So now I know that the two options (mac authorized)
I have within a couple hours of driving distance are not going to be good options. I'm going
to now look for a general amp tech and just ask him to recap it.

Finally my 2 questions:
Are there specific caps that I should request/purchase to keep the authentic mac sound, or is it just as simple as making
sure that I meet the required power requirements (I do have the service manual w/parts list)?

Also, if I'm currently at 65 watts...does that mean that I won't have as much headroom, but will still be able to
reach clipping, OR does that mean I won't be able to get to the "clipping zone" when the amp is only pushing to a max of 65 watts currently?