MC225 - how is this possible?

Discussion in 'McIntosh Audio' started by gregchristie, Jan 11, 2018.

  1. gregchristie

    gregchristie New Member

    Messages:
    15
    Hi all,

    After reading about the MC225 since I was a teenager, I finally bought one, in absolutely beautiful condition, with tube cage, and even original McIntosh branded output tubes.

    I received it today, replaced the 5751 that was in the first position with a tele smooth 12AX7. The 12AU7s were mismatched so I put in some RCA clear tops. I left in the pair of sylvania 12BH7s - they matched each other and had nice black plates.

    I opened up the amp and everything in there is original and clean. So on a whim I plugged in a source and a pair of test speakers and put it on a variac. I brought it up to voltage slowly until the filaments hit 6V and checked voltages. Everything was close, and NO hum or buzzing at the speakers - just the slightest tube rush hiss. I took it up all the way to 6.3 volts and the same.

    The only voltages more than a few percent off are at pin 5 of the output tubes. Instead of 0.8V, they were all between 0.35 and 0.4 volts.

    The sound was beautiful on my test speakers, no power supply noise even after an hour playing. Voltages after an hour were essentially the same as the first full voltage readings.

    How is it possible that this amp plays as is, without having to replace the Power Supply caps or the other electrolytics on the board? How are 50 year old electros NOT dried up and humming/buzzing? Are they just time bombs?
     
  2. Pio1980

    Pio1980 AK Member Subscriber

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    The doubler capacitors seem to go first, fwiw. That shows as lowered B+.
     
  3. damacman

    damacman Blown and Injected

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    Most likely it was used daily.
     
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  4. mnmmt

    mnmmt AK Subscriber Subscriber

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    Like a car, regular use keeps keeps things in operating condition. That is a keeper.
     
    unfairlane likes this.
  5. jlovda

    jlovda Things I loved from the 60's and 70's Subscriber

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    The big filter caps on the MC2200/MC2205 series SS amps just seem to keep going like the Energizer Bunny.
     
  6. jlovda

    jlovda Things I loved from the 60's and 70's Subscriber

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    Frequent use also keeps the unit warm and dry, greatly reducing chassis corrosion.
     
  7. gregchristie

    gregchristie New Member

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    Yes, perhaps it was used often enough to keep all components healthy. I'm still in disbelief. My plan for the next week is to run it for a little every other day or so, and repeat the measurements each time. That should provide some use-time for any drifted resistors to show themselves.
     
  8. cduarte

    cduarte AK Subscriber Subscriber

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    for what it's worth, the mc225 I rebuilt about 18 months ago sounded fine when I got it, but it didn't take that long for those aged capacitors to start failing. I used the Authenticap KTL3 250uf@250v caps for the voltage doublers, but they've now made available the KTL3-3, which is rated for 330uf@250v for better voltage regulation. I used the Authenticap KTL25 to replace the filter cap, which is 80/40/30/20@550v, 600v surge which added a nice increase in capacitance from the stock caps. I purchased them directly from Germany from this website https://www.die-wuestens.de/eindex.htm They mostly communicate by email, and they respond quickly and while this isn't a modern ecommerce site, once you total up what caps you need, they will send a paypal invoice which includes the added shipping, which is quite reasonable. I prefer to order directly from them, since the prices are less than half of what certain ebay sellers in the US get for those caps and shipping is quick enough and not costly at all, I've had a few different orders from them and it typically takes about 10 days to get to the east coast of the US. My standard practice these days when I rebuild vintage tube gear is to not even power them on before all electrolytic caps and certain other caps, especially the dreaded bumble bee caps that explode pretty violently and make quite a mess when they die. I also replaced the selenium rectifier with diodes and used a CL80 inrush limiter in place of the factory one. After replacing all of these components my mc225 sounded orders of magnitude better than it ever had previously. It was better than my other mc225 which had been partially rebuilt by audio classics, who left the bumble bee caps that act as high voltage snubber caps for the voltage doublers. I'm not sure why they didn't replace those, since they replaced the voltage doublers with mallory 250uf caps. I also replaced every original glass diode with new ones. I'd do the same if I ever get another mc225 but right now that's not likely since I already have one, and a pair of mc30s that are currently getting a lot of use.
     
  9. CohibaJoe

    CohibaJoe AK Subscriber Subscriber

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    Agree.....With a fine piece that you have...I would send this out for a Complete Restore regard less of how Clean and Nice sounding this is. Parts do get out of spec.
     
    Last edited: Jan 17, 2018 at 12:20 PM
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  10. gregchristie

    gregchristie New Member

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    15
    thanks all.

    I have replacement caps all on order. Authenticaps for the ps, atoms for the other electros. I also have vitamin q's lined up for when the bumblebees show their true colors (get it?) and start leaking. Plus the diode and thermistor of course.

    Two more tests and no voltage changes worth noting. But I will rebuild the power and bias supplies for sure before I trust running it without constant attention.
     
    Last edited: Jan 15, 2018 at 3:46 PM
  11. cduarte

    cduarte AK Subscriber Subscriber

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    did you order the 250uf or 330uf authenticaps for the voltage doublers? Also, since you're in there, I'd get rid of those bumbyle bees now. they weren't that great when they were new, and they don't age well. I had one explode on me and it really made a mess. It wasn't in my mcintosh gear, it was in something I'd sold a long time ago. BTW, on my mc225 when I replaced the bumble bees it made such a huge difference. You really haven't heard what that amp is really capable of until you do. Your ears will thank you. I'm still not happy that I had to sell my other one, but given that that was the only hifi piece I had to sell in the divorce, I should count myself lucky.
     
  12. gregchristie

    gregchristie New Member

    Messages:
    15
    I'm going with the 250's. Waiting on them to arrive.

    Thermistor got replaced yesterday, I will replace the bumblebees with Vitamin Q, but after the power supply, bias supply and the bypass electros.Then after the bumblebees, I'm fairly certain some of the matched resistors will no longer be matched.

    One step at a time, listening and taking notes along the way...
     
  13. lordgarth

    lordgarth AK Subscriber Subscriber

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    OP, when you get tired of boring sound coming out of that really old amp, send a PM & I'll find some use for it.:beerchug:
     

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