MC225 - how is this possible?

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

  1. gregchristie

    gregchristie New Member

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    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?
     

     

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  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 Subscriber

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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.
     
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  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.
     

     

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  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 Well-Known Member

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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
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  10. gregchristie

    gregchristie New Member

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    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
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  11. cduarte

    cduarte Well-Known Member

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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.
     

     

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  12. gregchristie

    gregchristie New Member

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    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...
     
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  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:
     
  14. cduarte

    cduarte Well-Known Member

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    excellent, I'm really glad to hear that. If yours has a selenium rectifier, please replace that as well if I haven't already mentioned that (I see that I did, but it's a pretty important because selenium rectifiers emit poison gas when they fail and keep in mind I said when, not if. It's really easy to do. You can look at the mc225 rebuild thread I did when I was doing the rebuild and after tuesday I'll be able to take more pictures of the inside. My tech has a mod that he does on MC240s that adds power and reduces distortion. He adds 2 more diodes to the power supply to make it a true full wave rectifier. I haven't yet had that done to mine, but once I do I will post pictures. That's part of his standard rebuild package for MC240s and he sees a 7%+ increase in wattage and dramatically lower distortion. I'd do it myself, but I'm not quite sure how he implements it. I'm very good at swapping parts, but doing mods like that is a bit beyond me unless I see one he's done already. I also struggle with soldering since my hand tremors have gotten worse over the last two years. I'm in the process of moving due to a divorce and fortunately was able to keep all of my hifi gear. If you need any help, please feel free to ask questions and I'll help as best as I can, but one of the best things about this forum is how many truly knowledgable people post here and they're all extremely helpful. Even Terry DeWick posts here from time to time and if you don't know who he is, he's the best McIntosh repair tech in the country, especially for vintage gear. My tech is extremely good as well and finding most issues is childs play for him.

    Once your amp is rebuilt you will have one of the best vintage tube amps ever made and it is better than most, if not all new tube gear made today. Right now I'm in love with my rebuilt and modified MC30s and am using them, but I always end up going back to the MC225. It's just that good. If you're running very efficient speakers like Klipsch or vintage Altec, I can't think of a better amp to power them with. BTW, the same holds true for my C11 preamp. It holds up very well even against modern McIntosh tube preamps like the C2500, where the only real advantage the C2500 has is the lower noise floor. At his request after I got it rebuilt, I put it in place of my brother in law's C43, connected it to his MC501 monoblocks and B&W 802s (which really need the nuclear power that those MC501 monoblocks provide) and the improvement was so huge that after about 5 minutes, his response consisted of him looking at me and saying F@#$% you, and then immediately found and purchased a C2500, which is also an amazing preamp. My only concern with that preamp is longevity. It will always be possible for a competent tech to rebuild the C11, but if McIntosh runs out of spare replacement boards that contain integrated circuits, it may become impossible to repair. I greatly prefer the simplicity of the vintage gear to the new stuff, and the lower cost both in purchase, but ease and cost of repair will always be lower.
     
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  15. Bill Ferris

    Bill Ferris Super Member

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    Do you folks take advantage of Mac providing the 125 volt power transformer tap change over ?
    I have done so on my MC 225, MC 30`s, & MC 60 as my house voltage can be as high as 123~125v and doing so gets the important heater voltages a lot closer to spec. and after doing the tap swap, I noticed the amps seem to run cooler as well.

    Just a thought.

    Regards, OKB
     
  16. Pio1980

    Pio1980 AK Member Subscriber

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    I did it on the three amps I refurbed for the gent in Atlanta.
     

     

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  17. Bill Ferris

    Bill Ferris Super Member

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    Yes Pio1980, I seem to recall you indicating that in another thread sometime last year.. :beerchug:
    Well worth the 5 minutes to do so, IMHO.
     
  18. Brice

    Brice AK Subscriber Subscriber

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    The MC225 has only a primary 117VAC with 2 black wires. No other tap here.
     
  19. Pio1980

    Pio1980 AK Member Subscriber

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    That should do ok.
     
  20. gregchristie

    gregchristie New Member

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    I'm consistently hitting 6.3 or 6.4 volts on the heaters without the variac in play, so unlike some other amps I've rebuilt (I'm talking to you, Quad II's!) this won't be an issue for my MC225.

    Spent two hours Friday matching resistor pairs and fours - some of the critical ones have drifted .

    The caps have all arrived, hope to get started for real on the refresh next week.

    And yes, the selenium will go...
     

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