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Old 13th November 2007, 05:42 PM   #1
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Default Tube Newb Mc Intosh questions.

I recoup 2 Mc Intosh MC40 out of service, I would like to re-wire it, for 15 to 25 w class A or AB1, which would be the best (simple) design based on 6L6 output tube, HT supply is 470V DC, provided by a voltage doubler, about 250V DC in conventional bridge).

http://www.freeinfosociety.com/elect...iew.php?id=500

Thank for some hints, tracks or links.

Richard.
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Old 13th November 2007, 06:00 PM   #2
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I would start by just rebuilding the original circuit but with more modern parts. It's a classic circuit design, very innovative for its time, and specific to the particular output transformer used. I also wouldn't mess with the operating points.

After it's up and singing, there are some potentially useful mods, most notably in the power supply and the cathode follower stage driving the finals. But get the original working first.
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Old 13th November 2007, 08:05 PM   #3
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Thank you SY

One of the amps has been hardly modified by the guy who gives them to me; he doesn’t remember exactly what he did about ten years ago, it’s why I ask all the this before re-wiring it as a classic MC40, the other one not in service is much less modified.
I will begin by the power supply and rewiring it as in the original schematic (voltage doubler), which rectifier do you recommend, the 150 uf doubler are they ok or a lower value will best to lower trafo transient current peaks, did a mosfet cap multiplier would be interesting for the input stages?
Are you sure that this class B design could sing?
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Old 13th November 2007, 09:09 PM   #4
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It was one of the best class B amps ever made. It's not my favorite amp, but it's still excellent and held in high regard by many (the collector value for a stock unit is mind-boggling).

I'd probably use high speed diodes for the power supply and modern 105 degree-rated electrolytics of a similar value to start with. Later, you can experiment with regulation, starting with the input stage.
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Old 14th November 2007, 03:46 PM   #5
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Ok SY, I will rewire them as they where, it could take some time...

I must change and replace some resistors, film caps and lytics, what do you recommend as general use resistors / caps and more particularly in critical parts, input stage, feedback, bootstrap, etc. Mills, Riken, Kiwame, Rel-Cap, Nichicon…

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