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Old 21st July 2007, 10:23 AM   #1
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Default mcintosh mc3500 clone

i have a question for the knowledgeable. was/is the linear technology employed in the mc3500 amp a stable and fiable approach for replicating. the amp schematic is readily available but i find the transformer winding, turns, iron mass information is scarce. these units are selling for way over 10000 dollars u.s. and i thought cloning one might be a worthwhile project. any and all thoughts on this subject needed. thanks
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Old 21st July 2007, 05:53 PM   #2
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Hi,
Cloning one is rather straight forward, if you have the original output transformers. And that is a big "if"! The outputs are a unique penta-filar design (5 side by side windings for the primaries) that would be cost prohibative to make now, but there are people that will do it if you are willing to meet their price. Dennis Hoyer of "Audio Transformers" comes to mind. Do a search on him.
Anyway, once you get the outputs, basically you will just have to start collecting the other parts. Plitron makes a 175-0-175 transformer rated at 750VA or 1000VA that would make a good B+ transformer. The other voltages are easy to come by with other transformers commonly available.
As for the front end on the MC3500, several "upgrades" can be done without changing the basic circuit. Like, using a CCS on the cathodes of the phase splitter and using a regulated negative supply rail. The original front end circuit does the job, but isn't that great. Do a search for "Zed Audio" and e-mail him. He has a pair of these amps that he has restored.
As for the output transformers, each of the primary windings is rated at 150 ohms CT. There are 4 output windings; 2 each of 4 ohms with 1 ohm taps, and 2 more windings at 1 ohm each. with this combination, out can achieve output impedences between 1 ohm and 64 ohms. Each output weighs around 50-60 pounds; they are heavy!
As for the ratings, they can do 500 watts RMS easy.
I have been collecting parts to make a pair for the last 4 to 5 years; I guess just to say that I have a pair. No pun intended.
Bottom line is, these Macs are a money pit to build, even if you already have the output transformers. Each of the output tubes will cost you $50 each, and if the amp isn't properly set up, the amp will eat them up. There are much better sounding amps that can be made (think triodes with no NFB).
Daniel
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Old 22nd July 2007, 11:31 AM   #3
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daniel, your final statement about being better units to build is what i was looking for. however, it seems you have also chosen to try your hand at a restoration. the main thing that peaked my interests is that people who know a great deal more about these amps than i are willing to pay 15-20k for them. that is what made me think they were or are superb. i was thinking of winding my own trannys to save the money. the specs are what is hard to come by. i have access to extensive motor winding equipment, ie vacuum tanks, ovens, pressure epoxy etc. i will google those sites you recommend thanks
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Old 22nd July 2007, 03:54 PM   #4
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hey-Hey!!!,
Just get a single sample ofthe output. Get it unwound by somebody skilled at it. Heyboer TX in Michigan comes to mind. They've done some good work for me in the past, and on outputs at least as complicated as the McIntosh's. They've been less expensive than Doc Hoyer too.
cheers,
Douglas
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Old 22nd July 2007, 07:22 PM   #5
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Hi again,
Don't get me wrong, any Mac will sound good and they spec out excellent, there are just other amp topologies that sound so much better.
I would venture to guess that the people willing to spend the crazy money for the MC3500's have a lot of money for spending on stuff like this and want to own the "rarest of all tubed Macs". I think only 1000 units were made. Add to it, that the Japanese and others go crazy for Mac gear. IMO you have rich people out there going for the hype of this unit.
These were the units that powered the original Woodstock music festival and the Grateful Dead used them also.
This is different from the hype that surrounded to "Ongaku" amp. The Ongaku sold for $75,000-$100,000. Was the unit itself worth this much? Probably not, but every report that I have read or person that has talked about it has said that it was one of the best, if not best amps they have ever heard. So, does this make it worth $100,000? Why not, if it brings musical ecstacy and you have that kind of money to spend.
Basically, what I am saying is that the MC3500's hype surrounds it's history and not the wonderful sound it produces. It sounds like any other Mac but more powerful, mainly due to all the feedback used in their circuit.
As for me, I happened to come across a pair of outputs; it's the only reason I plan on building them.
I have a manual on the MC3500 and besides describing the output transformer as "pentafilar wound", it also states that the primary is wound in 10 sections. This is a double "C" core transformer and the windings are on 2 bobbins, kind of like the original Williamson configuration. This means 5 sections of primary interleaved with 4 sections of secondary on each bobbin. You can calculate primary inductance by knowing that the primaries are each 150 ohms center tapped and that typically the Quality Factor of Mac outputs is around 75000. This will get you acceptable leakage inductance for the transformer. It's going to be a lot of math but doable. Look at RDH4 for the formulas.
Daniel
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Old 23rd July 2007, 08:45 AM   #6
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having a big fat trust fund would make obtuse purchases a lot easier. given the fact that in the sixties mcintosh hand made almost all of there products inhouse at the binghamton factory. i have seen the pictures of the woman winding the coils at there small workbench. assuming i only wanted to drive a 4 ohm load would make the transformer math and construction less troublesome. most of the other large wattage tube units i have researched were exotic and one of a kind. the mac's were produced for large scale public abuse. i figured this design to be rather stout and predictable. most of the originals are still working today. that is quite a testament of design. thanks for the input.
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Old 2nd May 2008, 09:44 AM   #7
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Default MC-3500 output transformer

Hi danFrank

It would be helpful to state the technical specifications with reference to the colour coding of the transformer. To be specific:

-- what is the step down for the primary relative to each of the secondaries

-- what is the phase shift

-- what is the induction of the primary

-- what is the DC resistance of the primary and each of the secondaries

It would also be useful to know the iron mass, but that would require a disassembly which in all likelyhood would be destructive.

Jan D.
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Old 2nd May 2008, 02:06 PM   #8
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Default Schematic

It seems that the MC-3500 output transformer schematic wasn't included in my previous post.

Please let me know how I can make it available.
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