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Old 21st November 2010, 05:08 PM   #101
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Class AB2, not class B, no wonder! Now you have to ask yourself why all this crazy OT configuration and massive feedback stuff.
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I'm shocked. You mean the OT is not even layer wound? OMG! I would envision a "correct" Mac OT as being layer wound and interleaved on a split bobbin. Even further, it should be on a long-E core or a cut core (or ultimately a toroid with progressive wind). Using the 50% cathode feedback scheme, both bifilar winding sets MUST have equal coupling to the output winding or HF distortion results from imbalanced leakeage L. Only way to do that is with a split bobbin with interleaves.

Although an el-cheeso method would be to multifilar wind the whole thing, and put up with massive distributed capacitance. I shure wouldn't pay $4500 for that. The "Erector Set" amplifier? One remotely possible scheme, but unlikely on a non long-E core, would be to try a progressive wind. This keeps adjacent turns of the winding with minimum AC voltage difference next to each other everywhere by careful proximity winding practice. Probably hand wound and with adhesive on the wire to make it stay in place (a very mechanically awkward winding is required although multiple PI sections are another way).

The original Mac OT was wound using enamel magnet wire insulation and had problems with HV breakdown. The adjacent strands in the bifilar pairs have ground and B+ on them, a recipe for trouble. Modern super magnet wire insulations are available now though that can withstand 1000's of volts.

But if one were going to try to clone the Mac, I would just use a Circlotron output scheme. Functionally equivalent. Better AC coupling, no DC problems, 1/2 the turns, could even multifilar the secondary in using ordinary TFE wire insulation on it (the secondary). And off the shelf OT candidates are readily available. This pre-occupation with duplicating the Mac OT is absurd. Especially in light of the class AB2 operation.

Why clone the Mac when it would be easier to do it better? (differential driver, "magic" Schade feedbacks, Circlotron output on a long-E core).
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Last edited by smoking-amp; 21st November 2010 at 05:17 PM.
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Old 21st November 2010, 07:11 PM   #102
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Smoking-amp,
The transformer was wound in layers with adjacent primary and secondary sections which is good practice, but there was no isolation between layers. With isolation each next layer has a smooth plane so the bobbin can be wound tidy all the way up.
My favorite core for push-pull output transformers is a single c-core (stacked when necessary), and that is what I use for that purpose exclusively. For a single c-core you can wind two bobbins, one for each leg of the core. This way you have accurate center taps.
The older generation Mac amps used c-cores as well but I think in a double core set up with a single bobbin.
The older Mac transformers used what was called Formvar wire, which is magnet wire with thicker enamel. It will work OK as long as the bifilar winding is tidy and there is no room for resonance. I am curious how one of the earlier transformers were wound as I would not be surprised if it would look much better. Apparently the modern EI output transformer is made on a budget basis, which is a pity in an amplifier where the output transformer is its heart and soul.
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Old 21st November 2010, 07:23 PM   #103
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Modern magnet wire has very good insulation. At work I was playing with the new HIPOT tester and twisted two strands of 32 awg wire together. It passed the 5KV AC/DC test with no arcing or leakage. However the coating is delicate. All it takes is a minor scrape to short against a metal core.
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Old 21st November 2010, 09:13 PM   #104
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it's my understanding, that the original Mc amps were C-core ...


McIntosh MC275 Amplifier Reviews | ProductWiki
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Old 21st November 2010, 10:37 PM   #105
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If I tape up a roll of enameled bailing wire and saw it in half, is that a C core?
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Old 21st November 2010, 11:14 PM   #106
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Them thar thangs is called bail-n coils around these parts.

I recall seeing a lot of old telephone loading coils made with steel wires. Must have been enameled. I don't think you can get grain oriented stuff without rolling it though, but permalloy wire would be interesting. Why cut it then. Use a cheap random wind toroid winder to put it over the (large diameter) wire coil bobbin.
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Last edited by smoking-amp; 21st November 2010 at 11:28 PM.
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Old 21st November 2010, 11:30 PM   #107
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I like to read debug of old Mac but sorry what to do with class AB nfb 75w tubes power ?
is not too much power ? the tubes have if I'm rigth 10db of overdriver vs sand...
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Old 22nd November 2010, 02:46 PM   #108
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Quote:
Originally Posted by smoking-amp View Post
Some questions for Rosinante:

How many output Watts are you looking for here?

And do the tubes have to be standard Audio ones like KT-88 ... or are NOS Sweep tubes acceptable?
Smoking-amp (and others), I appreciate this discussion though I understand precious little of it. I do not plan on cloning the legendary MC275, but do hope you guys figure out the (apparently somewhat mysterious) design decisions that were made. I accept the notion there are better designs now, and less complex designs, but I'm just not sure those old units were terribly more complex than other tube amps. It's not as though they are FILLED with miles of wire and thousands of components. They were different, and better, than all the others of that time. Again, I'm wondering if some of their wisdoms are still applicable. Bet they are. These were not a bunch of idiots.

Again, I am too ignorant to answer your questions. I can only express my goal in terms of sound, and of course that is very subjective. The best tube amps (according to my ear) have certain characteristics. Heck.....I'm wondering if all that current passing throught the 12AX7's is part of the secret. I hope to build an amp that has this "fat" -ness in the lower tones. Warmness throughout the range, but as this warmness covers those lower tones, you can get this "fat" -ness that is absent in most solid state gear. Again, a warmness that covers the entire range, and an "air" -iness in the mids and highs. An "organic" sound. And now for the hard part: These warm, fat, airy, organic features need to be achieved without flabbiness. Notes played through this amp need to retain all their amplitude, without that resonance stuff. If a rock drummer hits his kick drum, it needs to NOT sound like a kettle drum. I wonder if this helps.....hope it does.

I am keeping an eye out for a variac or some device with which I can regulate voltage to this Yaqin MC100B. Because the consensus wisdom seems to be that this improves sound, and perhaps tube longevity (which I don't care about). Replacing some caps might help too. This amp does not sound right yet. It's got some decent fatness in the bass, and some airiness in the mids and highs, but it's flabby. Not tight-sounding. And there is a tad too much brightness at the very top.

BTW, we're getting some cold weather in my location, and possible storms. If things get really frigid, I will put a fire in the fireplace for ambiance, and we'll huddle around this amplifier for warmth. This thing makes HEAT.
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Old 22nd November 2010, 03:10 PM   #109
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Quote:
Originally Posted by Rosinante View Post
Smoking-amp (and others), I appreciate this discussion though I understand precious little of it. I do not plan on cloning the legendary MC275, but do hope you guys figure out the (apparently somewhat mysterious) design decisions that were made. I accept the notion there are better designs now, and less complex designs, but I'm just not sure those old units were terribly more complex than other tube amps. It's not as though they are FILLED with miles of wire and thousands of components. They were different, and better, than all the others of that time. Again, I'm wondering if some of their wisdoms are still applicable. Bet they are. These were not a bunch of idiots.

Again, I am too ignorant to answer your questions. I can only express my goal in terms of sound, and of course that is very subjective. The best tube amps (according to my ear) have certain characteristics. Heck.....I'm wondering if all that current passing throught the 12AX7's is part of the secret. I hope to build an amp that has this "fat" -ness in the lower tones. Warmness throughout the range, but as this warmness covers those lower tones, you can get this "fat" -ness that is absent in most solid state gear. Again, a warmness that covers the entire range, and an "air" -iness in the mids and highs. An "organic" sound. And now for the hard part: These warm, fat, airy, organic features need to be achieved without flabbiness. Notes played through this amp need to retain all their amplitude, without that resonance stuff. If a rock drummer hits his kick drum, it needs to NOT sound like a kettle drum. I wonder if this helps.....hope it does.

I am keeping an eye out for a variac or some device with which I can regulate voltage to this Yaqin MC100B. Because the consensus wisdom seems to be that this improves sound, and perhaps tube longevity (which I don't care about). Replacing some caps might help too. This amp does not sound right yet. It's got some decent fatness in the bass, and some airiness in the mids and highs, but it's flabby. Not tight-sounding. And there is a tad too much brightness at the very top.

BTW, we're getting some cold weather in my location, and possible storms. If things get really frigid, I will put a fire in the fireplace for ambiance, and we'll huddle around this amplifier for warmth. This thing makes HEAT.
Since you are looking for a power amp you'll of course have a preamp to feed it. I'm reading all the characteristics you are looking for described as "warm" and "fat" and "airy" and "flabby"....

Why not set your amp goals to find one that doesn't do anything to the input signal to color it at all, just amplify it. You then use the controls on your pre- and use speaker placement to get the sound you want. Do you really want the amp to do anything other than amplify? Anything else "added or lacking" is a failure of the design. After all, the amp designer didn't intend for it to do anything else except reproduce the source material as closely to perfectly as possible.
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Old 22nd November 2010, 05:02 PM   #110
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That's a good suggestion, one I can understand, and is a big part of my current thinking. A power amp that is just a very obedient amplifier. It amplifies. Then, "cook" the sound using a preamp.

Yup, in theory, amplifiers are all obedient. In the real world though, they color the sound. And if they don't.....they sound sort of harsh. Clinical. There is a sound I am looking for, and it is warmer and more organic than most of the solid state stuff I have heard. All of it, actually.
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