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Old 11th April 2004, 11:10 AM   #1
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Default Mc275 opt manual rewinding

Glad to have found your forum about trans. Is anyone of you know about the mac unity coupled transformer? I'm 45 years old now and I'm working with this project since my high school days. This model is known until now as the best power amp ever and I'm very eager to have it for my personal use. Since we are poor and cannot afford to buy a McIntosh Mc275, I've decided to assemble the amplifier, and I have completed my circuit and chassis project. Until now, only the transformer is missing and I've done all the mathematics and exhausted interneting but still could not find the winding data. I have no plan to buy the original.I'd like to make my own and have already ask the official mac rewinder but too expensive for me.

I know some basics of the original opt such as bifilar technique, core size, the parallel type secondary, the tertiary winding, the interleaving , turns ratio, but anyone of you who knows about the exact no. of turns and the wire size. I, like to do it myself manually...
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Old 11th April 2004, 07:10 PM   #2
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Check out the Plitron Transformer web site (www.plitron.com). Plitron carries unity coupled toroidal O/P trafos that might be suitable for your need. As always, good "iron" is costly, but I think that Plitron's prices are not nearly as high as custom rewound "Mac iron" is.
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Old 11th April 2004, 08:22 PM   #3
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Thanks Eli, for mentioning the Plitron transformer (PAT 1070 UC)--you saved me the trouble.

I'm curious if anyone has any insights or advice on using the Plitron tranny in a McIntosh MC275 type design. The Plitron seems the best alternative to the $600 McIntosh transformers-- and outperforms it in some ways--but has its own limitations.

My own investigations seemed to reveal that the 1070 UC lacks two of the MC-275 transformer's windings--not the crucial bifilar windings, but others that supply boosted voltage to the driver stages.

This is not in itself a show-stopper, but means that the 275's design would have to be modified to provide the necessary drive voltage without help from the output tranny. Although I am diligently studying, such modifications are as yet beyond me.

If anyone has any corrections or suggestions, I'd be fascinated.

Happy Non-Denominational Holiday!

George Ferguson
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Old 11th April 2004, 09:03 PM   #4
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Hi,

Has anyone done a search for Mac OPTs here on the forum?

I vaguely remember someone suggesting a group buy for these items in custom made form.

It didn't happen as at least one or two companies had the iron available at a lesser than new (from MC Intosh) cost.

If nothing turns up here, maybe a google search would.

No matter what shows up, the Mac iron is pretty complicated as it is multifilar with multiple windings interleaved.

Cheers,
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Old 11th April 2004, 09:26 PM   #5
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Quote:
Originally posted by fdegrove
Hi,

Has anyone done a search for Mac OPTs here on the forum?

I vaguely remember someone suggesting a group buy for these items in custom made form.

It didn't happen as at least one or two companies had the iron available at a lesser than new (from MC Intosh) cost.

If nothing turns up here, maybe a google search would.

No matter what shows up, the Mac iron is pretty complicated as it is multifilar with multiple windings interleaved.

Cheers,

I've bumped the thread up to the top of the list, but here's a link just in case:

Mcintosh Output Transformers

Actually, the Plitron was the only current transformer folks discussed, but the thread ended without any specifics of how to use it. I see the Plitron has "feedback windings", labeled 10 and 11 on this scematic:

http://www.plitron.com/PDF/1070UC.PDF

Are these infact the "bootstrap" positive feedback windings for boosting drive voltage? Would these enable the Plitron to be an essentially drop-in substitute for the McIntosh tranny in the MC275 circuit?

Best,

George Ferguson
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Old 11th April 2004, 09:32 PM   #6
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Guys, check out Menno Vanderveen's writings. 1 of his books, whose title eludes me, explains the operation of the PAT1070UC (among several other "specialist" trafo types).

The large voltage swing needed to drive a unity coupled O/P stage IS a problem. FWIW, fairly recently, there was speculation by myself and other posters over on AA about using a PP Class "AB" driver when LARGE voltage swings are needed.

Another possibility is to use a Russian EL84M (7189 equivalent) with regulated B+ (say 150 V.) on the screen grid as an intermediate driver. Pentodes can swing closer to the voltage limits than triodes can and the max allowable plate voltage for a 7189 is 400 V. Phase splitting would be done before the 7189s and there would be a dedicated 7189 driving each "final".

BTW, what sort of NFB circuitry, other than unity coupling, does the MC275 employ?
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Old 11th April 2004, 09:48 PM   #7
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Hi Eli,

Thanks for the helpful reply.

Quote:
Guys, check out Menno Vanderveen's writings. 1 of his books, whose title eludes me, explains the operation of the PAT1070UC (among several other "specialist" trafo types).
Good tip. I have the book--"Modern High-End Valve Amplifiers"--and it is available on the Plitron site. It discusses the PAT1070UC, but leaves the circuit design to the reader. I asked Dr. Vanderveen about it, and he recommended that a newbie like me not tackle it, with its multiple feedback loops.


Quote:
The large voltage swing needed to drive a unity coupled O/P stage IS a problem. FWIW, fairly recently, there was speculation by myself and other posters over on AA about using a PP Class "AB" driver when LARGE voltage swings are needed.
What about using a HV drive for an electrostatic headphone? Like this one:
http://headwize2.powerpill.org/proje...ilmore_prj.htm

Quote:
BTW, what sort of NFB circuitry, other than unity coupling, does the MC275 employ?
Here's the schematic--maybe you can tell us?

http://www.kbapps.com/audio/schemati...osh/mc275.html

Best,

George Ferguson
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Old 11th April 2004, 11:37 PM   #8
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The 6S4 is interesting, in that it's a "small signal" triode that can take 550 V. on its plate. The caveat about triodes not being able to swing close to the voltage limits still applies. Here's a URL for the 6S4 spec. sheet (http://www.pmillett.addr.com/tubedat...%202/6S4-A.PDF).

The MC275 has a global NFB loop. There is a winding on the OPT that's connected all the way back to the I/P 12AX7's cathode circuitry.

Notice the use of DC coupling between the 'X7 and the phase splitter. Count the number of caps. in the signal path around the entire global NFB loop. I make that count to be 2, which is proper (as expected from "Mac") engineering practice.

Do you want to clone the MC275 or do want to learn from the MC275 and develop your own unity coupled design? Notice the 12AZ7 in front of the "finals". The 12AZ7 is a high mu/high gm tube very similar to the 12AT7. The mu of the 6S4 is less than that of of the 12AU7.

Here are some thoughts to mull over. For hackers, like us, the shorter NFB loops are, the better. Very linear, high gain, I/P stages that do not need any NFB can be built using CCS loading. A Schmidt (differential) phase splitter allows the NFB network to be DC coupled to the speaker winding of the O/P trafo. The non-inverting I/P of a differential stage is oh so convenient. Cap. coupling the phase splitter to 2X 7189s and cap. coupling the 7189s to the "finals" stays within the 2 cap. in a NFB loop "law". Large amounts of voltage gain are relatively easy with pentodes. Regulating the screen grid B+ of pentodes IMPROVES linearity.
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Old 12th April 2004, 03:10 AM   #9
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Smile unity coupling data

Thanks Eli for the advice, but buying plitron is too expensive for us.

Materials in our country are available and very much cheap and I'd like to wind it myself, hoping anyone could share winding data of unity coupling.
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Old 12th April 2004, 10:00 AM   #10
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hi,

yes, materials are very cheap in our country, silicon steel cost about u$1 per kg and copper wire at u$6 per kg for fine gage wires, #30 and up, myself makes traffos for my projects...

i doubt they will give away winding info for traffos such as the ones used for the macs, it could be part of their 'trade secrets'...

so, your best bet would be, if you can get hold of a good mac output traffo, you can get the dc resistance of the windings and from there we can work something up, maybe, it would be nice if we can get the turns ratio....

this can be done by injecting a low ac voltage, say 5volts at 60hz and measuring voltages at the other side...be very carefull though, high voltages will be present at the primary side...i am interested in the feedback winding....

physical examination of the traffo can give us clues as to how it was wound....reverse engineering, i believe the term is...

lastly, the quality of the iron available in our country will be a factor...but my guess is it should be ok, i am sure you are familiar with 'boosters' used by jeepney drivers driven by car radio's...some setups can realy sound good when not overdriven...

good luck!!!!

btw,
some great guy from this forum gave me these links, have fun reading them, it may help you, as it helped me, although i have been winding traffos since the 70's as a hobby.
Transformers and Coils (Chokes)

* KevinsGarage, Transformer Data,
Some basic info you might need if you have a vintage tranny:
http://www.kevinsgarage.50megs.com/about.html
* MIT Staff, The Use of Ideal Power Transformers for
Impedance Matching(from Pearl Audio site)
This article also has very good info on Push Pull theory:
http://ken-gilbert.com/images/pdf/Pu...Theory_MIT.pdf
* P.G. Doynov: Calculation of Outut Transformers:
http://www.valveheart.com/theory/transformer.html
* Doede Douma, A Double Secondary Out Put Transformer:
http://www.dddac.de/tp05.htm
* Elvis Rakic, How To Make Audio Output Transformers:
http://www.diyaudio.8m.com/Ot/ot_en.html
*Alessandro Coppi, Air wrapped Transformer Calculator:
http://www.autocostruire.it/trials/stargate.htm
* Gregg van der Sluys (ScorpioRising):
Winding your own transformers:
Part I http://geek.scorpiorising.ca/windingtransformers.html
Part II http://geek.scorpiorising.ca/windingchokes.html
*Max Robinson, Winding Coils:hands on advice
http://www.angelfire.com/electronic/...s/Coils-1.html
*RG Keen, OPT Design and Winding:
Located in the “Tube Guitar Amp Tech Pages” section here:
http://www.geofex.com/
*Jan Rowland, Roll Your Own Power Transformers:
http://members.tripod.com/~schematic...m/xformer1.htm
*Patrick Turner, Output Transformer Design Calculations:
http://www.turneraudio.com.au/htmlwe...esigncalcs.htm
*Transformer Basics, from Lessons in Electric Circuits:
http://www.ibiblio.org/obp/electricC...s/AC/AC_9.html
*Determining OP Transformer Impedance:
http://www.geocities.com/vintage_radio/outimp.htm
*Basics of Inductors:
http://www.hills2.u-net.com/electron/induct.htm
* VoltSecond and Mike Lefevre,
What does the Transformer Inductance do to the Loadline of the Tube? http://www.siteswithstyle.com/voltse...LOAD_LINE.html
?Gabe Velez, Transformers:
http://members.tripod.com/~gabevee/transform.html
* Michael LaFevre, Transformers and Phase Shift:
http://www.magnequest.com/tech2.htm
* Color codes (MarVac Electronics):
http://www.marvac.com/funpages/chokes.htm
* University of Surrey, Producing Wound Components
http://www.ee.surrey.ac.uk/Workshop/advice/coils/#turn
* David Berning, An Audio Amplifier Design Philosophy:
http://www.meta-gizmo.com/Tri/otlology/BERNINGS.htm
?Randall Aiken, OPT’s Explained
http://www.aikenamps.com/OutputTransformers.html
* Richard Sears, Trany Math:
http://richard984.tripod.com/transformer_math.htm
* Lundahl, Technical Papers from Lundahl:
very short pieces from Lundahl lab:
http://www.lundahl.se/papers.html
* Pete Millett, Power Transformers for Audio Equipment:
http://www.pmillett.addr.com/images/ax_power.pdf
* VoltSecond, Damping Ringing in LC Circuits: http://www.siteswithstyle.com/VoltSe...ringing_XFMRS/
Damping_ringing_in_xfmrs.html
* Plitron Site:
http://www.plitron.com/Pages/sitemap.htm.
Link takes you to site index. Under “Publications”.
A collection of articles including:
-Secrets of Output Transformers, Menno Van de Veen
-Measuring OPT Performance, Menno Van der Veen
-Theory and Practice Of Widebandwidth Toroidal OPT,
Menno Van der Veen
-Modeling Power Tubes and their Interaction with OPT,
Menno Van der Veen
* John Atwood (One Electron), Transformer Measurements:
http://www.one-electron.com/Trans_Tests.html
* Max Robinson, Understanding the Transformer:
http://www.angelfire.com/electronic/...nsformers.html
* Dean P Currier, A Biographical History of Induction Coils:
http://radiantslab.com/quackmed/Deanbio.html
* Edcor, Transformer Tek Notes:
http://www.edcorusa.com/gadgeteer/tek-notes_xformer.htm
Toroid Corp. of Maryland Tech.Bull. #1
( Application Notes on Rectifier Transformers) and #2
(International Line Voltages and Frequencies):
http://www.toroid.com/technica.htm
http://www.coilws.com/magneticandhow.html
http://www.maneng.com.au/dcchokes.htm
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