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Mcintosh Output Transformers

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Is anyone interested in a group purchase of Mcintosh type output trasformers to be custom wound by a winder of our choice? The hardest part of a great tube amp to obtain is the output tranny and Mcintosh arguably made the best ones. I have seen some originals for sale for an unreasonable $500 and up each. Do you have a preference as to what design, UL with cathode feedback windings which offer flexibility in design or full unity coupled outputs requiring substantial driver circuitry? Plitron transformers are nice but require extra solid state DC balance circuitry to work.
I'm thinking of a design that would allow the use of 6550 type tubes aka KT88, KT90, etc. with possibly one 5 ohm secondary for easier winding, 40% UL taps and 10% cathode feedback windings. 60-100 watts.
Any input?
diyAudio Senior Member
Joined 2002
Mac OPT.


Those were very fine OPT's for sure.
With the current craze for SE amps I'm not that sure you will find many takers.
Pricing policies?
Any idea on breakdown quantities and who's going to wind them?
Anyone you have in mind?
Why not improve on them in the process anyway?

ultranalog said:
being located in the old world, shipping would probably be the primary cost factor. I think I'd better have them made here.
Ditto. Iron is too expensive to send air, and 12 weeks surface is too long.

And the fact I only like UL amps where you have to power inefficient speakers (and why would you want those?) or for subs.

I wouldn't go so far as to say Mc trannies are the best either. Never heard a <i>great</i> Mc amp.

You might be on your own with this one I'm sorry to say.
diyAudio Senior Member
Joined 2002


Experience has it that if you look hard enough you'll always find some transformer company at reasonable distance from where we all live.
You can often have you transformers made to your own spec without adding considerable transportation expenses.


Couldn't help the grin on my face reading you're not a big Mac fan either.
The OPT's are probably fine though.
I've heard them recycled and couldn't detect that typical MAC sonic footprint.:(

salute folks

i am selling original mcintosh mc 30.
so you can take out xformers or use them to a new design.

i already modified my mc 75 having serious results.!!!!!!!!!!
(eliminated neg feedback,triode mode & class a with a flip of a switch.

High quality output transformers

I have just found this thread on McIntosh output transformers and thought that I would put in my advice and thoughts.

It would be much to expensive to have a small number of high quality transformers wound with special windings. A much better coice would be to use some of the new and truly outstanding Plitron transformers. These are of special design and many have cathode feedback windings. They are reported to have much better specifications than the McIntosh ones.

If I were to build a MC275 or similar circuit these would be my choice. The price is lower than having a limited number of custum ones built. All the engineering has already been done.

A link to the Plitron audio output transformers is:


There are other ways to provide cathode feedback in the output stages, but if you want to keep things simple then the Plitrons are the best way to go.

Best regards to all,

John Fassotte
Alaskan Audio
The cathode winding was used for feedback in the classic Mac circuits. The advantage was lowered distortion. The disadvantage was high drive voltage requirements, which Mac accomplished by a bootstrap of the driver stage (positive feedback).

Norman Crowhurst had an excellent analysis and comparison in his book and in several of his articles in "Audio Engineering."
diyAudio Senior Member
Joined 2002

Hi guys,

Another famous design using this topology is Peter Walkers' QUAD II amplifier.

Regarding the Plitron OPT: http://www.plitron.com/

In the books section you will find a book by the inventor Ir.Menno Vanderveen.
He's a Dutch engineer that developped this type of audio xformer.
You'll also find the articles that appeared in various audio mags.
Download them and read them,they're well worth the time spent.

richt said:
hi john

do you know what is the purpose of the winding on the cathode?
is a type of feedback or for damping factor?

thank you

SY is correct in his answer to your question. The damping factor will also be lowered slightly through the use of cathode feedback. A seperate transformer feed from the secondary of the output transformer and applied back to the output stage cathodes in the proper phase relation ship can also add cathode feedback.

Back in the days of 16-ohm loudspeakers it was possible to rewire the output transformer to ground the 4 ohm output tap. Then the old common and 16 ohm taps were connected to the output stage cathodes for feedback. The 16 ohm loudspeaker was then driven as normal. This configuration provides a balanced output. The disadvantage of this scheme is that when using the correct output tabs to drive 4 or 8 ohm loads there would be a unbalance generated in the output transformer due to the wire losses in the output windings becoming uneven due to the load not being spilt evenly between the two halves of the secondary winding.

It is still fun to experiment with this type of feedback. If driver bootstrapping is not used through use of a separate winding on the output transformer then the driver tubes have to be able to generate a considerable voltage swing on their own. As I recall I used 6CL6 tubes in the driver stage with KT88 output tubes with good results. The amount of increased drive voltage required from the driver is approximately equal to the AC voltage generated by the cathode feedback winding from any output stage cathode to ground.

Other types of balanced feedback can be added from the output or output stage to the intermediate stages using resistive networks, which also give good results. Many amplifiers use this method instead of separate windings on the output transformers.

John Fassotte
Alaskan Audio
One other thing- Mac's output stage allowed them to really run the idle currents pretty low, very close to class B, without problems with notch distortion. This operating method had advantages of extended O/P tube life, lower running temperature, and less magnetization problems in the O/P transformers.

There are, IMO, better ways to get this class B advantage (my triode amp uses screen drive and idles at 3 ma or so), but this was a good approach at the time.
diyAudio Senior Member
Joined 2002


There are, IMO, better ways to get this class B advantage (my triode amp uses screen drive and idles at 3 ma or so), but this was a good approach at the time.

And your not suffering from xover distortion?
Or perhaps your speakers are very efficient and you don't actually go out of Class A operation?

I'm puzzled...

No, my speakers are of modest efficiency (low 90s/2.83V/1m). It's just that screen drive is a particularly good way to avoid crossover distortion.

There was a review in "Audio" some years back of one of David Berning's amps that used this trick. In the review, they showed the I/V curves of the O/P tubes at low currents- very, very linear. I could probably dig out the reference for you in a day of so, if you're interested.
diyAudio Senior Member
Joined 2002

Hello SY,

Now you mention Bernings' name,I do vaguely remember something about it in the early nineties I think.

If it's not too much trouble,I would like to have a closer look on that article.
Would be nice to see if we have a use for it here at the forum.

Thanx a lot,:)
I seem to remember it also, would like to se it again. I have a pair of output transformers meant for the Mcintosh MA230 that I'm using in a project with either the new EH 7591's or a pair of 8417's. These tubes have a lower drive requirement than any others I know of. 20 to 26 volts peak which makes the driver stage easy. I plan to use The new JJ ECC99 for it to produce the 30 to 36 V needed with the cathode windings calculated in. Those EH 7591's look much beefier than the originals, probably 25 to 30 wartts dissipation if I just go by plate size. I cannot find the data for the EH 7591's anywhere. Thanks for all of the input to this thread. I think tube amps can still be improved from the present SOA levels to something better. The problem with Mcintosh amps IMHO is the previously mentioned bootstrap driver. This can be accomplished much easier with fewer stages and lower distortion with today's tubes and circuitry. A design such as this would be my ultimate killer tube amp.
diyAudio Senior Member
Joined 2002


Not so long ago I stumbled on a site comparing pixes of a regular USSR 6L6GC with these 7591A's.
I couldn't spot any differences.
Do these EH differ in any way?

Those EH 7591's look much beefier than the originals, probably 25 to 30 wartts dissipation if I just go by plate size. I cannot find the data for the EH 7591's anywhere.

I could find you a datasheet of the original 7591A if that is what you need.
Just let me know and I'll mail it.

Happy trails,;)
Thanks, Frank but I have the data for the originals. Yes, the EH 7591's look much like the Sovtek 5881/6L6WGC as I look at them in front of me with the EH 7591 plate structure being a bit bigger, and much larger than some originals that I have. These could be some nice tubes if the rest of the specs are like the 7591. No experience with them yet.
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