Output transformer for McIntosh 275 2x75W?

[johnheath]

Hi…

I am about to build a McIntosh 275 and while looking att different output transformers (Edcor) I find that they have different input impedance and I wonder what the difference would be… let's say 4k to 10k?

Schematic is attached


Best regards

/John

 

[johnheath]

Posting a link to Edcor web site:

https://www.edcorusa.com/cxppseries#/pageSize=25&viewMode=list&orderBy=0&pageNumber=2

 

[abbey road d enfer]

I am about to build a McIntosh 275 and while looking att different output transformers (Edcor) I find that they have different input impedance and I wonder what the difference would be… let's say 4k to 10k?

The OT of McIntosh amps is very specific. In addition to its usual primary (plate) and secondary (output) windings, it has an overall NFB winding (that feeds back to the cathode of the 1st stage), a center-tapped cathode winding, which is both a primary and a secondary (it acts as a cathode load, adding the cathode power to the anode power AND providing negative voltage FB from the anodes to the cathodes), and finally a winding that bootstraps the anode voltage of the driver tubes .
Add to that the fact that all these windings are sandwiched, you'll understand why these transformers are so expensive.
I don't think Edcor have anything that starts to resemble a McI. OT.

The recommended plate-to-plate impedance for a pair of 6550 operating at 450V with fixed bias is 3.5k. 3 to 4k would be adequate, 5k a little off and 10k totally out of whack.

 

[PRR]

> McIntosh 275 and ... output transformers (Edcor)

This Mac has, not one, not two (as in other Macs), but *three* primary windings with very specific interleaving.

If you do not know the basic effects of OT impedance, you have much to learn before you can understand why Mac did such a strange thing.

Edcor has nothing even close.

I don't think Edcor would want to touch this job as "custom", because in addition to trifilar (perhaps hexfilar) windings, two secondaries have to be tucked-in at many levels. I'm not sure they can wind from two spools at once, and am sure they are not set up for this many spools at once.

AFAIK, only MacIntosh (and maybe a hi-end Japan product) ever had the tooling to do this work.

I know how much a Mac 275 must be worth by now. (I sold my 240 at the bottom of the market.) But that is the only way you can get these OTs. You can't run "ordinary" OTs with this much gain-stage around it.

 

[CJ]

Kapton wire works well here, pri and sec wires wound right next to each other, 

how to improve this OPT?  maybe a C core,

 

[johnheath]

Thanks you all

I saw the different coils in the schematic and after looking at "inside" photos of the MC275 there was no sign of these windings so I started to think that it was something with the OPT it self... which it seems to be.

About the the primary impedance of the Edcor OPT... I really needed the answer that Abbey Road d enfer gave me...

Thanks

/John

 

[lassoharp]

Dennis "Doc" Hoyer specializes in the MC OTs

http://www.roger-russell.com/autran.htm

 

[nachoclown]

Hello, I need to know the colors of cables of mcintosh output transformer M-316 of a mc-275 GORDON J GOW edition. Are 13 cables and are not similar to the regular mc-275 M-216. Thank you