Manley Vari Mu Input XFMR 6756

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no continuity between any pins, maybe the wrong epoxy, John Hardy had a problem with this in the first discrete opamps, epoxy too hard, temp coef breaks leads due to expansion, usually potting compound is a bit more pliable than this stuff, heat gun did nothing to soften it up,

full props on the mu can, had to fight like a tiger to get that thing apart, watching out for the razor sharp edges we know and  love when it comes to mu cans, :'(



 

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piece of copper clad pc board material in there to space the core off the can,  do not think the copper provides any shielding as it is not grounded, and they usually uses "soft" copper for nested cans,


 

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outer layer taken off, had about 600 turns of some fine wire,

then this middle wind (coil had 3 sections) which is a bi fi that kept breaking, so no turns count, seemed like around 200 T each wire, maybe 300,



 

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inner wind was about 2000 turns of real fine wire, like guitar pickup size,

is this a step up or step down transformer?  seems like a step up would make sense if we are going into vacuum tube grids, but what about overload? i bet they have a volume knob on that vari mu,

usually bi fi is used for the input wind like a Peerless K-241-D,  maybe good for CMMR and linkage,
but maybe they wanted good balance for the push pull amp so that bi fi wind might be for the grids, making this a step down transformer,

maybe some flux calcs will solve this, do not know why they have three winds, maybe one is for metering, or maybe the bi fi wind was a wire shield, lots of questions,  ???

 

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Cool ,
Manley really didnt want anyone anyone reverse engineering that . :-X
Would soaking the bobbin in solvent make the unwinding/turns counting any easier and breakages less likely ?
Is it possible to once you know wire gauge ,dimensions and coil resistances of the original to work things out better?

Im all excited as my manual coil winder is arriving today ,actually I ordered three as I hope to get a few friends into having a go at winding too. Ive re purposed a step counter and a microswitch as a turns counter ,goes upto 100,000 counts,costs next to nothing .I'll probably fit the three winders with modded calculator counters in the end ,as the ability to program the calculator for doing interleaving, counting  up or down turns ,and perform calculations is a handy bonus.

Interesting ,quite a bit of dectective work involved in figuring it out .
 
had a garbage can full of wire and a bobbin,  what to do?  measure the wire, we have a turns count for 1 of 3 winds,  look up the wire size, and we have a bobbin with tape marks on it from which we can measure the build with the calipers and get turns estimates for the two unknown winds by using wire tables with turns per sq in,  DCR, ...


you could do this input on a Neve lam with two coils and get a little better CMMR  and maybe extend freq range to 40 K. use poly crystal wax instead of resin, those nickel lams need to breathe, when you hard dip them the lams can not stretch due to magneto-restrictive forces,

if using stock core, wind the bi-fi section with one wire reverse wound then connect F-F for better CMMR,

 

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here is the print with the wire calcs, did not think we chopped 2000 T of the outer layer, which thru us off as it made no sense to have unequal turns making us think we had a third wind for something, but by looking at the bobbin, we must have chopped 2000 on the outside also, thin wire, #44 like guitar pickup wire, so a lot gets chopped with each cut me thinks,

what is max input level for this machine? they are hitting the nickel core pretty good which is probably where the "color comes from. They make a cleaner version now, the nu mu, which has bigger iron IIRC.

1:5 means they hit the grids hard so there must be some voltage division inside the box.

primary turns could possibly be a bit lower on the original, but you will saturate at 2 volts rms with 600 turns,

enjoy!

 

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Very interesting, as usual, CJ.  I had a Vari-Mu in recently that had one side lower level than the other.  Turned out the input transformer had no continuity on one side of the primary.  Desoldered it and was getting ready to email Manley for a replacement, but decided to test it "outside the box".  Continuity was back!  Installed and re-soldered it, worked perfectly.  Got lucky I guess.
 
Thanks CJ!
Think your right about the colored sound of Manley stuff. Some people dig it, some don't, but I do think its a result of the transformer designs. I want to describe it as "squishy". I've noticed they're output transformers seem to get loaded down pretty easily which is probably due to the small size. I like big cores, big wire, big sound!!!!
 
Yes, squishy matches the units I've heard.  The vintage classics all have less squish, excepting the UA 176 types which also suffer from tiny squishy transformer syndrome. 
 
mjrippe said:
Very interesting, as usual, CJ.  I had a Vari-Mu in recently that had one side lower level than the other.  Turned out the input transformer had no continuity on one side of the primary.  Desoldered it and was getting ready to email Manley for a replacement, but decided to test it "outside the box".  Continuity was back!  Installed and re-soldered it, worked perfectly.  Got lucky I guess.

I had one with one side of primary went bad.  Tested "outside the box" but no joy!
Best,
Bruno2000
 
the outside wind seemed to have had lower turns than the inside, perhaps they experimented with offshore iron for a brief spell,

this would cause a mismatch which could probably be dialed out with the balance control.

some copper screens might get their freq response up a bit, but if it is their best selling product, then don't fix it,

here is their vari mu - non squish model>

http://www.manley.com/products/view/MNUMU

 
Hey CJ, is there any way to wind hum bucking coils on an EI core, or do you just have to use two bobbins on a UI for that? My winds are mostly 1/2 secondary -- Primary ---1/2 secondary.  Maybe reversing one half of the secondary? or would that just cancel the signal?
 
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