Not all imported OPTs are good

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Not all imported OPTs are good

Postby 6a3fan » Mon Mar 26, 2012 11:29 am

When my wife studied in the UK in 1997, I asked her to buy me a pair of output transformers from a rather large and well known manufacturer. It was for a 2A3 amp, primary 2500 ohms, secondary at 4 and 8 ohms.

When the OPTs arrived, I installed in the 2A3 amp I built which previously used transformers I made myself using the 1 1/4 by 2 inch cores from some chokes I bought in HK. I was never happy with the new transformers as I thought the highs were a bit ragged. I went back to the home made OPTs. Later on I installed the UK OPTs in a 300B amp I made, carefully adjusting the operating point to use the 2500 ohm primary (the 300B notes show the tube under different operating conditions including for 2500 ohms).

Well, the amp disappointed, and the amp and the OPTs remained in storage for some time. Now that I have an HP signal generator and Tektronic oscilloscope on long term loan, I subjected the OPTs to some tests, specifically testing the OPTs to a 10 kHz square wave, and looking at the response on the 'scope. To put it kindly, the QC of the manufacturer needs improvement. One of the OPTs has a very ragged response, while the other, while squarer, has a rounding off on the leading edge showing some high frequency roll off.

My homemade transformers do much better, showing a squarer leading and trailing edge with very minimal ringing on the same test. Thinking that I can do better than the British manufacturer, I disassembled the OPT, to rewind it. It was rather disappointing to see the way the tranny was made. In fairness, the core was high quality .4 mm laminations. The core former, branded Former, was very well moulded (UK spelling) , in 120 by 1 3/4 size.

But the winding left much to be desired. I understand that the fill window needs to be filled up, meaning lots of turns, to bring up inductance for good low frequency response. The window in the transformer I disassembled was only about 70% filled. Also, there were only 3 secondary sections and 2 primary sections. Again, it is my belief that more interleaving will improve high frequency response. When I started unwinding the core, I found that the insulation between the primary and secondary sections was only .1 mm thick. I understand that you need a thicker insulation at this point, not so much for electrical insulation, but to lower distributed capacitance, to reduce unwanted ultrasonic oscillations that result in ringing. Finally, the primary section was scramble rather than layer wound.

While British and other foreign manufacturers may have access to better quality materials, they are hobbled by high labour costs. I'm sure that if I got the premium grade, the transformer would be better made. Indeed, you get what you pay for. From now on, I will stick to local transformers, who have the benefit of lower labour costs. The tranny and the chokes I have bought from Edrel have been very well made.

I have forgotten how to post photos on this site, but if anyone wants to see the traces of the British OPT, I can email the photo to you.
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Re: Not all imported OPTs are good

Postby tony » Mon Mar 26, 2012 12:16 pm

i hope it is not a Sowter opt.... ;)

or maybe that opt was meant for a guitar amp.....how was the gapping implemented?

But the winding left much to be desired. I understand that the fill window needs to be filled up, meaning lots of turns, to bring up inductance for good low frequency response. The window in the transformer I disassembled was only about 70% filled. Also, there were only 3 secondary sections and 2 primary sections. Again, it is my belief that more interleaving will improve high frequency response. When I started unwinding the core, I found that the insulation between the primary and secondary sections was only .1 mm thick. I understand that you need a thicker insulation at this point, not so much for electrical insulation, but to lower distributed capacitance, to reduce unwanted ultrasonic oscillations that result in ringing. Finally, the primary section was scramble rather than layer wound.


design of set opts are more of an art than science, they are oftentimes closely guarded secrets....you can reverse engineer then, but then you are just limited to cloning those, something an experienced rewinders like Rey Luis and Mang Rod and others can do....

set opt's are a balancing act, you have both dc and ac flux present at the same time....requirements can be contradictory....
more turns means higher inductance, but then your dc flux can suffer....
more interleaving means lower leakage inductance, but then shunt capacitance takes a hit....
there are not many forums that actually tell you how to wind one....
but this is a good start: http://turneraudio.com.au/education+diy.htm
Last edited by tony on Mon Mar 26, 2012 12:37 pm, edited 1 time in total.
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Re: Not all imported OPTs are good

Postby ambel » Mon Mar 26, 2012 12:34 pm

OPT winding is considered to be a black art. Chances are those OPT's were wind by a machine.
You're right that their only advantage is their material M6 imagine if their using 50% or 80% nickel
With that lousy work what a waste.
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Re: Not all imported OPTs are good

Postby 6a3fan » Mon Mar 26, 2012 1:39 pm

The air gap was a .1 mm polyester insulation. The way the winding was done, with some overlaps on the layer wound secondary and the uneven windings, it looked like it was done by hand.
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Re: Not all imported OPTs are good

Postby tony » Mon Mar 26, 2012 3:12 pm

the McIntosh mc275 used jumbled windings.....

the idea is to get high enough inductance, (lesser chance for core saturation, this is why SET opt's always end up bigger than an equivalent pp opt), the leakage inductance and shunt capacitance very low, and the self resonance way above the audible frequency range, 60khz is a good number...
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Re: Not all imported OPTs are good

Postby tony » Mon Mar 26, 2012 8:42 pm

now that we are on the subject, the best core material as far as grain oriented silicon steel GOSS goes is the M2, notice that as the number after the M increases, the quality in terms of losses decreases......cost likewise decreases....
Last edited by tony on Tue Mar 27, 2012 8:57 am, edited 1 time in total.
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Re: Not all imported OPTs are good

Postby brady » Tue Mar 27, 2012 7:59 am

we once dis-assembled a PS transformer of a UK-made electronics. It was too hot and I feared for the nearby capacitors.

we did another winding and used the same core. it now runs cool. I think the UK designer/winder was operating at the wrong point in the BH curve. I can't remember now how many turns per volt he was using
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Re: Not all imported OPTs are good

Postby tony » Tue Mar 27, 2012 8:53 am

he was operating at a higher point in the BH curve......any designer can choose operating points anywhere below the knee of the magnetization curve, as long as the core does not saturate........operating at a point lower in the BH curve, magnetizing current is low and therefore heating is likewise low, and vice versa....

now as you load up the transformer, more current is drawn and operation moves up the BH curve and the transformer heats up some more.......

of course you can increase the number of the primary turns, but that entails using more copper, and voltage regulation suffers as a result.....

so i think we can not say right or wrong here.....it is just a matter of priorities....

it is a matter of economics, copper will always cost more than steel....,

btw Brady, care to join the 35k challenge?
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Re: Not all imported OPTs are good

Postby brady » Thu Mar 29, 2012 7:21 am

"btw Brady, care to join the 35k challenge?" heheheh. Don't know enough...

and btw, even the junkyards where I usually find film/oil can caps, terminations, knobs, etc... have jacked up their prices...
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Re: Not all imported OPTs are good

Postby tony » Thu Mar 29, 2012 12:00 pm

you are too modest.....i am eager to see your power supplies..... ;)
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Re: Not all imported OPTs are good

Postby 6a3fan » Fri Mar 30, 2012 8:15 am

UK AC is 50 hertz. Could this be a factor for the UK power tranny running hot? Or should it in fact run cooler at 60 hertz?
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Re: Not all imported OPTs are good

Postby alexg » Fri Mar 30, 2012 8:41 am

6a3fan wrote:UK AC is 50 hertz. Could this be a factor for the UK power tranny running hot? Or should it in fact run cooler at 60 hertz?


Running hot for a power transformer means that the size of the core is a bit small for the requirement. The core material might also be a factor (inferior core material, hotter power tranny).

I have seen local transformers used in restorations gets very hot!
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Re: Not all imported OPTs are good

Postby dudong » Fri Mar 30, 2012 9:21 am

alexg wrote:
6a3fan wrote:UK AC is 50 hertz. Could this be a factor for the UK power tranny running hot? Or should it in fact run cooler at 60 hertz?


Running hot for a power transformer means that the size of the core is a bit small for the requirement. The core material might also be a factor (inferior core material, hotter power tranny).

I have seen local transformers used in restorations gets very hot!


Unless its repaired/rewound by Edrel :)

I had him rewind the power transformer of my Lafayette LA310 and it now barely gets warm, difference is night and day. :clap:
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Re: Not all imported OPTs are good

Postby tony » Fri Mar 30, 2012 9:39 am

UK AC is 50 hertz. Could this be a factor for the UK power tranny running hot? Or should it in fact run cooler at 60 hertz?


a traffo designed for 50 hz will have more primary turns than a comparable 60hz design...having said that, the uk traffo can actually run cooler if only for this reason.....the difference in number of turns in most cases will not be that big so that some makers actually rate their traffo as 50/60hz......

even an M6 taffo can run hot, it all depends on the design and the amount of copper the builder put into it.....

heat alone is not a determinant of transformer quality, getting a transformer that doesn't heat up much is a matter of preference..... :D

transformers can be designed in more ways than one, what is your pick?
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Re: Not all imported OPTs are good

Postby 6a3fan » Fri Mar 30, 2012 2:53 pm

tony wrote:

transformers can be designed in more ways than one, what is your pick?


How do you like your steak? Medium rare and with only salt and pepper as seasoning for me.
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Re: Not all imported OPTs are good

Postby 6a3fan » Mon Dec 16, 2013 8:32 pm

Reviving an old thread. Now that I know how to do it, the photo posted below shows traces of two output transformers of the same model type built by the same British manufacturer and bought at the same time. As stated in my first post above, the quality control of the manufacturer leaves much to be desired. One of the OPTs shows a good trace of a 10kHz square wave, but the other trace is very ragged. These transformers were used in my 300B amp, and needless to say, this amp didn't get any use in my system as the amp was nearly unlistenable.

Image

Apologies for the poor quality of the photo.

Postscript: I rewound the offending transformer and got a much better amp even if the two output transformers are not exactly the same. But I still prefer my 2A3 amp.
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Re: Not all imported OPTs are good

Postby tony » Tue Dec 17, 2013 10:22 am

are you by any chance referring to Sowter transformer in your first post?
what model was that? i am curious...... ;)
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Re: Not all imported OPTs are good

Postby 6a3fan » Tue Dec 17, 2013 10:26 am

The OTPs are not Sowter. I'll PM the manufacturer and model number.
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Re: Not all imported OPTs are good

Postby tony » Tue Dec 17, 2013 10:32 am

got it...
since you make your own transformers, you can buy good quality cores like M6/Z11 from edrel for your OPT's,
and RM18 for power transformers....
sometimes you can get lucky and buy surplus cores, if you know how they look like,
0.35mm or gage 29 is good, zebra striped even better...
the 0.5mm are also good for power transformer...
happy hunting... :)
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Re: Not all imported OPTs are good

Postby 6a3fan » Tue Dec 17, 2013 10:35 am

A trip to Rosario, Cavite is in my plans for some cores and formers. Perhaps, since we live in the same area, we can make the trip together?
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