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Power transformer (advise needed)

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Hi,

I’m planning to build a stereo 2A3 SE amp (JE Labs) and I’m currently gathering parts. One of the things I still need is a power transformer. In The Netherlands there are several companies that will wind a transformer to customer specifications, but none of them list what you actually have to specify.

I’m sure that if I just specify the voltages and currents I need for the different taps I’ll get a really decent transformer, but I would like a great transformer. Browsing the Internet I’ve found a number of things that might be relevant to performance and I would love to get some advice on what to ask for.
  • EI-coreor C-core?
  • Core material (M6?)
  • Regulation?
  • Smallair gap (to handle mains DC)
  • Operating temperature
  • Electrostatic shield
  • Flux density
  • Transformer impedance (rectifiers like to see a minimum impedance, but does it make sense to aim for that minimum)?
  • Anything else?
With kind regards,
Tom van der Laan
 
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Tom,

Air gaps have an adverse effect on trafo performance and increase cost. If DC on the power mains is truly a problem, use "Da Blok".
 

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Hello Tom,

I would recommend just providing the documentation of the JE Labs 2A3 design to the winder and let them work with that.

I would recommend an EI core. Paying for M6 is generally not necessary. In a class A design with everything all penciled out for you, the transformer regulation won't be too critical.

The electrostatic shield is a nice idea and should only add a few Euros. Specifying flux density will drive up the price; be sure you're ready to pay for it! I think you typically see 12,000 Gauss listed as an OK value for a PT, with some of the crappy ones varnished in maple syrup being closer to 15,000. I've heard references to a paper from Marantz back in the day that made a strong case for sticking with 5,000-7,000.

I do not believe that you are going to get a very low DCR transformer that also has low flux density, but others on here may be able to chime in about accomplishing that.
 
@Eli,thank you for the input. I just measured the DC on the mains in our house andit fluctuates between 0.1V and 0.35V. That should not be a problem I believe.So, no air gap in the power transformer.

@Audiowize,just handing the design to winder and let them determine what I need was myinitial plan. That was until I started ‘researching’ power transformers on thenet and found some information that made me second guess my plan. There areapparently people out there who have power transformers wound on ultra Hi-BC-cores normally used in top of the line OPT’s. Thanks for the advice!

Tom
 
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Those trannies without shielding are not first choice for any tube amp.
First choice is C-core or double C-core or toroid trannies. The sound is very different- much better than conventional transformers. Due to the low winding resistances (attention to tube heaters: maybe they need current limiting resistors additional to the rectifier tube, see data sheet of the tubes) or due to the much better magnetic material qualities. Most conventional EI or other cores use cheap lamms- thats not the way excellent sound is achievable. Maybe a cheap and heavy transformer- but sound isnt good.
Search for the better core materials, search for C-cores or toroids( but they are to be avoided in signal- tranformers). And check before, whether your power surge is without DC and CLEAN. A dirty power source with DC is not welcomed for toroid trannies. They tend to vibrate and resonate.
 
i use H18 0.35mm thick lams for EI power traffo...
H50 is the lowest grade, is 0.5mm thick but can also be used,
just remember they will be bigger than a comparable H18 lams...

electrostatic shielding is a must..they are not so expensive to implement,
can be done two ways, copper foils or single layer small #30 wire on end of which
connects to the cores/chassis and the other end left hanging but well insulated..
 
i have very limited access to C cores so i am unable to comment...

since i diy all traffos in my tube amps, i design them very conservatively, where a traffo can do with a 1 1/4 in core, i use 1 1/2 inch core, if stack can be 2 inches, i use 3 inches, i run them at anywhere from 0.6 to 0.8 Tesla....
using cores with bigger winding window, i am able to use thicker gauge wires and takes care of dc resistances, which for tube amp
duties are more than sufficient...
i use full wave voltage doubler rectifiers which allowed for lower voltages in my traffos and that takes care of insulation issues very well too...

pictured here is a 2A3 set i did many years ago, the power traffo used taps in secondary, 250, 300, 350 volts ac, 5U4 hybrid rectifier..today i would add a 400 volt tap so i can be able to challenge the JJ 2A3 to higher plate voltges...

An externally hosted image should be here but it was not working when we last tested it.
 

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Those trannies without shielding are not first choice for any tube amp.
First choice is C-core or double C-core or toroid trannies.
It's funny, every time I've built a tube amp with a toroidal power transformer, I've ended up removing it and going back to EI type transformers. The transformer posted in that link has a flux band, and unlike toroidal transformers, it will have predictable nulls.


I actually believe that toroidal transformers are the cheap choice, and that they are to be avoided at all costs. After all, how much does it cost to "stack" a toroidal transformer? How much does the bobbin cost on a toroidal transformer? How much does the varnish vacuum impregnation cost on a toroidal transformer?
 
There are things which are necessary to obtain good performance from a toroid power trafo. The high bandwidth of toroids is a negative in this role. Ferrite beads that suppress the inevitable trash riding on the mains are very much in order. Toroids behave poorly in the presence of "standing" DC. If a substantial amount of DC pollution is present in the power mains, definitely install "Da Blok".

The low stray magnetic fields of toroids is a definite plus.

BTW, some toroids have a copper flux band around the perimeter. Surprise (NOT), they cost more.
 
Hi,

I’m planning to build a stereo 2A3 SE amp (JE Labs) and I’m currently gathering parts. One of the things I still need is a power transformer. In The Netherlands there are several companies that will wind a transformer to customer specifications, but none of them list what you actually have to specify.
but none of them list what you actually have to specify.
PM member E55L2 here: Login
He is from Holand, Martin will guide you, he have huge experience and can made everything you need.

[*]EI-coreor C-core?
It is said EI OPTs have the old fashioned warming tube sound and C core a more neutral sound, for power trafos perphaps it be less relevant, but in the Yamamoto A08 45 amp all transformers are C core;
 
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It's funny, every time I've built a tube amp with a toroidal power transformer, I've ended up removing it and going back to EI type transformers. The transformer posted in that link has a flux band, and unlike toroidal transformers, it will have predictable nulls.


I actually believe that toroidal transformers are the cheap choice, and that they are to be avoided at all costs. After all, how much does it cost to "stack" a toroidal transformer? How much does the bobbin cost on a toroidal transformer? How much does the varnish vacuum impregnation cost on a toroidal transformer?
Its not about the cost of all the materials, its about the price for the labour and complex winding machinery of a toroid what makes them more expensive than the cheap conventionals.
If they are to be avoided at all costs, why so many good amps today use them? Why they havent been used extensively in the past is a clear thing: they were much more expensive.
Yes, this trannie has a flux band. But a good toroid is shielded all around and that shielding is much more efficient than a single flux band.
Toroidals are more expensive to produce, but an excellent EI core can be expensive, too because of the cost of the lams. But most of them have cheap lams.
My own experience is: toroids generate much lower hum (almost non existing, if its a completely shielded type), they have significantly lower winding resistance which results in more dynamic and lively sounding (thats pure physics, too). Their sound signature is much more neutral and dynamic, whereas an EI have more of a flavour (depends on the lams used).
Most reputated companies use toroids nowadays and thats for the reasons written above.
So if you dont like or cant use toroids, use at least C-cores. They outperform EI or other conventional ones with ease.
 
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I’m planning to build a stereo 2A3 SE amp (JE Labs) and I’m currently gathering parts. One of the things I still need is a power transformer. In The Netherlands there are several companies that will wind a transformer to customer specifications, but none of them list what you actually have to specify. <snip>

You might check transformers offered by Thomas Mayer.
He has a range of power supply transformers; one of those would likely suit your wishes nicely.
These transformers are high quality (well impregnated; double screening; outer flux band; quiet operation).
Reasonably priced.
VinylSavor: transformers
Scroll down.
 
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Status
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