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Power transformer (advise needed)
Power transformer (advise needed)
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Old 1st December 2017, 05:21 PM   #1
Tom1973 is offline Tom1973  Netherlands
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Default Power transformer (advise needed)

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

Last edited by Tom1973; 1st December 2017 at 05:57 PM. Reason: Corrected some typo's
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Old 1st December 2017, 05:49 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by Tom1973 View Post
One of the things I still need is a power transformer.
Dual mono is worth the cost.
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Old 1st December 2017, 06:17 PM   #3
Eli Duttman is online now Eli Duttman  United States
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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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File Type: jpg Da Blok Schematic.JPG (21.1 KB, 327 views)
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Old 1st December 2017, 06:45 PM   #4
audiowize is offline audiowize  United States
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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.
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Old 1st December 2017, 07:10 PM   #5
Tom1973 is offline Tom1973  Netherlands
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@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

Last edited by Tom1973; 1st December 2017 at 07:17 PM.
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Old 1st December 2017, 07:52 PM   #6
gideon1990 is offline gideon1990  Netherlands
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TME has some nice tube amp transformers, example:

https://www.tme.eu/en/details/tsl100...stening/indel/
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Old 1st December 2017, 08:03 PM   #7
Schmitz77 is offline Schmitz77  Germany
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Quote:
Originally Posted by gideon1990 View Post
TME has some nice tube amp transformers, example:

TSL100/001 INDEL - Transformer: mains | TME - Electronic components
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.
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Old 2nd December 2017, 01:27 AM   #8
TonyTecson is offline TonyTecson  Philippines
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Power transformer (advise needed)
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..
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Old 2nd December 2017, 10:24 AM   #9
Schmitz77 is offline Schmitz77  Germany
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In my opinion and experience they arent able to match with good C-cores or toroids soundwise. Try it out for yourself. The better is the enemy of the good parts.
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Old 2nd December 2017, 08:50 PM   #10
TonyTecson is offline TonyTecson  Philippines
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Power transformer (advise needed)
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...

Click the image to open in full size.
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File Type: jpg IMG_8461.JPG (553.1 KB, 215 views)
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