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Power transformer for SET: toroid vs EI core

Hello,
I am building an SEP (not SET) 307a tube amp and the power transformer question arose.
I need secondaries of [email protected], [email protected], [email protected]

Your advice would be appreciated:
IE core vs Torroid vs R-core?
Does it make any difference if I get single unit with multiple secondaries or separate units for each secondary?

thanks,
Herman
 
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I've used EI's and toroids as power transformers in amps and haven't noticed any major differences in terms of performance. There seem to be more sources for inexpensive toroids, but these (for aesthetic reasons) usually end up under the top plate of an amp, making the whole thing bigger. EI's (with pretty end bells) can sit on top in plain view.

I think toroids are supposed to have a more compact magnetic field. Good in theory. It's hard enough to find heater taps on toroids though, much less heater taps with multiple voltages.

Oh, I misread part of the post. You shouldn't have a problem finding an EI with the necessary voltages. If you want to use toroids, you'll most likely end up with multiple transformers. No problems using multiples as long as it's all wired safely.
 

ericj

Member
2008-12-08 10:24 pm
Be sure to put ferrite beads on the primary wires of toroid power trafos. Toroids are wide bandwidth and suppressing trash riding on the AC mains is important.

Yes, or just use an AC inlet filter that includes a common mode choke. Or put a common mode choke in front of the transformer.

Is there noticeable performance gain from using separate transformers for B+ and heaters?

It's neither here nor there. You either have enough juice or you don't. Either way works.

If you're not totally married to your voltages, you could get something like this reasonably priced antek: AS-1T250 - 100VA 250V Transformer - AnTek Products Corp

It has a pair of 250vac secondaries that you can put in series, and in real world use you might get near 520vac anyway.

Two 6.3vac 3A secondaries - load one of them down with a couple 0.22ohm 3w resistors and its' a 5vac 3A secondary.

It's true that toroids are not the prettiest. You can get covers for them, but Antek charges $28 for theirs which seems like a lot of money to put a hat on a $35 transformer. There are some ebay vendors, but a 100x52mm cover that would fit over that toroid is still about $18 shipped $10-13 shipped off of aliexpress. You could get creative and just find anything that will fit neatly over a 4"x2" donut.
 
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ericj

Member
2008-12-08 10:24 pm
Edcor will sell you an EI core with Really Pretty blue end bells for $58 that will work for you in a similar fashion:

https://www.edcorusa.com/xpwr231

At any rate, when using resistors to drop excess voltage from a heater secondary, be prepared for the eventuality that you may need to take some measurements, solve ohm's law, and order different resistors to get exactly what you're looking for. Or just make sure you're within the allowed margin of error for your tubes.
 

Rod Coleman

Member
2004-07-25 6:55 pm
UK
Ericj, thanks for your answers. For the common mode choke, what mH and Amp rating would you recommend?

Herman

Hi Herman

The CMC should be rated for the peak current in all cases.

It's safer to install it in the secondary circuit, mounted near to the transformer.

If you don't have it, search for "PSUD2" application (Windows and Linux/WINE app). This helps model your power supply, so you can find the peak current in your B+ circuit. Then, add a little margin for safety, and search Mouser/Farnell/RS/Allied etc for

"Ferrite toroidal common-mode choke"

1A, 2A and 1mH to 10mH are commonly available, aim for something in that region. Mount it in a plastic housing for safety.

If you use a mains filter, I recommend removing the built-in caps from mains L and N to safety-earth (these are used to stop emissions from noisy equipment getting OUT into the mains - and thereby failing EMC regulations like EN 55022 & CISPR 22). But for us, these do more harm than good, (when you connect your DAC, for instance).

Compared to an EI, a toroidal trafo can easily have ten times more leakage capacitance from primary to secondary, and this is not helpful for noise transmission. Especially if your neighbours have solar-power up-converters.
 
IE core vs Torroid vs R-core?

There's a good summary of toroid transformer talking points here: Tortran - Toroidal Power Transformer Advantages - Bridgeport Magnetics Group

Superior regulation is often the most persuasive advantage over EI construction, but class A power amplifiers are not demanding in this respect. The toroid's reduced size, weight, and magnetic field can still influence your decision. Capacitive coupling can be suppressed in either type by including a shield, but split-bobbin EI types combine low capacitive coupling with low cost.
 

ericj

Member
2008-12-08 10:24 pm
There's a good summary of toroid transformer talking points here: Tortran - Toroidal Power Transformer Advantages - Bridgeport Magnetics Group

Superior regulation is often the most persuasive advantage over EI construction, but class A power amplifiers are not demanding in this respect. The toroid's reduced size, weight, and magnetic field can still influence your decision. Capacitive coupling can be suppressed in either type by including a shield, but split-bobbin EI types combine low capacitive coupling with low cost.

"Low Cost" being relative here. Antek toroids are extremely economical, compared to "classic" style EI core transformers from Hammond, Edcor, Mercury, etc.
 
Lundahl mains transformer

Maybe this article interested you:

What makes a Lundahl mains transformer so unique?

C-Core3.GIF

 
Is there noticeable performance gain from using separate transformers for B+ and heaters?

I have thought that there is a benefit when using separate transformers not to have the rectifier pulses of the B+ be so easily magnetically coupled to the heater winding and vice versa if rectifying the heater supply.
 
I have thought that there is a benefit when using separate transformers not to have the rectifier pulses of the B+ be so easily magnetically coupled to the heater winding and vice versa if rectifying the heater supply.

Good point, but I would expect a significant benefit from separate transformers only when raw AC heater power is used in low-level stages. In other cases, the waveform distortion component gets filtered out along with the usual (much greater) voltage ripple.