Difference between SE output transformer and push-pull output transformers - diyAudio
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Old 7th March 2009, 03:05 PM   #1
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Default Differennce betwee SE output transformer and push pull output transformers

Hello here,

I am new to the tube amplifier world, i have a tube guitar amplifier but now i want to make my own stereo HIFI tube amp ( i think i am going to base it on an EL84 tube frst ).

I have red a lot and i am now looking into the different parts, it is quite easy to get a good power tansformer for a reasonable price so thats not a problem, even though i live in the Netherlands ( i have searched a lot ). But when i came to the subject of the output transformer things got blurry.

Hammond creates two types of output transformer for single ended and one for push pull. What is the difference between the two? i wanted to build/desing a single ended vestion but these transformers are almost three times more expensive ( 70,- vs 200,- euros).

I cant imagine that there is only a price difference so is there anybody here that is able to explain the difference? and maybe someone knows where to buy other brands of output transformers which are less expensive.


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Old 7th March 2009, 03:31 PM   #2
hilbren is offline hilbren  Netherlands
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The differents is very simple. Pushpull trafo's don't have a bias current. that way the magmatic field in the core is AC only.

The reason single ended transformers are more expensive is because they have to handle a DC current. The won't saturate like a normal transformer would do.

More information in dutch at http://forum.zelfbouwaudio.nl/
For the single ended output transformers: buy a aurexx crystal 1 at marktplaats.nl (you also get a set of el84 + ecc83 with that for ~150 euro's)
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Old 7th March 2009, 04:01 PM   #3
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Hi Hilbert,

I am no expert on this matter but I hope I can give some advice.
The difference between single ended and push-pull operation is the magnetic field in the transformer.
Assuming class A operation you have a constant current running through the primary winding of the transformer. Push-pull circuits (and the transformers) are designed that way that equal currents are running in opposite directions through the primary winding of the OT. This is why the B+ is connected to the center-tap of the primary winding. The resulting magnetic fields cancel each other. So in a perfect world there is no magnetic field induced by the DC current through the primary winding in push-pull circuits. The transformer only needs to handle the magnetic field induced by the AC currents.
Therefore the core can be smaller, the whole transformer is easier to design. This is different in single-ended circuits:
In single-ended operation there is a constant magnetic field generated by the idle current through the primary. To avoid saturation the core has to be made with air gaps in between. This causes further problems with bandwidth etc. Since a single ended transformer has to handle a larger magnetic field without running into saturation the core is usually much bigger and the design is more complicated - hence the higher price.

Maybe somebody more into transformer physics will argue against my simple/naive view of the matter...

You're living in the Netherlands, so shipping should not be that expensive from Germany. I would look at "Ask Jan first" site, he is selling good transformers for a reasonable price. They are open-frame but he sells covers, too.

I have not tried to buy Edcor transformers in Europe, but they have bargain prices in the US.

I hope this helps.

Best wishes,
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Old 7th March 2009, 04:02 PM   #4
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Oh, I see hilbren replied while I was typing...
Sorry for the redundant post...
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Old 7th March 2009, 05:01 PM   #5
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Hi Hilbert

If you are thinking about the EL84 SE I recommend you to have a look at the RH84SE, good for a couple of watts.


For cheaper output transformers you can contact 'radio twenthe' in Den Haag. They cary the Lowe line of transformers, which are quite good for the money. Here you have a list with their range of products.


The AU 4A would be a good choice for the above amplifier.

my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!
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Old 7th March 2009, 05:33 PM   #6
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Hello People,

This is why i love the internet...

This drops the costs for my first tube amplifier to the affordable regions. I´ll post some pics when I have completed my first tube amp.

Lots of thanks!
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Old 7th March 2009, 07:23 PM   #7
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Hi Eric,

While looking at the transformers (AU 4A), these have an output resistance of

0-5-15 ohm

I am not yet familiar to these numbers, as far as i know this transformer has an resistance of 0, 5 and 15 ohm. I have 8ohm speakers, how do i change the resistance from e.g. 5 to 8? do i have to put a resistor in series between the transformer and the speaker?

Thanks in advance.
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Old 7th March 2009, 07:36 PM   #8
Pano is online now Pano  United States
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Default Mind the gap

As they say in the London tube stations "Mind the Gap!"

A SE transfo needs a gap in the magnetic circuit to handle the DC bias. A gapped core needs to be bigger to than P-P or the same power rating. So they are usually bigger and cost more fro the same power rating.

If you're building a SE amp, you need a SE transformer with gap, for sure.

Otherwise, not.
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Old 7th March 2009, 08:29 PM   #9
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You may use a PP transformer for SE amp, just add one transistor constant current source that takes a control voltage from cathode resistor. I did that, it works like a charm. The drawback is power consumption is twice higher...

But if you counter-modulate the CCS like I did in my Alligator amplifier you will get more of output power from a single triode.

The Devil is not so terrible as his math model is!
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Old 7th March 2009, 11:19 PM   #10
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Hi Hilbert

The value of 0 - 5 - 15 means that to obtain the given primary impedance you need to connect either a speaker of 5 ohms across 0 and 5, or a speaker with 15 ohms across 0 and 15. If you connect an 8 ohm speaker between 0 and 5 your primary impedance will raise, while if you connect the 8 ohm speaker between 0 and 15 your primary impedance will decrease. This can be calculated, and maybe you even know that... here is a link that explains it all


In practice: a loudspeaker is almost never a nice 8 ohm load over the whole frequency range... 8 ohms is a nominal value, but the true impedance may be lower on some frequencies, while higher on others. This changing impedances of the speaker will be reflected to the primary of the transformer, which will therefore also change. If you want you can tell us what speaker you are using, and then we may tell you if the EL84 SE will have a chance to make that speaker sing! There are no hard rules in defining what a good combination is, but expecting the EL84 to play a large electrostatic panel is unrealistic.

Hmm... I wrote about practice: connecting an 8 ohm speaker to the 5 ohm output of the transformer will probably work very well indeed!
my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!
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