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Output transformers

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Just a thought:

Would it be possible to use ordinary mains transformers as output trannies for lo-fi (guitar) tube amps? I believe a 230/9V transformer would be about 5,2k/8 Ohms, if my calculator serves me.
Should be pretty ok for a single ended EL84 / 6BQ5 amp, right?

What about DC core saturation, air gaps etc.?
EI core or toroid?
VA rating? I´m thinking 3-4 watts output power.

Remember, this would not be for Hifi use!!

Is there anyone who has done this before and is willing to share some experiences? I´m new to tubes, and I would like to fool around a bit with cheap stuff before I pay big bucks for Lundahl transformers and such.


Toriods work great as far as turns ratio and impedances fit. I had to chance to audition that myself and would say they come close but do not reach famous trannies like Tamura and Tango .

And, correction, Lundahls are dirt-cheap if you consider quality. They do match Tango quality (yeah, i know :) , it depends on th model)
Audio or Power Trannies


It is possible to use a transformer designed for power supply use as an audio output transformer. But it is not recommended unless you desire to amplify fairly low frequency signals such as those used in servo amplifier to supply less than 400 HZ to drive motors or such things.

The problem you are going to run into is fairly limited frequency response due to core and winding losses and also poor matching of the output tubes to the load. As to whether or not the results are suitable for your application would be a personnel choice.

I personally would spend a extra few bucks and get a proper transformer. Your proposed application will have to have good frequency response well beyond the range of a typical power transformer in order to reproduce a correct harmonic structure of the individual and combined notes. The steel in power transformers is just not designed to do this.

John Fassotte
Alaskan Audio

have to object partially. With EI cores you are completely right, but with toroids you are not.

My friend Manfred measured toroid power transformers misused for OPT purposes and found them to have bandwidth of several hundred kHz. The toroid core itself does its job unexpectedly, extraordinarily well. Another friend of mine who winds his own transformers lately remarked "if i only could get my hands on cores as good as toroid ones but C-cores atleast come close". This friend is an objectivist, a measuring guy, BTW.

What Tango, Bartolucci, Lundahl and TMK also Tamura uses are those C-cores coming close to toroids.

Where the problems come in with a toroid is interleaved winding, shield foil windings, having the windings exactly matched DC-wise. But usually the turns ratio is exactly 1:1 if the winding has a center tap, and usually the turns ratio primary2secondary is kept well (what i would expect from an automatically manufactured device).

C-cores have basically the same properties. They add the option of having the airgap desired. They have the drawback of not reaching the same coverage a toroid trannie does have.

Where another problem enters with toroids is that proper turns ratio and impedances are not that easy to find as the intended purpose of the trannie is a different one. AFAIH and read in the linked asylum thread, it is more difficult with speaker output transformers, but not a big problem with line outputs, line inputs and interstage trannies.

As transformers have the unpleasant habit to have a preferred impedance situation different to the one you are intending to use, they got bad rep with many people.

But if a trannie has the load impedance at the secondary and the output Z i prefers to see at its primary, it is happy and makes you happy.

So it is advisable to design the trannie's environent accordingly.

Generally (as mentioned above) toroids have no airgap and saturate at low DC currents. So using them for SE is tricky. Using them for not-balanced-enough PP also can give inferior results. Using them for balanced-enough PP can give extraordinary good results, second best only to worldclass transformers desinged exactly for the purpose.

Having said this and having read the original question of Fuling, i would guess that a power toroid OPT PP (not SE !!! :( ) amp with a pair of triode-wired EL84 would be an amplifier vastly better than Fuling's non-HIFIexpectations are. If the amp is made fully differential including the output stage (common CCS for the power tubes), it could be an amp frightening all but the best retail products to death.

As far as my Ratheiser tube manual reports, EL84 work best with 10K of load plate2plate, both in pentode and in triode mode.

Trannies are ratio devices, they translate prim.impedance to sec.impedance. So i would try the toroid out with the tubes driving different secondary loads (2 to 16 Ohms) and check the amp's frequency response for responses in the higher frequency range and then decide whether the trannie is suited for the application.


stay away from EI power trannies for SE amps unless your expectations are really low.... :)
Different types of cores give different results


Yes I agree. I did not consider the toroid transformers. I don't know exactly why since I use them exclusively in all my equipment. Perhaps when tubes are mentioned I go back to the old days when toroids were not readily available and assumed the transformers in question contained staggered E/I cores.

I have never measured the actual performance of a toroid power transformer at frequencies other that 60 Hz. Thus I have no experience with these when these are used as a output transformer.

Thanks for opening my eyes.

John Fassotte
Alaskan Audio
Forget mains toroids- what about o/p toroids!

I've been looking at bulding a PP EL34 with custom wound (toroid) o/p trannies. Surely the question is how purpose designed o/p trannies (toroid Vs EI) compare.

Why is the discussion centered around using toroids not designed for the job?

Is there a shortage of toroid manufacturers able to wind o/p trannies?

I believe the original intent of the original post was likely to save money by using whatever transformer was available that may be able to perform the function of a output transformer.

If a person is willing to send enough money to have a custom transformer wound then it is very likely that using a transformer designed as power transformer would never be considered. However it is possible to use such a transformer in a limited number of applications with fair to good results. The quality of the results depending on the construction of the transformer and the type of core material used. I myself, and most likely most others would not recommend using a power transformer as a output transformer unless there was no other choice.

There are indeed plenty of manufactures that can supply standard output transformers or do some custom design work. Building a output transformers is quite a art and some manufactures who design mostly power transformers may not be able to do the best job due to lack of experience with specialized winding techniques required. This may tend to limit the number of manufactures who would undertake such a task since it would take them away from the mainstream of where they generate income. If output transformer quality specifications are very high then the number of manufactures willing to undertake the task will likely be very low.

John Fassotte
Alaskan Audio
If you want to have a transformer that you can saturate and use for distortion a very good source is PA amps. Bogen,Grommes,etc. They were not made for HiFi but for voice amplication. They are common and should be inexpensive, use common tube types and will work great in guitar apps from what I have been told. Also small SE OPT from consoles that had SE for stereo and a PP for bass are great for SE guitar amps. Consoles should be easy to find and cheap to buy. Good Luck!
A great source for outputs that are great for guitars are PA amps from the 50s. As a matter of fact you would just need to overhaul the amp and be ready to go. PA amps usually had output iron set up for reproducing the human voice( limited bandwidth ) and were not very large and so easy to saturate.(not all of them,but the size of the output iron is your guide) Another source are the old consoles as they often had small SE outputs run by 6BQ5s and could easily be put in a 6V6 amp and since they were mostly stereo you would have an extra transformer. Old record players like Rodgers have amps that are about ready to go as is.
The core of a toroid is wound from a continuous strip of metal like a roll of sticky tape, and so the grain goes along it's length. Tops for trannies.

An EI core has the flux going along the grain then it has to turn the corner and go across the grain. This may be part of the reason they are not as salubrious to sound as are toroids.

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