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O.T. tone

1/7/1998 8:46 PM
PAUL C	O.T. tone I'm thinking of trying that matchless project out , but I'd like some info on what output trans to use and how to get the right sounding one for what I'd like.This subject is the biggest problem for me in the tone dept. I was talking with Dave Funk when he was still here in Nashville about Trainwrecks. He told me one of the biggest secrets in the amp is the custom O.T. that Fischer designed for the amp. You can copy the design, use the same layout and parts - but you'll never "get it" without his trans. We all know about plexi transformers and the cool ones used in tweed fenders and how you just wont get it right without 'em. Just how would you describe the tone effect of the output transformer? What does core saturation sound like? How does it combine with the tone of the tubes? what about the size of the core and it's effect on the low end? What about the windings and the inductance this causes? what about the range of primary imp. that can be used for a tube and it's tone effect? Yada yada yada... Sorry about the amount of "whats" and "how's" in this post, but I want to get a grip on the effects of every single parameter. Numbers - specs - and charts just don't tell me enough about what the dang things sound like. They cost to much to just get a bunch with close, but different, specs to try in a circuit to judge their effect. I would really like to get hard core with this subject so everybody please respond with any info. Thanky kindly Paul

1/7/1998 8:46 PM

PAUL C

O.T. tone

I'm thinking of trying that matchless project out , but I'd like some info on what output trans to use and how to get the right sounding one for what I'd like.This subject is the biggest problem for me in the tone dept. I was talking with Dave Funk when he was still here in Nashville about Trainwrecks. He told me one of the biggest secrets in the amp is the custom O.T. that Fischer designed for the amp. You can copy the design, use the same layout and parts - but you'll never "get it" without his trans. We all know about plexi transformers and the cool ones used in tweed fenders and how you just wont get it right without 'em. Just how would you describe the tone effect of the output transformer? What does core saturation sound like? How does it combine with the tone of the tubes? what about the size of the core and it's effect on the low end? What about the windings and the inductance this causes? what about the range of primary imp. that can be used for a tube and it's tone effect? Yada yada yada... Sorry about the amount of "whats" and "how's" in this post, but I want to get a grip on the effects of every single parameter. Numbers - specs - and charts just don't tell me enough about what the dang things sound like. They cost to much to just get a bunch with close, but different, specs to try in a circuit to judge their effect. I would really like to get hard core with this subject so everybody please respond with any info. Thanky kindly Paul

1/7/1998 10:04 PM
R.G.	When I wrote up the first edition of the Tube Amp FAQ, similar questions (although not as complete) showed up a lot. What you have asked, translated into transformer-geek language, is "How do I completely describe the equivalent circuit of a transformer and the circuit it resides in?" To be truthful, there isn't all that much mystery about transformers, but it's not like the rest of your everyday electronic parts. Transformers are susceptible to electronic modelling, and once you get the model correct, you can twiddle the values until you get the "tone" you want, including nonlinear effects. The later versions of SPICE include nonlinear transformer models for exactly this use. You won't like the answers, primarily because of size. To understand a transformer's effect on tone, you have to be able to model the whole power amp/tube/OPT/speaker chain and account for the effect of changes in the OPT model, then synthesize back to real hardware once you get the response you like. You've asked for a couple of semesters equivalent worth of information on transformer modelling and design linked to a course on the design of the output stage of a tube audio amp. I suggest that if you really want to know this stuff, you find a copy of Nathan R. Grossner's "Transformers for Electronic Circuits", which is out of print, but available at many technical libraries. I put this reference in the Tube Amp FAQ to answer this kind of question. You can model any transformer as a shunt primary capacitance across the primary winding, a series leakage inductance to the primary winding, a series resistor equal to the winding resistance, a nonlinear inductance representing the primary inductance, with a nonlinear resistor in parallel with the primary inductance to represent core losses, primarily from eddy currents. Then an ideal "perfect transformer" to convert the voltages and currents correctly, a series secondary winding resistance, a series secondary leakage inductance, and a shunt capacitance across the secondary. A shunt capacitor from primary to secondary completes the model. Get those component values correct, and you can accurately model everything about any transformer. There are no mysteries hiding in there. The component values are all measurable, and to a certain extent predictable from the start. Any transformer can be copied, Fischer and his ilk to the contrary. So - tone effect of a OPT? first - what does the base transformation ratio do to the reflected loading on the tubes as a function of frequency, including speaker loading. This is fairly independent of the transformer model, depending only on that "ideal transformer" in the middle, but has a big effect on how the tubes put out power. Next - What are the values of the model components? That is, how much leakage inductance, shunt capacitance, and core loss is there? At what points in the excitation does the core start going into saturation, and from the composition of the iron, what is the irreducable energy loss per cycle to magnetizing losses, which shows up as pure third harmonic distortion. Core saturation sounds like any soft limit on a signal; its effect on tone also depends on the symmetry of the limiting. You get primarily third, but smaller amounts of fifth and seventh harmonics on pure tones. Combine with the tone of the tubes? I have a problem with that, and I'm not just being difficult. Define "tone"... The power response of the tubes will be affected a lot by the degree to which the reflected loading on the plates matches the "power transfer sweet spot" for the tube, and this is a function of frequency, depending obviously on the speaker impedance curve and the other parasitics in the model. The size of the core and the number of turns have a direct effect on the low frequency response, but they affect it by changing how much the primary inductance loads the tubes at the lowest frequency of interest. Good designs make this NOT be a consideration in most cases. The winding inductances are entirely subsumed into primary and secondary inductances and have no effect on tone whatsoever - except to the extent that the physical location and sectionalization of the windings contribute to the leakage inductance and shunt capacitances. The effect of the loading on the plates IS a major contributor. Each tube type has a power response curve, power out at a given impedance. There is also a curve of distortion versus loading. In general, the sweet spot of max power is not the sweet spot for lowest distortion, so changes in loading cause the amount of power out to change as the amount of distortion changes, too. Changes in plate loading will cause big changes in tone - and speakers all by themselves have impedance versus frequency curves that vary by four or more to one. To get a good grip, first get some good background. There is not enough room in this forum to type in what you've asked. Get a book, preferably Grossner, but any other that describes the basics of transformer modelling; then I can point you to some books on transformer making that will give you an idea on how to change the things you do in making one that can change those parasitics. A final thought. If the totality of what a transformer does to tone can be modelled by the ideal transformer and some non-ideal components, could you take a transformer with very small parasitics, close to ideal, and add in external "parasitic" components and make it look like any one of a number of less ideal transformers? Yep. You can add inductors and caps to OPTs to make them look more like some transformer you like better, as long as you're not haveing to add negative inductance and/or capacitance.

1/7/1998 10:04 PM

R.G.

When I wrote up the first edition of the Tube Amp FAQ, similar questions (although not as complete) showed up a lot.

What you have asked, translated into transformer-geek language, is "How do I completely describe the equivalent circuit of a transformer and the circuit it resides in?"

To be truthful, there isn't all that much mystery about transformers, but it's not like the rest of your everyday electronic parts. Transformers are susceptible to electronic modelling, and once you get the model correct, you can twiddle the values until you get the "tone" you want, including nonlinear effects. The later versions of SPICE include nonlinear transformer models for exactly this use.

You won't like the answers, primarily because of size. To understand a transformer's effect on tone, you have to be able to model the whole power amp/tube/OPT/speaker chain and account for the effect of changes in the OPT model, then synthesize back to real hardware once you get the response you like. You've asked for a couple of semesters equivalent worth of information on transformer modelling and design linked to a course on the design of the output stage of a tube audio amp.

I suggest that if you really want to know this stuff, you find a copy of Nathan R. Grossner's "Transformers for Electronic Circuits", which is out of print, but available at many technical libraries. I put this reference in the Tube Amp FAQ to answer this kind of question.

You can model any transformer as a shunt primary capacitance across the primary winding, a series leakage inductance to the primary winding, a series resistor equal to the winding resistance, a nonlinear inductance representing the primary inductance, with a nonlinear resistor in parallel with the primary inductance to represent core losses, primarily from eddy currents. Then an
ideal "perfect transformer" to convert the voltages and currents correctly, a series secondary winding resistance, a series secondary leakage inductance, and a shunt capacitance across the secondary. A shunt capacitor from primary to secondary completes the model.

Get those component values correct, and you can accurately model everything about any transformer.
There are no mysteries hiding in there. The component values are all measurable, and to a certain extent predictable from the start. Any
transformer can be copied, Fischer and his ilk to the contrary.

So - tone effect of a OPT?
first - what does the base transformation ratio
do to the reflected loading on the tubes as a function of frequency, including speaker loading. This is fairly independent of the transformer model, depending only on that "ideal transformer" in the middle, but has a big effect on how the tubes put out power.

Next - What are the values of the model components? That is, how much leakage inductance, shunt capacitance, and core loss is there? At what points in the excitation does the core start going into saturation, and from the composition of the iron, what is the irreducable energy loss per cycle to magnetizing losses, which shows up as pure third harmonic distortion.

Core saturation sounds like any soft limit on a signal; its effect on tone also depends on the symmetry of the limiting. You get primarily third, but smaller amounts of fifth and seventh harmonics on pure tones.

Combine with the tone of the tubes? I have a problem with that, and I'm not just being difficult. Define "tone"...

The power response of the tubes will be affected a lot by the degree to which the reflected loading on the plates matches the "power transfer sweet spot" for the tube, and this is a function of frequency, depending obviously on the speaker impedance curve and the other parasitics in the model.

The size of the core and the number of turns have a direct effect on the low frequency response, but they affect it by changing how much the primary inductance loads the tubes at the lowest frequency of interest. Good designs make this NOT be a consideration in most cases.

The winding inductances are entirely subsumed into primary and secondary inductances and have no effect on tone whatsoever - except to the extent that the physical location and sectionalization of the windings contribute to the leakage inductance and shunt capacitances.

The effect of the loading on the plates IS a major contributor. Each tube type has a power response curve, power out at a given impedance. There is also a curve of distortion versus loading. In general, the sweet spot of max power is not the sweet spot for lowest distortion, so changes in loading cause the amount of power out to change as the amount of distortion changes, too. Changes in plate loading will cause big changes in tone - and speakers all by themselves have impedance versus frequency curves that vary by four or more to one.

To get a good grip, first get some good background. There is not enough room in this forum to type in what you've asked. Get a book, preferably Grossner, but any other that describes the basics of transformer modelling; then I can point you to some books on transformer making that will give you an idea on how to change the things you do in making one that can change those parasitics.

A final thought. If the totality of what a transformer does to tone can be modelled by the ideal transformer and some non-ideal components, could you take a transformer with very small parasitics, close to ideal, and add in external "parasitic" components and make it look like any one of a number of less ideal transformers? Yep.
You can add inductors and caps to OPTs to make them look more like some transformer you like better, as long as you're not haveing to add negative inductance and/or capacitance.

1/7/1998 10:54 PM
Stephen	R.G., Yeah..well..that's what I'd call a comprehensive reply! I'm out of breath just reading it Could you point _me_ at some books on actually _making_ OT's. I have the recent series in "Valve" also RDH, and Terman. And we know where the Peerless archives got to but understandably, they are not on public display. I'm gradualy finding my way, but I just have this mad hope that somewhere there is a reference which gives real examples of high quality OT designs and things like the effect of different thicknesses of interlayer and interwinding insulation...and all that fine detail OT production stuff which takes so many years to pick up in tiny bits and pieces. Cheers, Stephen

1/7/1998 10:54 PM

Stephen

R.G.,
Yeah..well..that's what I'd call a *comprehensive* reply! I'm out of breath just reading it :-)

Could you point _me_ at some books on actually _making_ OT's. I have the recent series in "Valve"
also RDH, and Terman. And we know where the Peerless archives got to ;-)

but understandably, they are not on public display.

I'm gradualy finding my way, but I just have this mad hope that somewhere there is a reference which gives real examples of high quality OT designs and things like the effect of different thicknesses of interlayer and interwinding insulation...and all that fine detail OT production stuff which takes so many years to pick up in tiny bits and pieces.
Cheers, Stephen

1/8/1998 8:19 AM
Doc	Mentioning the name Peerless, after R.G. discussed some core/magnetization parameters helped me recall something which may help in describing why certain high quality OPT's sound decidedly better than other high quality units. The Peerless archives, original engineering drawings and notes, were purchased a while back by the owner of Magnequest. I remember discussing with him, in reference to some Peerless 20-20 Plus designs, that special alloy sheeting with superior properties, as far as audio transformers go, was used for core laminations. This material is not currently manufactured (maybe in small quantities at outrageous prices) and the remaining stock was bought up by people in the know. Getting back to what Ken Fischer is quoted as saying, with respect to building a Trainwreck clone and not having the exact output transformer to make it sound the same, and the existence of various other uncomplicated amps with something a little extra in their sound like Matchless, leads me to believe that the iron could be the key. A month ago, I was at a club where a guy in the band was playing thru a 4xEL84 Trainwreck amp and a 4x10 Celestion loaded Mesa cabinet. It had a great sound. The owner let me take a peek behind the nice hardwood cabinet. The transformers were not huge. They looked to be the right size for the power,etc. Since transformers can be copied using the same core size, wire sizes, number of turns, interleaving materials & thicknesses, etc., and still not sound the same, maybe it's the iron alloy which increases the octane just enough to make a pleasing audible difference? Doc

1/8/1998 8:19 AM

Doc

Mentioning the name Peerless, after R.G. discussed some core/magnetization parameters helped me recall something which may help in describing why certain high quality OPT's sound decidedly better than other high quality units.

The Peerless archives, original engineering drawings and notes, were purchased a while back by the owner of Magnequest. I remember discussing with him, in reference to some Peerless 20-20 Plus designs, that special alloy sheeting with superior properties, as far as audio transformers go, was used for core laminations. This material is not currently manufactured (maybe in small quantities at outrageous prices) and the remaining stock was bought up by people in the know.

Getting back to what Ken Fischer is quoted as saying, with respect to building a Trainwreck clone and not having the exact output transformer to make it sound the same, and the existence of various other uncomplicated amps with something a little extra in their sound like Matchless, leads me to believe that the iron could be the key.

A month ago, I was at a club where a guy in the band was playing thru a 4xEL84 Trainwreck amp and a 4x10 Celestion loaded Mesa cabinet. It had a great sound. The owner let me take a peek behind the nice hardwood cabinet. The transformers were not huge. They looked to be the right size for the power,etc.

Since transformers can be copied using the same core size, wire sizes, number of turns, interleaving materials & thicknesses, etc., and still not sound the same, maybe it's the iron alloy which increases the octane just enough to make a pleasing audible difference?

Doc

1/8/1998 10:20 AM
R.G.	The iron alloy does have an effect, and it's tied up in that business about the BH curve and nonlinearities. If you drive a transformer from a voltage source, 0 ohms impedance, then there is no distortion of the secondary voltage as a result of the BH nonlinearities, as the source can provide any current to keep the voltage correct. If however you use a source with a real impedance, like the plate impedance of a pentode, then the nonlinearities demand current, and the plate impedance then limits the current available, so the voltage waveform is distorted on both primary and secondary. Unfortunately, we need the transfomer BECAUSE the tube has internal impedance, so we can't just wish that away. As a sidelight, this is one of the classical arguments for triode output tubes over pentodes or beam power tubes during the golden age hifi years - triodes have a much lower internal impedance and hence lower the distortion of the transformer. What you CAN do is to do some fairly simple tests to map the BH curves of the iron you have, [sidelight: if you find a "magic" transformer, much more "tone" (whatever that means to you) than any other, you can do the work to measure the BH curve nondistructively on the core properties that it has, and then go duplicate them.] and then either get different iron or introduce air gaps to change the effective BH curve of the iron to make a core nonlinearity that matches whatever sounded good. You may not be able to matche the iron perfectly, but it's the core properites, not just the iron, that you're looking for, and there are things you can do there. There are a number of grades of transformer iron, but I doubt that there are larger laminations made especially for audio these days, as there is effectively no money to be made; the money is all in laminations for power transformers. There are several grades of good, linear, high permeability silicon iron made for power transformers, and I suspect that these are what ALL new manufacture OPT's come from. Note that this may (and probably will) result in a core that is bigger than the one you're copying, you may have to rework the windings to get the necessary primary inductance, shunt capacitancs, leakage, etc, etc. to duplicate the response of an iron core you can't get. Also - there are other ways to introduce the specific nonlinearities that make for a good sound if you can ever define what "good" is.

1/8/1998 10:20 AM

R.G.

The iron alloy does have an effect, and it's tied up in that business about the BH curve and nonlinearities.

If you drive a transformer from a voltage source, 0 ohms impedance, then there is no distortion of the secondary voltage as a result of the BH nonlinearities, as the source can provide any current to keep the voltage correct. If however you use a source with a real impedance, like the plate impedance of a pentode, then the nonlinearities demand current, and the plate impedance then limits the current available, so the voltage waveform is distorted on both primary and secondary. Unfortunately, we need the transfomer BECAUSE the tube has internal impedance, so we can't just wish that away.

As a sidelight, this is one of the classical arguments for triode output tubes over pentodes or beam power tubes during the golden age hifi years - triodes have a much lower internal impedance and hence lower the distortion of the transformer.

What you CAN do is to do some fairly simple tests to map the BH curves of the iron you have,

[sidelight: if you find a "magic" transformer, much more "tone" (whatever that means to you) than any other, you can do the work to measure the BH curve nondistructively on the core properties that it has, and then go duplicate them.]

and then either get different iron or introduce air gaps to change the effective BH curve of the iron to make a core nonlinearity that matches whatever sounded good. You may not be able to matche the iron perfectly, but it's the core properites, not just the iron, that you're looking for, and there are things you can do there.

There are a number of grades of transformer iron, but I doubt that there are larger laminations made especially for audio these days, as there is effectively no money to be made; the money is all in laminations for power transformers. There are several grades of good, linear, high permeability silicon iron made for power transformers, and I suspect that these are what ALL new manufacture OPT's come from.

Note that this may (and probably will) result in a core that is bigger than the one you're copying, you may have to rework the windings to get the necessary primary inductance, shunt capacitancs, leakage, etc, etc. to duplicate the response of an iron core you can't get.

Also - there are other ways to introduce the specific nonlinearities that make for a good sound if you can ever define what "good" is.

1/9/1998 1:19 AM
CMD	It is possible to obtain the thinner laminations for OT construction. After some considerable time, I found that Euro-Laminations Ltd make them. The material they make is called 35M6 which is 0.35mm grain oriented silicon steel. They are certainly thinner than most main transformer cores meaning that the eddy current losses will be lower .....thus a better frequency response than you are likely to get from thicker laminations. The lamination size which I obtained is called EI.96 which is about the right size for up to a 50W push pull design. They do make other sizes. I obtained a 3dB point of 40kHz with the transformer I made from these. Quite a surprise. The address is: Euro-Laminations Ltd Europa Works, Atkinson Way, Foxhill way, Foxhills Industrial Park, Scunthorpe, DN15 8QJ England Tel: +44 1724 282929 Fax +44 1724 282930

1/9/1998 1:19 AM

CMD

It is possible to obtain the thinner laminations for OT construction. After some considerable time, I found that Euro-Laminations Ltd make them. The material they make is called 35M6 which is 0.35mm grain oriented silicon steel. They are certainly thinner than most main transformer cores meaning that the eddy current losses will be lower .....thus a better frequency response than you are likely to get from thicker laminations.

The lamination size which I obtained is called EI.96 which is about the right size for up to a 50W push pull design. They do make other sizes.

I obtained a 3dB point of 40kHz with the transformer I made from these. Quite a surprise.

The address is:
Euro-Laminations Ltd
Europa Works,
Atkinson Way,
Foxhill way,
Foxhills Industrial Park,
Scunthorpe,
DN15 8QJ
England

Tel: +44 1724 282929
Fax +44 1724 282930

1/7/1998 11:55 PM
PAUL C	Great Scott, man... how fast do you type?! It's very cool of you to go into such depths on topics I know you must see several times a month. Thanks... I want to take this all the way. I'm tired of wondering if I'm going about tweekin' and dinkin' with designs and circuit values when my "answer" could be in the O.T. I know this stuff isn't magic, but tone to me is. I'm just trying to figure out if I need to change the O.T. in the magic recipe, or add more "eye of newt"! By the way - I saw a Book called "Transformers and Tubes in Power Amplifiers" by Menno vander Veen. Ever heard of it? Is it worth the $26

1/7/1998 11:55 PM

PAUL C

Great Scott, man... how fast do you type?! It's very cool of you to go into such depths on topics I know you must see several times a month. Thanks... I want to take this all the way. I'm tired of wondering if I'm going about tweekin' and dinkin' with designs and circuit values when my "answer" could be in the O.T. I know this stuff isn't magic, but tone to me is. I'm just trying to figure out if I need to change the O.T. in the magic recipe, or add more "eye of newt"!

By the way - I saw a Book called "Transformers and Tubes in Power Amplifiers" by Menno vander Veen. Ever heard of it? Is it worth the $26

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