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previous: PAUL C I'm thinking of trying that matchle... -- 1/7/1998 8:46 PM

Re: O.T. tone

1/7/1998 10:04 PM
R.G.	Re: O.T. tone 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.

Re: O.T. tone

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.

Replies:

Stephen R.G.,Yeah..well..that's what I'... -- 1/7/1998 10:54 PM
PAUL C Great Scott, man... how fast do you... -- 1/7/1998 11:55 PM