Heyboer and MM 18W Output Transformers

[Mike K]

I should probably get my a$$ in gear and actually test these things. I've had them sitting around for a while.

Any specs you want to know or comps you think I can realistically make, LMK. I'll see what I can do. I dug up some pics of an OG RS 18W from a Reverb auction so we can qualitatively compare. Some specs are listed in the sheet shown with the original.

I'll start off with a few pics. Here's the original:










And here's a side-by-side comp of the two modern contenders (Merc on the left, Heyboer on the right) showing the laminations:






Right off the bat you can see the Heyboer uses much thinner laminations and I would guess a different type of steel for the core. I won't be getting into any metallurgic analysis, but one can tell a little by the color. Certainly the difference in core laminations is a major thing. One would think that the Heyboer would have much less eddy current losses due to those very thin laminations, but they aren't the same as an original and perhaps they are trying to compensate for something else, such as a different core material.


The Merc is a true clone except that it has leads instead of the solder tabs. It seems the lamination directions and lead termination are both reversed so as to try to maintain the same as the original, although the frame is flipped (so the leads come out the bottom). The secondary has the same configuration and the primary has the 43% screen taps.

The Heyboer has only the essential leads and the secondary is set up as a standard Gnd 4/8/16. The primary has no screen taps. None of that matters to me as far as actual use, but perhaps it has some influence on how the coil is actually wound.


A couple other visual notes - the Heyboer uses a what appears to be a paper bobbin (it's all wrapped but there's no indication that it's a plastic bobbin like the Merc. The Merc uses a modern plastic bobbin that looks to somewhat emulate the original. We can't really see where it counts, but it visually creates a more square coil, especially on the outside, much like the original RS.


More to come...

[Mike K]

DCR Measurements for both:








DCR Measurements for the Heyboer are much closer to those stated on the datasheet in the OP. But I'm betting Merc actually measured a RS that they liked the best. There's quite a bit of imbalance between each leg of the primary on theirs, percentwise, much more: 23 vs 6%!

[mozz]

Dc imbalance but what about inductance imbalance ?

[Mike K]

Dc imbalance but what about inductance imbalance ?

I will try to measure this. One might assume DCR would be proportional wire length which would be proportional to turns. I'll first measure voltage ratio at each leg, which should give us a verification of turns ratio.

[Helmholtz]

With OT's I like to measure:

- Effective primary impedance with rated load.

- Effective primary loss resistance with designated secondary shorted.

- Full output primary inductance at 50Hz (or 60Hz) with primary voltage corresponding to rated output.

- Primary leakage inductance(s) @1kHz with designated secondary shorted.

- Primary capacitance @100kHz.

I've also done small signal frequency response measurements (arrangement depending on operating class).

[Mike K]

With OT's I like to measure:

- Effective primary impedance with rated load.

- Effective primary loss resistance with designated secondary shorted.

- Full output primary inductance at 50Hz (or 60Hz) with primary voltage corresponding to rated output.

- Primary leakage inductance(s) @1kHz with designated secondary shorted.

- Primary capacitance @100kHz.

I've also done small signal frequency response measurements (arrangement depending on operating class).

Keep in mind I don't have a LCR meter.

Also, I'm not engineering transformers. I simply buy them and listen to how they perform. I am an engineer, although not an EE, so this is somewhat interesting to me just out of curiosity but in reality I want to build a test circuit and listen to them.

Please outline test procedures for each of the measurements above and I'll see what I can do. If there is a method to measure (and calculate) with a DMM, scope and/or signal generator, those are my current available tools.

[mozz]

Dc imbalance but what about inductance imbalance ?

I asked this because depending on how it's wound you could have a dc resistance imbalance, yet inductance could be balanced. But if both are imbalanced you are not going to have symmetry, but whether that matters in a guitar amp or is a auditory asset I'd be curious.

[Mike K]

I asked this because depending on how it's wound you could have a dc resistance imbalance, yet inductance could be balanced. But if both are imbalanced you are not going to have symmetry, but whether that matters in a guitar amp or is a auditory asset I'd be curious.

Me too. It's always touted that these imbalances in PP amplifiers create more second harmonic distortion (if real). It's also worth noting the traditional use of this OT is on a circuit with an imbalanced PI (all are really). So in that sense, one might get a different result in how they phase this relative to the PI (again assuming at this point that imbalances are real). You might, in one sense, get a more balanced output and in another and extremely imbalanced one depending on if it was phased to be additive or subtractive. The usual response I get when asking the manufacturer about this is: "Try it both ways and see what you like better."

I intend to wire this up on my proto chassis with the Trem channel of the 18W. I'll leave the normal channel off as I don't like it much no matter what OT it has. I have a Heyboer PT which is a z mount but my chassis only accepts standups so I'll need to make a little bracket to hold it. I'm trying to decide if I can come up with a (safe) way to mount and switch the OTs or if I just solder them in and out. I typically do the latter.

[Helmholtz]

Keep in mind I don't have a LCR meter.
.

Keep in mind that I just listed what I would do (and have done) to determine essential OT performance parameters

Please outline test procedures for each of the measurements above and I'll see what I can do. If there is a method to measure (and calculate) with a DMM, scope and/or signal generator, those are my current available tools

Effective primary impedance and loss resistance can be calculated from DMM measurements.
Loss resistance determines efficieny/power loss.

Primary inductance at rated output determines bass response.
To measure I connect a 50Hz voltage of around 200V to one half-primary of the unloaded OT and measure the current to determine the impedance @50Hz. 200V across one half-primary corresponds to 20W in class A with an Raa of 8k.
Requires an isolated variac and AC current meter.

Primary leakage inductance and capacitance determine HF response.
Requires an LCR meter.

You can do comparative frequency response measurements with the OT in your test amp using a variable frequency generator and a scope.
Connect appropriate load resistor to secondary, feed signal to the PI input and scope output voltage.
Amp should not use NFB and signal must be clean.

[Mike K]

Keep in mind that I just listed what I would do (and have done) to determine essential OT performance parameters

Right, I understand these but I would guess 99% of amp builders are not measuring all of them, or maybe none.

Effective primary impedance and loss resistance can be calculated from DMM measurements.
Loss resistance determines efficieny/power loss.

If I'm not mistaken, this will all come from DCR measurements and voltage ratio measurements?

Primary inductance at rated output determines bass response.
To measure I connect a 50Hz voltage of around 200V to one half-primary of the unloaded OT and measure the current to determine the impedance @50Hz.
Requires an isolated variac and AC current meter.

Unfortunately my variac was water damaged but it would not go that high. Our mains are 117VAC @ 60Hz. I can simply use a small PT which should produce around 200 VAC @ 60Hz from the secondary. I have a bunch of different ones around. I'll cap all the other leads off and wire it to a terminal block so it's somewhat safe. I do have a current probe for my DMM but not for my scope.

Primary leakage inductance and capacitance determine HF response.
Requires an LCR meter.

I could buy an inexpensive one but they all seem to have limitations. Some only measure at 1kHz. From what I see, a decent handheld is around $300. I've never had the need for one, so I'm not sure it's a great investment for me.

You can do comparative frequency response measurements with the OT in your test amp using a variable frequency generator and a scope.
Connect appropriate load resistor to secondary, feed signal to the PI input and scope output voltage.
Amp should not use NFB and signal must be clean.

I've done this before, it's mildly useful. The OTs tend to be pretty flat clean and the 100-5k range but often have audible differences driven.

I acquired some documents on transformer design and it seems to me from browsing that significant aspect of harmonic distortion comes from the core material and design specifics. I've not read it thoroughly yet, but there were plots in the document showing this.

[Helmholtz]

Right, I understand these but I would guess 99% of amp builders are not measuring all of them, or maybe none.

I think it's a shame that the data isn't provided by OT manufacturers.

If I'm not mistaken, this will all come from DCR measurements and voltage ratio measurements?

Correct.

The OTs tend to be pretty flat clean and the 100-5k range..

Exactly .
In some case you might find that bass response varies with output level.

I acquired some documents on transformer design and it seems to me from browsing that significant aspect of harmonic distortion comes from the core material and design specifics. I've not read it thoroughly yet, but there were plots in the document showing this.

OT saturation and distortion is unlikely with a tube amp as a pentode/tetrode essentially acts as a current source, limiting primary current.
At low frequencies primary impedance drops (depending on OT design) and causes early distortion in the circuit (i.e. the tubes) but hardly in the OT itself.

[Mike K]

OT saturation and distortion is unlikely with a tube amp as a pentode/tetrode essentially acts as a current source, limiting primary current.
At low frequencies primary impedance drops (depending on OT design) and causes early distortion in the circuit (i.e. the tubes) but hardly in the OT itself.

This seems contrary to the observation that distortion character changes with core size. I also could dig up some clips of two transformers on the same circuit, same guitar, same speakers both running the same primary load but one which causes a fizzy sound when distorted, the other which sounds very clear. This clearly comes through in recording, but IRL, the one that did not fizz had an incredibly bright resonance in the room that I would say sounded analogous to a large bell being rung close to my ear. I tend to note this (the breakdown in clarity i.e. fizz) in almost every transformer though if I'm able to drive it with enough signal. It just seems the "better" ones can take far more and compress and sustain more rather than fizz out.

[Helmholtz]

This seems contrary to the observation that distortion character changes with core size.

Where's the contradiction to my statement?

[Mike K]

Q: What are the things about output transformer that cause the differences in tone? How do differences in output transformer construction combine with tubes to give differences in tone? How do I design/modify an OT for a tone I like? How do I duplicate the tone of a OT I already like?

A: 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. First, define "tone" unambiguously...

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. Poor designs make it a critical factor, and you hear the poor design as either core distortion or low frequency restriction. 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 FAQ 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. The iron alloy also has 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 match 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 well enough.

http://www.geofex.com/tubeampfaq/tub...m#transformers


Seems like there are a number of things that can be done to tweak the non-linearities and parasitics of the transformer to make it sound more "interesting". Many of these things lie within the core.


I'm a firm believer that most of this stuff was found by accident due to material availability and/or manufacturing tolerances. It just so happened that we have 70 years of that sitting in front of us in terms of musical gear and many musicians have either stumbled upon or actively looked for that top 0.01%. It's evolution by natural selection. Now some people try to reverse engineer those properties and replicate them in high volume. We've done the same with pickups, guitars, circuit designs and speakers.

Anyway - trying to measure these things in gross terms like frequency response or impedance ratios may be relatively inconsequential as it's likely the things we are not readily able to measure as well as the sum total of other metrics that may cause real, audible changes.

Ultimately the true test came down to the one that would have selected those that were "better" out of vintage gear: listening and playing. So I believe that's where the true test comes today.

[Helmholtz]

To measure I connect a 50Hz voltage of around 200V to one half-primary of the unloaded OT and measure the current to determine the impedance @50Hz. 200V across one half-primary corresponds to 20W in class A with an Raa of 8k.

I should have mentioned that this method gives the inductance of one half-primary.
For full primary inductance multiply by 4.

To directly find full primary inductance use 400V across full primary.