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Thread: Characteristics of Guitar Amplifier Output Transformers, and criteria for design

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    Senior Member SoulFetish's Avatar
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    Characteristics of Guitar Amplifier Output Transformers, and criteria for design

    I'm trying to better understand magnetic circuits and have been doing some informal reading. I've found discussions and threads in this forum to be insightful; particularly in inspiring me to step back and get some better fundamental knowledge on the subject. Quite frankly, I had some uniformed assumptions that were just wrong. I'm sure I still have some (and there's obviously much more to learn) but I'm starting to piece it together and I'm getting those light bulb/"Oh Shit" moments when it makes total sense.
    I've been mostly reading about Mains type power transformers, and have found the articles on Rod Elliot's site to be really great. They are a really good primer in laying some groundwork for understanding some of the other info I've found in some other sites.
    But, I've been thinking about Output Transformers lately, and would like to have a no bullshit conversation about what we are really looking for in OTs for guitar amps.
    Most of the "output transformer shootouts" that people have put out there aren't that informative at all, I find. Everyone I've seen pits one manufacturer against another and one is left to choose which manufacturer's iron sounds better.
    I would rather understand the different material characteristics and construction techniques and set up the pepsi challenge under those conditions to see if I preferred one method/lamination grade/interleave over another.
    Really, I'd like to try my hand at maybe designing one from the ground up. I think that will help me understand how different materials react under different conditions.
    And I think you guys are just the sort of folks to talk to about it. We need, like, a bat symbol send out for R.G. with threads like this.




    So, where do I begin? I imagine I start with deciding what the power and bandwidth requirements will be?
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    Senior Member Malcolm Irving's Avatar
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    This is a good source (but I expect you may already have found it):

    http://jensen-transformers.com/wp-co...rs-Chapter.pdf

    and this one:

    https://ieee.li/pdf/introduction_to_...chapter_12.pdf

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by Malcolm Irving View Post
    This is a good source (but I expect you may already have found it):

    http://jensen-transformers.com/wp-co...rs-Chapter.pdf

    and this one:

    https://ieee.li/pdf/introduction_to_...chapter_12.pdf
    Thanks for those links, Irv. I also tracked down a pdf of Lee’s “ electronic transformers and circuits “.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    Supporting Member jmaf's Avatar
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    There's a text by RG somewhere where he analyzes old and new trafos and finds subtle differences in old iron and as usual for RG in the process he yields several knowledge pearls. I can't find it now, it's not the top results at Google. You should definitely give his other texts a look as well.

    Sergio Hammernik of Mercy Magnetics talks about rust in transformers in a column for a mag, that PDF can also be found online, which is a second resource that comes to mind about old iron actually being good.

    Just my 2 cents about one of the aspects that isn't discussed very much. Like some wines, good transformers age well, so that's a rich field for research. What's in old iron?
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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by jmaf View Post
    What's in old iron?
    Impurities, those magic impurities. Like what's in old copper wire, for pickups and transformers. Our local pickup builder-repairman is constantly scanning for the oldest rolls of wire he can find "because that's necessary to get that old-fashioned sound." Modern metal is too pure, too good.

    Good advice on the OT's, why reinvent the wheel. Besides being very time consuming, and wondering exactly what it is you're hearing (takes double blind, um, make that double deaf tests), bound to be expensive.

    AND may I add it's good to see you back jmaf, where have you been? It's been a while.
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    Supporting Member jmaf's Avatar
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    Quote Originally Posted by Leo_Gnardo View Post
    AND may I add it's good to see you back jmaf, where have you been? It's been a while.
    Thank you, Leo. Great to chat with you again as well.

    Been on and off several projects, times have been a bit rough here in Brazil. But as they say here, I'm still above the ground and below the heavens.

    Certainly wanna be more in touch with the great discussions and folks on this forum.

    Cheers!
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    The short answer to your topic is: there's no specific criteria for designing a guitar OT. Few simple facts from the past decades illustrate that very well:
    1/ Many different types of iron were used
    2/ Many different types of winding configurations were used
    3/ Most of the time the basic criteria for the OT (especially for the big manufacturers) was the price - the cheaper, the better.

    That's why you can see and OT made from 0.5mm PT grade laminations, one primary/one secondary, without any interleaving. On the other side you can see (not very often) HiFi grade OTs made from M6 iron, many pri/sec, heavily interleaved.

    My sincere advice to you is don't bother designing a guitar OT from zero. If you're into a specific type of sound better find out what specific OT was used in that amp. Also if you think that designers of a specific amp spent endless hours designing that specific OT to achieve that specific sound you'll be as far from the truth as you can get. However if you would like to learn how an OT is generally designed and learn new stuff that's entirely different story. There's room for many experiments in this field.
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    Don't beat yourself up if you don't get transformers clearly in your mind quickly. It can take years for the information from the texts and tinkering to soak in. It did for me anyway.

    Here are some quick and dirty maxims to think of.

    > There is a big inductor on the primary side. This inductor is the model for the loading of the core that your incoming signal must pay to get through the transformer. It is the fundamental low frequency limit on the transformer. The low frequency rolloff point at half power happens at the frequency where the primary inductor's impedance is equal to the reflected load from the secondary.

    Example: If you have an OT with a speaker load, and you've get the winding ratios right, then the speaker load impedance (e.g., 8 ohms, 4 ohms, etc.) is reflected into the primary side by the transformer. For a pair of 6L6's, the plate to plate loading is often set at 4400 ohms. So if you want a half-power bass point of 40Hz so that your guitar's 82Hz low string is a full octave above the rolloff, you instantly know that the primary inductance must be no smaller than
    Zl = 4400 @ 40hz = 2*pi*40hz*L
    or L = 4400/(2*pi*40) = 17.5 henries.
    That's a minimum, and for hifi work they like the half-power point to be lower than a tenth of the actual signal response. In this case, for 1/10 of 82Hz, the inductance needed gets to be 85.4H.

    Things get out of hand rapidly, as making ~100H inductors that can handle tens to hundreds of watts requires BIG cores and lots of copper windings.

    That sets the low end. Doing this stuff well means picking big enough iron and stacking the Es and Is in a way that maximizes the magnetic properties of the stacked iron and minimizes the contribution of air gaps.

    The above is for continuous AC transformers or push-pull. Single ended designs get yet more complex because the magnetic field in the iron has to be able to put out a half cycle of the maximum maximum output energy in a lowest-frequency half cycle. There's only one output device to pull power on its active half cycle, so the energy for the "relax" half cycle of the output device can't come from anywhere but the magnetic field in the core. This gets out of hand in the iron design, as a practical matter, because you must include an air gap, and designing the air gap is tricky and may well need multiple passes.

    > For the high end, the fundamental limits are set by a combination of the leakage inductance and interwinding capacitance, as well as funny stuff like interwinding balance and cross-winding coupling. These are all properties of the winding method, mostly.

    Leakage inductance is caused by the magneto-motive force ( or MMF; that's the current in the primary windings times the turns) forcing a magnetic field to form that energizes the core. Iron cores are not complete "conductors" of magnetic fields, and free space is not a perfect "insulator" of magnetic fields, so some of the MMF sent into the primary coils leaks out into space and does not get inside some of the secondary windings.

    To combat this, special winding schemes are cooked up to intermix primary and secondary windings and layers interspersed with one another so that leakage MMF has a hard time getting out into free space without coupling with the intermixed secondary wires. Ideally, you'd wind the primary and secondary wires side by side, so that every primary wire was parallel with every secondary wire. That's not possible in a world where we want 20-30 times as many primary turns as we want secondary turns, unfortunately.

    The next best thing is to wind a layer of primary, then a layer of secondary, then another primary, another secondary, etc. That is GREAT for reducing leakage, but is a real PITA to actually wind, and makes capacitive coupling worse. One of my first tasks as an engineerling was to derive the equations for leakage inductance from the physical geometry of coils of X width and Z winding height. Then I had to go wind the trans former in the lab and explain why I got it wrong. It was very instructive, and in only another year or so I found where I'd gone wrong with that.

    As a practical matter (there's that word again) commercial ventures can't make layer per layer interleaves. They subdivide the primary and secondary into sections, and wind, for instance, three or four layers of primary, then a couple of secondary, then some more primary, then some more secondary until the right number of turns is on. This can be shown (I finally got the equations right) to reduce leakage by the square of the number of alternations between primary and secondary. Generally, splitting primary and secondary into two sections each or two/three will reduce leakage by a factor of four over just primary over secondary.

    > Interwiring capacitance fights you at every step. If there is a voltage difference between two conductors and insulation between them, capacitive current flows through the space between the conductors. This is true for even two side by side turns of copper wire in a single turn of wire. The intra-and inter-winding capacitance is a distributed thing that depends on where each wire is actually put in the coil. Random winding makes this unpredictable (if smaller - no parallel wires) and different for each article. Layer winding maximizes it. Sigh.

    > The general figure of merit in an OT was defined as the "Goodness Factor". No, I didn't make that up, that's what it was called in the OT biz back in the 50s and 60s. This was the ratio of the primary inductance to the leakage inductance. An OT with a primary inductance of 100H and leakage of 10mH (representative values, these) gives a GF of 100/0.01 = 10,000. This is the range you want to shoot for if you're designing hifi OTs.
    Write up your last will and testament before trying to get to that with an SE design.

    > Other complications happen if you're doing push-pull as anything other than Class A. When half a winding turns off, as it does once each half cycle for Class AB or B, that winding's coupling and magnetic effects (and inductance!) vanishes as it quits participating in the magnetic goings-ons. Not being very careful to make each half-primary have beautiful, complex interlayering/interleaving with the entire secondary causes little glitches to happen in the output right at the spots where current in the primary halves turn on and off. This imposes a fundamental limit to how low crossover distortion can go in a Class AB push-pull output stage. It is the fundamental reason for the "unity coupled" McIntosh OTs and the big hoopla in the hifi market it caused. Guitar amp designers mostly ignore this as an issue, as the silly guitarists LIKE distortion. Let'em live with it. And it's too expensive to fix, anyway, so the makers said.

    Hope that helps rather than confusing issues. Yell with questions.
    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by Gregg View Post
    My sincere advice to you is don't bother designing a guitar OT from zero. If you're into a specific type of sound better find out what specific OT was used in that amp. Also if you think that designers of a specific amp spent endless hours designing that specific OT to achieve that specific sound you'll be as far from the truth as you can get.
    I appreciate the advice. Look, I've though about leaving the whole thing alone and walking away from the topic all together. There's a part of me that has a sneaking suspicion that when the dust settles, and we near the ass end of this conversation, I'll already be 3 orders deep and waiting for more supplies so I can wind my own OT.
    ...ugh. (Don't let me go down that rabbit hole, guys)

    However if you would like to learn how an OT is generally designed and learn new stuff that's entirely different story. There's room for many experiments in this field.
    Yeah, more this ^^ I want to be a little more purposeful in my approach to this area of the design. I generally think learning empowers creativity, and the creative part of this is one things I enjoy most about music electronics.
    (even though I'm not breaking any new ground here)
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    Bent Member Chuck H's Avatar
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    Maybe a good place to start from an amateur perspective would be to reverse engineer the OT from an amp you like the sound of (as much as possible without destroying one I suppose) and then apply your research to HOW those physical properties affect the end result and WHY those results sound good. In doing this you're bound to discover some construction aspects you'd like to hear tweaked a little
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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by R.G. View Post
    Don't beat yourself up if you don't get transformers clearly in your mind quickly. It can take years for the information from the texts and tinkering to soak in. It did for me anyway.
    Yeah, I have a lot of questions that kind of stretch out over all the different factors at play. For instance, primary inductance is an important factor in audio transformers, where it is of no real design consideration in power transformers? The inductance needs to increase with loading impedance on the driving signal. This seems to indicate a relationship between inductance and magnetizing current? Also, making practical sense of BH curves in the context of voltage/current/frequency conditions. For instance, is there risk of saturation at low signal levels in high permeability cores? Anyway, I have many more, but at this point I'm trusting the learning process. I know it will come together. So let's start off where you did.

    Here's what I'm designing for:
    2XEL84s with a p-p load impedance of 10k2 and a peak sine wave output of around 17.5W. But square wave output at full drive, i'm guessing +25W maybe? Let's account for alternate tuning and the rare bass player who may want to plug in and set the -3dB at 20Hz.
    That means I require a primary inductance of around 81.5H rated for min 25W. (Ideally)
    Now What? (lets make some big iron)
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    Bent Member Chuck H's Avatar
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    Hammond 1609

    The 1608 at 8k has a very good rep for 18W type builds and it's what I have in my personal (prototype) amp. I solicited Heyboer for the production model and the amps are apples to apples. The Hammond sounds better. Maybe try one and see what you think, then reverse engineer it if you want to play with that. The transformer is rated for 10W, but trust me when I tell you that it will handle anything 2Xel84's will throw at it under any operating conditions. I have that from a tech at Hammond and I've experienced it for myself.
    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Gregg View Post
    Also if you think that designers of a specific amp spent endless hours designing that specific OT to achieve that specific sound you'll be as far from the truth as you can get.
    True.
    Amp designers work on the *electronic* side of the problem , and just *order* transformers from an Industry supplier, leaving him to work the details.
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    Senior Member Malcolm Irving's Avatar
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    Quote Originally Posted by SoulFetish View Post
    … . For instance, primary inductance is an important factor in audio transformers, where it is of no real design consideration in power transformers? ...
    Primary inductance is in parallel with the load reflected from the secondary, so for an audio transformer the tubes supplying the OT have to provide current into that parallel inductance as well as into the proper load. At low audio frequencies, the reactance (2.pi.f.L) of the primary inductance is low and the extra current causes the primary voltage to drop (due to voltage drop across the output impedance of the tubes - if you like).

    For a PT, the internal impedance of the mains supply is very low (say 1 ohm or so). The mains is almost a perfect voltage source. In this case some reactive current is 'wasted' into the primary inductance, but the primary voltage holds up and the secondary (load) side is happy. There still needs to be a reasonable primary inductance (and hence reactance at 50 or 60 Hz) otherwise the primary would draw very high current and overheat.
    Last edited by Malcolm Irving; 05-15-2018 at 12:29 PM.
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    Senior Member Malcolm Irving's Avatar
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    Quote Originally Posted by SoulFetish View Post
    ... The inductance needs to increase with loading impedance on the driving signal. This seems to indicate a relationship between inductance and magnetizing current? …

    Yes. We can think of whatever is driving the primary as a Thevenin Equivalent, i.e. voltage source in series with an internal impedance.

    Then the magnetizing current is Vs / (Zth + Zmag)

    where Vs = source voltage, Zth is source internal impedance, and Zmag = magnetizing reactance ( = 2.pi.f.L ).
    Last edited by Malcolm Irving; 05-15-2018 at 11:40 AM.
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    Senior Member Malcolm Irving's Avatar
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    Quote Originally Posted by SoulFetish View Post
    ... Also, making practical sense of BH curves in the context of voltage/current/frequency conditions. …

    In the B - H curve, H is proportional to magnetising current and 'rate of change of ' B is proportional to primary voltage.
    For a sine wave, the 'rate of change' is just another sine wave (but advanced by 90 degrees, i.e. a quarter of the wave period).

    In a PT, for example, the primary and secondary voltages are close to sine waves, but the B-H curve means the current into the primary is distorted. The primary inductance (also called the magnetizing inductance) is a non-linear inductance (due to B-H).

    The B-H curve shows the non-linearity and hysteresis between primary voltage and magnetizing current.

    The frequency of the signal is how many times 'we travel around' the B-H loop per second.

    I hope some of this is helpful and I apologise if I am 'teaching my grandmother to suck eggs'.
    Last edited by Malcolm Irving; 05-15-2018 at 12:31 PM.
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    That means I require a primary inductance of around 81.5H rated for min 25W. (Ideally)
    Lp values this high are quite realistic in guitar amplifier OTs. But only around rated output power and at low frequenies (max. ac flux). I measured Lp of around 40 different OTs at 650Vpp and 50Hz and found Lp to lie between 60H and over 300H. But when measuring with an LCR meter @ 1kHz, the values were lower by a factor of 5 to over 10. (The reason for the different values is that the LCR meter measures at very low flux amplitudes where the ac µ is close to the initial permeability. The ac µ rises strongly with flux amplitude up to the onset of saturation.)

    The results seem to indicate that the major design concern was to avoid excessive power loss from high magnetizing current at max. output, while the bass response at low output power may suffer. Of course NFB can take care of this.
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    In transformer design, everything depends on everything else, so getting an optimal transformer is an exercise in optimizing many things at once. Accordingly, with so many ways to make things better in some sense, there are many ways to look at the transformer and its drive and loading. I mention this not least because Malcolm has presented some views that are correct. Here are some other views that strike me from your questions.

    Quote Originally Posted by SoulFetish View Post
    For instance, primary inductance is an important factor in audio transformers, where it is of no real design consideration in power transformers?
    That particular question is answered in my mind as a question of objectives. Primary inductance is still a big deal in power transformers, but it's hidden behind other factors that are quoted. For PTs, the "bass response" is more subtle. You know the frequency response needed going into the design, so that's not the issue. The issue is how much of the incoming AC current leaks through that primary inductance, and correspondingly primary inductance is manipulated to get a "goodness factor" measured in dollars.

    A power transformer will be fed a single frequency at a relatively fixed size for 100% of its life. A better primary inductance "bass response" means that the amount of electricity that unavoidably leaks across the primary winding as a result of the primary inductance impeding it. The primary inductance spec is tied up in the "magnetizing current" or "efficiency" specifications. Big primary inductances make magnetizing current smaller, and to the PT customer, that makes the dollars spent on electricity over the lifetime of the trannie lower. It also makes the transformer run cooler (in concert with several other factors), so primary inductance still matters, it's just hidden as a specification.

    It would still be good if the primary inductance was huge, but huge costs money in terms of iron, copper, and labor.

    Interestingly (to me anyway) is that primary inductance being too small in power transformers is what got line frequency wall warts made illegal. The unavoidable primary current leakage got noticed by the protectors of the planet in California. Wall warts that stay plugged in all the time "spend" this electricity all the time, and the multiplication exercise of X zillions of wall warts at Y milliwatts each, times Z hours plugged in came up to "Oh my &deity. we have to stop that!" and they legislated that no wall wart could have a no load current bigger than "too small to be a line frequency transformer", and also got the EPA to issue the same regs for the country. This had the effect of making only switching power supplies be legal. And so the world was saved from primary inductance.

    PTs are all about power transfer (duuuh...) so their specs get wrapped up in volts, amps, phase angle and power loss terms. Fidelity is of little concern, so the funniness of BH curve bending and minor BH loops gets ignored, where it can't be for output trannies. Primary inductance and even leakage inductance gets hidden in the power specs.
    The inductance needs to increase with loading impedance on the driving signal. This seems to indicate a relationship between inductance and magnetizing current?
    Absolutely correct. Primary inductance is always across the driving signal and must be energized before any signal gets to the secondary. It's a price that has to be paid. To minimize magnetizing current, you maximize inductance.

    More subtly, you minimize the area of the BH loop in the core. Transformer designers work with a chart of flux density (B) versus magnetomotive force (H). The more closely the graph of B versus H approaches a straight (and vertical!) line, the better the material is. The slope of the BH graph at any point is proportional to the primary inductance at that flux density, so higher slope is better. Real materials do not retrace the same path on the BH curve going up and coming down, making each cycle of signal be an odd, squashed-S loop. The area inside the loop is representative of the energy wasted per cycle in iron losses.

    And put another way, the inductance varies at different levels of flux density. Yes, that means that an OT has distortion all on its own, due to the nonlinearity of the iron's magnetization. The more the iron "eats" of your signal (i.e. the lower the magnetizing inductance), the more the iron distorts your carefully prepared audio.
    Also, making practical sense of BH curves in the context of voltage/current/frequency conditions. For instance, is there risk of saturation at low signal levels in high permeability cores?
    I already got into that a bit. A BH curve is just a plot of how much magnetic flux density (magnetic field intensity) exists per unit of MMF. Flux density used to be measured in (maginary) field lines per square foot/inch/yard/etc. Now it's measured in Teslas, after the guy who had the nerve to read a newspaper inside a lighting generation machine. Whatever the units, it's still magnetic field intensity. MMF is measured in Oersteds, after a guy I don't have a good anecdote for at the moment. It's units are abstract as well, but it's proportional to ampere-turns, and so designers universally think of H as ampere turns times some funny constant to make the units come out right.

    There is no concept of frequency in the BH chart. Iron has no frequency, and BH charts are ways of making statements about the magnetic material. Frequency gets into the whole picture because you have to drive the iron to a certain amount of ampere-turns to get a certain flux density, and how fast you can get there depends on how hard you can drive the primary inductance (slope of the BH line). It's always V = L *di/dt, so how fast you can get to a certain I depends on how much voltage you can impress on the total number of turns around the iron. All this is external to the material itself, and BH curves are about the material.

    And yes, there is some issue with saturation of high permeability materials with low signals, but as with all things magnetic, it happens a funny way. Ferromagnetism happens BECAUSE unpaired electon spin directions in the atoms of iron, nickel, and cobalt can be oriented by a magnetic field. This orientability is what lets an M field "flow" through ferromagnetic materials more easily than they flow through free space. When all the spins are in random directions inside the material as they are in virgin materials, making a few of them line up to follow a magnetic field is easy, so you get a lot of alignment for little energy input. That means, lots of field density for little MMF, so the BH slope is high and the inductance is high. As you get more and more of them aligned, it gets harder to get the NEXT atom's spin aligned because the easy ones to align are already in line. So you get incrementally less alignment and field strength per unit of MMF trying to align them., And the inductance is resultingly less. At some point, all of them are lined up, and with no more easy alignments to be made, the material is saturated, and any additional MMF can create B only at the rate it does it in free space. That's saturation.

    Some special recipes for iron, nickel, cobalt, aluminum, copper, and other metals and elements can make it easier for alignment to happen than in a pure sample of the big three (iron, nickel and cobalt), and so there exist special alloys with higher incremental inductance as reflected by steeper slopes on their BH curves. This being the real universe, there is no free lunch and you have to pay for this with lower saturation flux density, higher losses in terms of bigger area inside the BH loop, or both. So yes, there are materials with insanely high per-unit inductability ( I just made that word up) but these often saturate at low flux densities, so you can have high inductance, but low saturation. Still depends on how many turns and how big a lump you're driving with signal, though.
    2XEL84s with a p-p load impedance of 10k2 and a peak sine wave output of around 17.5W. But square wave output at full drive, i'm guessing +25W maybe? Let's account for alternate tuning and the rare bass player who may want to plug in and set the -3dB at 20Hz.
    That means I require a primary inductance of around 81.5H rated for min 25W. (Ideally)
    Now What? (lets make some big iron)
    And this is where designing gets easier. You're designing for your own edification, so you can spend as much on iron and copper, and winding labor and insulating materials as you'd like. If your job is designing transformers, you get pay raises and promotions in return for making the next design return higher profit, not for making it pleasing.
    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Supporting Member Jazz P Bass's Avatar
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    At a lecture, Oersted passed a compass over a current carrying wire.
    The compass needle was deflected.
    (What the heck was that!)

    When the current in the wire was reversed, the compass needle also swung the other way.
    (Holy, Moley!)

    So the man is credited with the 'discovery' that a current carrying wire has a magnetic component,

    In actual fact, to this day, no one knows exactly what he was trying to prove at that lecture.

  20. #20
    Senior Member Malcolm Irving's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    At a lecture, Oersted passed a compass over a current carrying wire.
    The compass needle was deflected. …
    Oersted having shown that electric current creates magnetism, Faraday had the idea 'well maybe magnetism should create electric current'. He put a large coil around a big permanent magnet, and measured current in the coil using a galvanometer. There was no current. But then he noticed that there was a current when he removed the coil from the magnet or replaced it on the magnet. Faradays law: induced voltage is proportional to 'rate of change of flux linkage'.

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    Design is one thing. Build is another - at least here in the UK. I was discussing transformer design with an amp builder who had done the design legwork and then went from factory to factory to get the things built. The best deal he got was to meet the minimum order of 10,000 stampings for each lamination type. The amps that used the transformers had a short run so he's left with lots of stock.

    Even when I've approached companies to build short runs of replica transformers they've directed me to current standardized sizes using off-the peg components, as anything custom is way too expensive for the small numbers involved.

  22. #22
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by Malcolm Irving View Post
    Primary inductance is in parallel with the load reflected from the secondary, so for an audio transformer the tubes supplying the OT have to provide current into that parallel inductance as well as into the proper load. At low audio frequencies, the reactance (2.pi.f.L) of the primary inductance is low and the extra current causes the primary voltage to drop (due to voltage drop across the output impedance of the tubes - if you like).
    .
    See, I knew I came to the right place. This makes complete sense. Thank you, this is a great exclamation!
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    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    Even when I've approached companies to build short runs of replica transformers they've directed me to current standardized sizes using off-the peg components, as anything custom is way too expensive for the small numbers involved.
    If you need a small run of something at maybe half UK prices (without quality loss) you should check some central and/or eastern EU countries where you can find small businesses that will accept small orders without any problems. Knowing a local also helps a lot.
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  24. #24
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by R.G. View Post
    Now it's measured in Teslas, after the guy who had the nerve to read a newspaper inside a lighting generation machine.
    R.G. c’mon man, I obviously know who Tesla is! I don’t remember the lightning thing though, I wasn’t at that show.
    https://youtube.com/watch?v=9vwHuCC6nP8
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    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  25. #25
    ric
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    Quote Originally Posted by SoulFetish View Post
    R.G. c’mon man, I obviously know who Tesla is! I don’t remember the lightning thing though, I wasn’t at that show.
    https://youtube.com/watch?v=9vwHuCC6nP8
    Well yeah, he's the guy building those cars and flying rockets.

    What I don't get is which is the right OT ...Hammond 1609 or 1608?

    From simple recipes to building your own induction cooktop and genetically engineering the ingredients, it's all here.
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    Quote Originally Posted by Gregg View Post
    If you need a small run of something at maybe half UK prices (without quality loss) you should check some central and/or eastern EU countries where you can find small businesses that will accept small orders without any problems. Knowing a local also helps a lot.
    I needed some gearbox parts for an old Harley and sent the worn ones to a machine shop in Poland to get them replicated. Heard nothing back for over 6 months and then a greasy parcel arrived. I was knocked out by the quality, finish and fit. Proper materials, properly hardened.
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    So.... What's my next move? Choosing a core lamination grade and begin the process of sizing it out?
    From what I've been reading, it seems that there are more than a few designers/builders of transformers who prefer to use non grain oriented steels for guitar OTs. (I understand that you'll find M6-M50 type lamination, with a variety of winding styles historically).
    For instance, I know that Heyboer uses M50 for many OTs, and a few designers prefered M19. Any thoughts either way on this?
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    Since you're asking about iron grade here's your first idea for experiments: wind a bobbin and use the same one with several types of iron to see (hear) the difference. Use a miked cab to capture the sound otherwise it will be all subjective due to ear fatigue as well. Also make sure the amp has the same settings at all times.
    Finding laminations retail in quantities for single transformers is not easy though. Most of the time the manufacturer will sell them as 20-22kg stack from specific grade.
    Over the years I've used M330 (0.5mm) and M6 (M165-35S according to DIN, EN 10107 - this is how it's known in Europe) grade iron and the results were quite good however I wasn't trying to replicate any specific transformer sound.

  29. #29
    Old Timer J M Fahey's Avatar
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    Don´t overthink it

    As Enzo says, these are guitar amps, not NASA Mars Mission Landers.

    You must *really* strethc design limits and carefully balance conflicting requirements in Hi Fi transformers, and that´s fine, not audiophoolery by any means, but on *Guitar* amps?
    Why try to:
    * lower distortion from 0.5% to 0.2% when tubes by temselves have 5 to 7% distortion ... when *clean* , and are usually clipped anyway?
    Reach 50kHz so 20kHz response is still flat when Guitar speakers drop at 24dB/oct above 3500Hz?
    And a speaker reaching 4500Hz (Italian Jensen) is deemed "unbearably nails-on-blackboard harsh/icepicky?"
    * guitar lowest frequency is >80Hz and in many respected amps (Marshall) it´s attenuated below 160Hz?
    Mind numbing below 700Hz or so in VOX amps.
    * transformer self resonant peaks which in Hi Fi amps must be tamed and when present held >>25 kHz become not that important when considering speaker bandwidth.
    * hard to achieve low phase shift to allow higher NFB without breaking into oscillation is not important in amps which either have relatively low NFB factor to begin with (Fender) , attenuate/kill higher frequency NFB (which is where problems appear), think Tweed and Marshall Presence control, or plain have NO NFB at all (VOX and many others).

    An OT must be very horrible to damage Guitar sound ... and in many cases restricted bandwidth may be a bonus.

    I wind my own, interleave a little within reason, from 1/2 Pri - Sec - 1/2 Pri to **at most** 4 primary sections interleaved with 3 Secondary ones, and that for Hi Fi amps, consider Marshall does 1/2 Pri in one continuous wind, 0-4 ohm winding , 1/4 Pri , 4-8-16 ohm winding (which has exact same number of turns as 4-8) , 1/4 Pri so it´s 5 sections in total.

    Champ transformers are a joke ... on paper .... are wound for 200Hz minimum frequency ... not that bad considering the tiny light speakers usually fitted to them also fall like a brick below 200Hz, so ...
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  30. #30
    Senior Member SoulFetish's Avatar
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    I appreciate this, Juan. I know you know your way around iron, and this is practical advice.
    I'm actually not interested in achieving the kind of fidelity required by hifi designs. But, in this particular case, I do want to design for (relatively) wide bandwidth, avoid saturating the core, and operate at full drive output without breaking a sweat.
    As far as some of the classic amps you mentioned go, IMO they often were able to overcome some of their deficiencies and sound good, I don't necessarily think they sounded good because of them. (Although, I'm really surprised at the 700Hz figure for voxes. I wouldn't have guessed that.)
    At the other end of the spectrum, I currently have a transformer which was made using M6/-1dB@20Hz to -1dB@20kHz/probably heavily interleaved. I'm not sure if I like this type of OT for a guitar amp... I'm not sure if I don't either. I would like something to compare it to. Something in between this and not close to a champ.
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  31. #31
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    But, in this particular case, I do want to design for (relatively) wide bandwidth, avoid saturating the core, and operate at full drive output without breaking a sweat.
    Most of the time guitar OT's can go down to 50 Hz at full power without problems and saturating the core is not so easy. Concerning the top end 15kHz is more than enough and is not difficult to achieve. You can use these as a start. In calculations shoot for -3dB. For guitar OT-1dB is overkill.


    Couple of examples from real guitar OT's (and what you'll actually get from calculations at -3dB):

    1/ 50W - iron sizes EI96x32-40mm,
    primary 1600-1800 turns/ 0.25-0.3mm
    secondary 60-68 turns/0.8-1.2mm
    2/ 100W - iron sizes EI96x45-60, EI108x40-50mm, EI115x35-45mm
    primary 1000-1200 turns/ 0.35-0.45mm
    secondary 52-57 turns/1.2-1.5mm
    The wire thickness for the secondary is for a single 4 Ohm winding which means that if you have two secondaries in parallel you must divide that by square root of 2 etc. All wire thicknesses are without insulation/lacquer.

    After you get your numbers it's time to choose the winding configuration and there are many. Then you have to see what is the actual wire thickness (including the insulation/lacquer) and do some calculations how many turns will fit in one layer on your transformer bobbin so they can spread equally in all layers. Some corrections may be necessary but if you get to that stage we'll help.
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  32. #32
    Senior Member Malcolm Irving's Avatar
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    In a band context, with a Bass player, I think it sounds better when the low frequencies of the guitars are attenuated. Especially when there are two guitars, their low frequencies just muddy the sound and 'get in the way' of the clarity of the real bass line. (Keith Richards was famous for 'inventing' the five-string guitar by throwing away his low E string. )

    For a solo jazz player, in the style of Joe Pass, a bit of hi-fi low frequency could be good, though.
    Last edited by Malcolm Irving; 05-19-2018 at 12:49 PM.
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  33. #33
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by SoulFetish View Post
    (Although, I'm really surprised at the 700Hz figure for voxes. I wouldn't have guessed that.)
    I was also amazed when I calculated it

    See for yourself: original VOX AC30 from the 60's



    Check Bright Channel volume control and its coupling cap, at low volume you have C1 500pF and VR2 500k .
    Use this handy online calculator:RC pad corner frequency upper and lower cutoff frequency calculation filter calculate time constant tau RC voltage power calculator capacitance resistance - sengpielaudio Sengpiel Berlin

    Using those old series nominal values you get some 639Hz cutoff (at mild 6dB/octave, so you *still* have some Bass and Low Mids, just quite attenuated) , with modern normalized values it would be 470pF and 470k = 720 Hz. In practice same thing.

    And that is with volume set low; when set to "10" capacitor also sees R9 and R7 to ground (depending on Normal Channel Volume setting, but which can be assumed is set to 0 if unused) so cutoff frequency starts at some 1200/1400 Hz

    I BET Chris Jennings did not fire up his slide rule to calculate a cutoff frequency, but most probably had somebody playing his prototype LOUD and tried different capacitors until he found one which cleaned the distorted sound a lot .... tried and true design technique

    FWIW revered Trainwreck amps are *basically* "a Fender amp with an extra tube for gain/sustain and a VOX type strong Bass cutoff for clarity"



    Notice 0.002 coupling cap and 180k grid resistor at the third triode, which is also a cold cathode clipper, cutting below 440Hz.
    The rest of the circuit is again a basic Fender with tweaked (Marshallish) Tone Control values.

    Like at Mc Donalds, where they can offer a couple dozen "different burgers" using basic 6 or 7 "components".
    Actually less, because they *always* need to use the bun and at least 1 patty
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  34. #34
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    Most of the time guitar OT's can go down to 50 Hz at full power without problems and saturating the core is not so easy.
    True, but an OT being down 3dB@50hz at full power may be down 3dB@500Hz at low power.
    Saturation headroom for a given core increases with the number of primary turns.
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  35. #35
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    True, but an OT being down 3dB@50hz at full power may be down 3dB@500Hz at low power.
    I didn't quite get that?
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