Choosing an output transformer

[moonbird]

Originally Posted by paulorr1

In the original post I thought the poster wanted some real rule of thumb stuff....like plate resistance tube spec relating to primary OPT winding Impedance as listed in the Hammond catalog for example.

I am interested in ALL METHODOLOGIES that people use -- please share yours!!

I have learned alot (and continue to) and received great advise from this post so far. Most importantly about all the things I needed to learn which I am in process of (better than watchin the boob tube ). My wife and I discuss (and laugh about) this post/forum all the time. The ecclectic nature of this post is JUST what I wanted -- thx to all. Keep it coming

[BudP]

In the original post I thought the poster wanted some real rule of thumb stuff....like plate resistance tube spec relating to primary OPT winding Impedance as listed in the Hammond catalog for example.

Yup, exactly that. However....... life is usually much more complex than a quickie thumbnail riule allows for and transformers are no exception.

If you want your OPT to actually match your tube, across the frequency response band and you want to be the person who determines what to buy, what Johan and I have portrayed is what the rule of thumb must contain. Otherwise, you have to go to a designer, like Johan, I, Per Lundahl, Dave Slagle etc, or the fine folks at Hammond, Edcor, James, etc and have them do the work for you.

The Hammond catalog specs will get you close enough that your amp will work and you will recognize the sounds being played through it. That is as close as a rule of thumb choice can get you, and there is NOTHING wrong with this approach. For prototyping and casual building for proof of concept, I recommend that do go through Hammond or other generalist manufacturers. For something more revealing, the devil is in the details and you have to know about them and be ready to use them.

Bud

[moonbird]

That what I have learned from this thread - thanks Bud for summing it up so well!!

[Johan Potgieter]

Same from me, Bud - you obviously know your magnetic material' behaviour.

A question then from me regarding the frequency response of core material. My take is that basically this depends on the thickness of the laminations - the thinner the better. But these days where just about every winder is into power transformers (50Hz or 60Hz), and those doing disco equipment output transformer rewinding not really wiser, how do I tell the h.f. capabilities of a steel? You might have picked up that for hi-fi stuff I prefer using C-cores (thin sheet); are they otherwise the same?

Some info in this regard will be appreciated (or a web-site I overlooked).

Thanks

[BudP]

Hi Johan,

Well, you are quite knowledgeable yourself. I only have some postulates to pass on, concerning core material and high frequencies.

1.) The thinner the core material the finer grained will be the planar magnetic fields, constructed within the window of E/I core. I think this is true of wound core also, but the layers of core are not edge laid into the window with C core etc. Also, I have never found a C core with thicker than 0.006 material so I really have no way to compare the theoretical mag field structures between the two. It is entirely possible that C core is superior here, with only two very large fields, rather than many very thin ones.

2.) Commercial E/I core materials are not in the picture, for being the transformation vehicle of signals, much above 400 Hz. Anything thicker than 29 gauge M6 is useless by 250 Hz. They still act as a ferrous bounding box. They still dictate the speed with which a magnetic field can be created within the window, and thus potential signal rise time, but a direct antenna to antenna event is the true transformation activity above these frequencies. Again my study of C Core in this respect is very limited, but I have not found any reason to think that power loss curves for wound core differ from E/I core materials. I am still discussing non nickle iron core materials here.

3.) My findings on core material frequency response limitations, or, just how far up the frequency response curve core is causing distortion in the transformed signal, are as follows. In a general sense M50 to M19 250 Hz, M6 & M3 400 Hz, 48% nickle iron 3500 Hz, 80% nickle Iron 7500 Hz and Amorphous core 18,000 Hz. Again, these come from power efficiency curves. All core will respond to all frequencies applied to it, but the effective square area of usable flux drops at a faster rate than the square area required by the frequency in question and so, power transformation from primary coil through core to secondary coil, is negligible above those general numbers expressed above.

4.)What I glean from this is that you want to study antenna theory and the dielectric composite number called dielectric constant. In short, if you use commercial core you must utilize antenna shadowing characteristics and very tight capacitive coupling, so as to make the best use of both E Field and B Field activities. If you use Amorphous core you still want to utilize antenna shadows, but you want the least possible capacitive coupling. You can see these two extremes at work in my audio transformers and those of my esteemed colleague Per Lundahl. Working somewhere in between these two poles requires a delicate balancing act, when it comes to choice of how much capacitive coupling to use and what sort of dielectric constant characteristics you want to incorporate. Two very good practitioners of this balance are Pieter Truffant of Tribute Audio and Dave Slagle of Intact Audio. Which route you take to high performance is really based more upon taste than any large performance differences, once you have understood how to make commercial E/I core work properly for audio concerns. I can assure you that in any case, power transformer E/I construction is the antithesis of what you want for Audio.

5.) All gap structures are your enemy and you want to push them as far away from the coil as possible and have them be uniform, but not contiguous through a stack of core material, in close proximity to a coil.

6.) So, in a general sense, what you do with your coil is as important as what you do with your core. You must match the relative permittivity (for want of a more useful term) of both your magnetic and dielectric circuits, to achieve high resolution, transient faithfulness, and overall low distortion.

Bud

[LinuksGuru]

Originally Posted by Johan Potgieter

A question then from me regarding the frequency response of core material. My take is that basically this depends on the thickness of the laminations - the thinner the better. But these days where just about every winder is into power transformers (50Hz or 60Hz), and those doing disco equipment output transformer rewinding not really wiser, how do I tell the h.f. capabilities of a steel? You might have picked up that for hi-fi stuff I prefer using C-cores (thin sheet); are they otherwise the same?
Thanks

Even very old (manufactured in 50th - 60th) high-quality 0.3-0.5 mm silicon steel lamination works fine in audio frequency transformers. Output transformers in vintage Pioneer SM-83 (manufactured by Tamura at the end of 60th) have virtually flat response up to 50 KHz (Pioneer claims 100 KHz but my digital input interface ESI Juli do not allow to measure over 50 KHz).

Taking surplus cores doesn't make any sense (at least for me) unless you precisely know what kind of material it is and what are its characteristics. This is not only permeability f(flux curve), but also permeability curve f(DC current over coil, flux) and "a"/"am" coefficients f(flux, DC current over coil), etc. I'm in doubt you can find this kind of data for material taken from scrapyard. In fact, you can barely find even for today's ferromagnetics. H-Fi audio-frequency transformers market is tiny, so no metallurgy factories takes real care about it. They have a number of alloys in their product line, which may work great, or may not, its all up to your skills.

Without CAD-based simulating of all this data + coil/winding geometry + amplifier output stage characteristics, constructing good audio frequency transformer have the almost same possibility as winning big bucks in lottery (although I cannot exclude there are talents who can). I have a software (BA TrafoCalc CAD) which upon evaluating all these input parameters can placate a poorly designed transformer, which, under certain circumstances, can produce as much as 3% of nasty 3rd order harmonic distortions, therefore, turning any amplifier into junk box.

[thoriated]

The entire design of an output transformer is a balancing act. If you use extreme perm core, like amorphous C core, you cannot make use of capacitive coupling or low dielectric constant insulating materials because the result is a Q peak in frequency. Hence you find primaries and secondaries wound side by side in these designs. This does limit the amount of internal gradient information that the transformer can maintain. These types of transformers measure very well, when built properly, are very clear with great dynamics and transients.

Using commercial core requires you to utilize capacitive coupling for all frequencies above 400 Hz in the M series, 3500 Hz for 48% nickle and 7000 Hz for 80% nickle. When properly built these types have superior performance in internal gradient information, are also very clear with very rich transient structures. They do not measure as well as the amorphous core units and have an overall "softer" presentation due to the slightly higher distortion they produce.

This is all fine, but let's suppose I wanted a 25-35 Watt 6.6-8K PP primary, 0-4-8-16 secondary transformer using one of these more exotic core materials. Whom would I contact, and are any reasonably priced (say, not more than $100 US apiece)?

[kevinkr]

Originally Posted by thoriated

This is all fine, but let's suppose I wanted a 25-35 Watt 6.6-8K PP primary, 0-4-8-16 secondary transformer using one of these more exotic core materials. Whom would I contact, and are any reasonably priced (say, not more than $100 US apiece)?

I was with you until you mentioned price, nonetheless you might want to talk to Jack Elliano at Electra-Print about your needs - he has wound a number of custom OPTs for me and I can honestly say I have been completely delighted with the results.

Jack might be able to build you something for not too much more than your target price point.

Electra-Print.com Audio Transformers

No affiliation, just a happy customer.

Edcore is another one to take a look at - many here swear by their products:

EDCOR Electronics Corporation

Note that high nickel content or amorphous cores will drive the cost way up from where you say you want to be. (My last Japanese made amorphous cores were over $600 each) For your budget you can get M6 which should be just fine in the right hands...

[BudP]

I agree with Kevin, Edcor and Hammond are quite good. Not cutting edge in information retention but certainly as good as anything from the commercial realm in the 50's. You can PM me, but I will be about 50% above your target price, the amount of retained data will be worth the difference, if you have a set of speakers that will reveal it, most mid fi and up will do so.

Bud

[jrenkin]

Originally Posted by BudP

I agree with Kevin, Edcor and Hammond are quite good. Not cutting edge in information retention but certainly as good as anything from the commercial realm in the 50's. You can PM me, but I will be about 50% above your target price, the amount of retained data will be worth the difference, if you have a set of speakers that will reveal it, most mid fi and up will do so.

Bud

Bud, I don't want to PM without permission but have a question regarding your transformers. I have been reading and enjoy your detail, but my high energy physics background is way rusty. On a practical side, you previously mentioned some sites, such as Rhodes as having transformers I assume are your design. Are these the Onetics? Are they suitable for an SE 300b design as offered on these sites or are you recommending something else for thor above? If so, can I PM you about options? I have a few possibilities lined up, but I am interested in what you might say and how they might compare. Although I should be satisfied with simpler, less expensive parts, I can't. I like good stuff, of course within some reason. Thanks. Sorry for the hijack.