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2A3 SETH S-240 Monoblock:
Mike LaFevre on
MQ|Peerless S-240-A Output Transformer


There is a lot of discussion on the MQ Forum concerning the MQ Peerless Nickel S-240-A and it's ability to handle unbalanced dc plate current so I wanted to take a look at this excellent iron a bit more in depth.

MagneQuest S-240-A in Permalloy - M6 is a valid option

The M6 version is also a very viable alternative
and sounds excellent in its own right

First of all, specsmanship and numbers can be awfully confusing. Trying to compare the numbers from one manufacturer to another is maddening to say the least. Many of our competitor's do not even publish a number for the amount of DC Unbalanced Plate Current their designs can handle. They give you no real-world guidance. And in another example of specsmanship, some companies do their power ratings at 30 hertz all the way up at the knee of the BH curve (16Kg).

The MQ Peerless S-240-A was rated from the factory for 20 watts @ 20 hertz. And it can deliver this full-rated power while only using approximately 75 percent of the available flux density as defined above. If we rated the S-240-A on an AC only basis (as many/most of our competitors do) then even at 20 hertz, the 240 could deliver 32 watts of power (with no unbalanced dc plate current) if we allowed the flux density to go up to the vaunted 16Kg mark that many others use as their standard. And, if we rated the S-240-A at 30 hertz (instead of 20 Hz), we could publish a power rating of 72 watts for the S-240-A.

This is exactly how many of our erstwhile competitors establish and publish their AC power ratings for their push-pull output transformers - At 30 hertz with no unbalanced dc consuming any of the available flux (16kg). So,if you did an apples to apples comparison then the S-240-A is capable of delivering greater than 350 percent of it's factory published power rating. How's that for robust?

Still, I like Peerless' method; rate it at the tougher 20Hz standard while using only about 3/4 of your total flux capacity thus leaving some in reserve for good measure. When you publish a power rating that puts a significant amount of the flux capacity in a "reserve fund," then you can also fully accommodate the worst case conditions published for your design. In the S-240-A, with the max published unbalanced plate current and the full power rating at 20 hertz instead of 30 hertz, even with the worst dc conditions and at the toughest frequencies, Peerless can still deliver the full rated power of 20 watts while producing a max flux density of approximately 16kg.

SETH 2A3 MonoNow let us reintroduce the 9 mils of unbalanced current max spec'd by Peerless in their catalog. The S-240-A can deliver 20 watts at 20 hertz while carrying 9 mils of unbalanced plate current @ approximately 16Kg flux density. If Peerless had rated this unit at 30 hertz you could still MORE than double the factory's 20 watt power rating while simultaneously carrying the 9 mils of unbalanced plate current; in other words, you're not going to hurt this puppy with 9 mils of unbalanced dc current flowing through it. Actually, it's better than what some of our competitor's units can do at 30 hertz, this with no dc at all figured into their flux density calculation!

So let's use their numbers of 30 hertz and 16 kg and apply it to Jeff's amp for a comparative contrast. Let's conservatively say the SETH 2A3 puts out 7w rms**. Now with 9 mils of unbal plate current and an ac signal frequency of 30 hertz, we are only using about half of the available flux density (8kg versus 16kg).

I calculated some other parameters of the S-240-A. For example, at 7 watts @ 30 hz and with 9 mils of unbalanced plate current, the copper circuit temp rise will be 1 degree C above ambient and the iron core will have a temperature rise of 2 degrees C above ambient. The total losses of the iron plus copper circuit combined will be .18 watts (18/100th's of one watt in losses). This example of 7 watts at 30 hertz with 9 mils of unbalanced current only consuming 1/2 of our available flux density tells us that even with an ALL nickel core, you are not beating up the nickel. You're not running the high perm material at the same 15 or 16 kilogauss that other companies spec for their nickel, mu-metal, and amorphous iron based transformer cores. Nope, with Peerless you're treating the nickel with kid gloves, relatively speaking.

All that said, because the Peerless was conservatively designed in the first place, even under the most adverse published conditions it will still deliver the performance that others can only deliver under their most ideal conditions.

Of course, it is still wise to try to either buy matched tubes and\or make or allow a provision to adjust for a minimum amount of unbalanced plate current between the two output tubes.

- Mike LaFevre


** Jeff thinks Mikey "underrating" Lafevre is following habit here; this is a 10w amp before clipping.



Which Peerless S-240-A? Nickel or M6 or Pinstriping...    by Mike LaFevre

What are the advantages of the nickel alloy laminations over the M6?

Well, some LOVE nickel and the signature sound of this alloy and other folks like the sound of M6 better than nickel. My job as a designer\builder is to offer logical, competent options when available which leaves you the job to choose between the options, sometimes a tough task.

Mike LaFevre thinking about Permalloy  vs M6Subjectively speaking, the nickel lams have a bit more color, a bit more warmth, and a relaxed, almost fatter-larger image size. It also offers perhaps a bit more laid back sound quality - an "easy" quality to it.

On the other hand, the M6 has a bit more transparency, greater sense of speed, and is a bit "thinner" sounding, perhaps what some would call more "neutral" . The M6 can sound really wonderful and responds well to changes in coil design.

Of course, these are only my subjective impressions; others might hear things differently.

My advice if you're not sure, then get the M6. It is very good. Remember that an OT is more than what the core material is. What you hear are MANY variables all working together; but the choice of core material influences and has a definite effect on the subjective sound qualities of the transformer.

The M6 will give you more headroom (perhaps not so critical in a 10 watt application). But in other applications, most of the high perm materials cannot be operated at the same flux density levels as M6 or M4 or High B core materials. So if you're a responsible designer you should de-rate the nickel cores power wise to keep the total flux density within the range of optimality for that particular core material. Yet it's not so clear cut to me that other than a preference statement, it makes much sense to say A is BETTER than B assuming proper use of the subject core materials.

Though in some apps, say small signal level apps where the coil should\must be kept small (you have large nominal impedances), then the high perm materials are perhaps even more attractive as they can produce more L per cubic inch of trans than say M6. But here we are also not talking about large voltage excursions across the coil.

Of note, this matrix assumes that you also have a good coil design, good materials inside the coil, good workmanship (important to sound quality), and that the part has been properly specified no matter what core material you choose i.e., would I rather have an M6 S-240-A or a 125 series Hammond built in cobalt or nickel? I'll take the 240... any day of the week.

Ultimately, you can ask folks their opinions, folks who have "ears on" experience, and still get a range of preferences. For instance, when I asked Jeff Lessard if he generally preferred nickel or M6, he said he prefers cobalt (Not a fair answer Jeff ..heh). Though, obviously, he is quite happy with all nickel S-240's. Wavelength's Gordon Rankin almost always prefers M6, M4, M3 over nickel, though he too likes cobalt. Dan (Bottlehead) LOVES nickel and cobalt; he actually prefers nickel almost always (where it is logical to use it) over M6 or M4. And when you throw Pinstripe (Nickel/M6) units into the mix, well some folks really like the pinstripe sound. Gary Dahl has reported well on this combination.

So, it is impossible for me to construct an ordinal ranking of "goodness" for these different core materials without taking into account the personal preferences and tastes, not to mention the applications. Yep, probably not the conclusive answer you were looking for....




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