

There is a lot of discussion on the MQ Forum concerning the MQ Peerless Nickel S240A 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.
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 realworld 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 S240A was rated from the factory for 20 watts @ 20 hertz. And it can deliver this fullrated power while only using approximately 75 percent of the available flux density as defined above. If we rated the S240A 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 S240A at 30 hertz (instead of 20 Hz), we could publish a power rating of 72 watts for the S240A. This is exactly how many of our erstwhile competitors establish and publish their AC power ratings for their pushpull 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 S240A 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 S240A, 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. Now let us reintroduce the 9 mils of unbalanced current max spec'd by Peerless in their catalog. The S240A 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 S240A. 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, mumetal, 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
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