SET Asylum Single Ended Triodes (SETs), the ultimate tube lovers dream.

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Re: SET and OTL

Posted by kurt s on December 25, 2006 at 22:27:43

In Reply to: Re: SET and OTL posted by Paul Folbrecht on December 25, 2006 at 07:16:02:

"Bear with me, if you don't mind - are you saying that the output devices will not each conduct 100% of the signal?"

To be precise, I am saying that not ALL of the output devices are conducting for 100% of the signal to full rated output power of a sine wave. If it did, it would meet the definition of class A. It does meet this definition for a little under 4 watts of output power. Above that and then one half of the output devices are cut off while the other half is further conducting to generate more power. This class of operation is defined as class AB, where there is appreciable amount of power operating as class A, but if it will go beyond that power to its rated power it is then acting mostly like class B (one side is now cut off). A large bias current makes a big overlap in the crossover and it's therefore pretty much a hybrid, AB. Perfect class B operation would have no overlap, where at the 0V output point both sides just barely cut off simultaneously. No real practical amp does that because you really can't make it work well and not cause bad "notch distortion". So push-pulls in the real world are either class A or AB, with varying amount of bias for how deep the class A operation is set. For the M60, that level is a little under 4 watts, again.

Now 4 watts of class A operation is pretty good, because that is more power than the full output power rating of a typical 2A3 SET, which is a true class A amp that clips at about 3.5 watts. The M60 is still a class A amp at the point the 2A3 quits, and then it will go on to 60 watts if need be, but not as a class A amp anymore.

One could also label the M60 as a 3.7 watt class A amp with 60 watts for dynamic headroom.

Kurt

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