KOTriode wrote:

Amp at close to clipping level 20.4W, I am surprised that sine wave looked pretty symmetrical. Still has couple chassis left for the Melquiades, the problem is the 400V supplies which required extra power supplies. This interstage amp is so much simpler and easy to build at the expense of the interstage cost.

KOTriode,

the 20.4W is about right. Hoiwever the absolute power is not so important in my view, even though it might be important in case of lower sensitivity that you use. The total power and the amount of distortions is not so indicative in my view to characterize the operation of amplifier. To me it would be more important to see the image of the be trace at let say 22W. I mean I would like to see the first sight of clipping and I would like to see that with your given load the clipping is absolutely symmetrical. If not, and if the loading of the tube that I use is give to me the right sound then I would plays a bit with current/voltage balance to make sure that the right and bottom clips are happen at the very same time and congruous by shape.

Also, you do not specify the frequency of the trace. Did not go over the conversion of your scope time marking into frequency but looking at the sleight asymmetry of the bottom half I think it was relatively low frequency signal. It always useful to know what the frequency and operating current was as it gives a perspective over the inductance of the transformer.

One more thing I learned as I was working on Milq. I tend to put the amp at fill power, get the very first clip by driving the generator lower and lower, make the clip symmetrical and then trace what section of my amplifiers responsible for the clip. An amp might be clipping by driver stage or by output stage. You need to make sure that the clip you get is purely transformer limitation and nothing else.