Monk -- Note too that the test was run at 25 watts with both channels operating, rather than at rated power output.
I have run my own performance analysis on many restored Fishers of virtually every stereo vacuum tube offering. In the receivers (B or C), the OPTs -- for the power the receiver represents -- are very good down to 30 Hz, permitting full power at low distortion down to that level. Below that point, full power is usually available to as low as 25 Hz, but distortion starts rising rapidly below 30 Hz as core saturation becomes a big factor below this frequency. Achieving this performance requires matched output tubes that draw the correct quiescent current for the bias voltage provided by the Fisher design. Of course with new foreign manufacture tubes today, the chances of that are slim to none, and we know where Slim went........
To improve performance on the low end would require significantly bigger OPTs, making the already heavy receivers even bigger and heavier, so a compromise was struck, which was really a good one. The bigger transformers of the C units can theoretically perform somewhat better at normal listening levels (assuming the same speakers and the program material demands it), precisely because they are bigger, and the C units produce more power. But then, this would theoretically be true with any quality unit of larger power.
Probably the best transformer capability to power output ratio in main line stereo Fisher gear came with the SA-100/X-202 models. Operating properly, they are capable of full power low distortion output down to 25 Hz, and even have quite respectable performance down to 20 Hz at up to about 10 watts power output. The transformers of the later X-202(B and up) series are a very close second. The transformers of the C receivers are smaller than those used in 7591 X202 series amplifiers.
Dave
