Zenith Royal 3000-1 121-374 Transistor Mod

Just for fun, I decided to look at the audio waveform from the final output and trace it back each stage. I am extremely good at discriminating audio waves and the slew rate seemed off spec to me. The driver transistor (121-375) seemed loaded and the second harmonics were all but gone, but then again, it was driving an ancient phase-splitting transformer (T12), however the designers may have intended it to work like this. I figured a slightly larger beta gain in the first audio stage (121-374) would give those harmonics a large enough kick. The quickest way to do this without even lifting the soldering iron, or changing the biasing resistors is to replace the transistor, so I decided to replace 121-374 with an AC128, and sure enough, it made a huge improvement to the sound giving it a presence.

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