Wow, nice work!!! Prettier than a dr-103 inside. I get around 478 on the plates of my 81 non ckt board 2 input dr-103 with 124vac. I installed a bias pot but took it out and went stock when I tried RG's voltage bucker. It sounded WAY better and all voltages then matched the published schems, 460 on the plates, no redplating with russian el34's. Have you tested it for clean sine wave giving you 120 watts? Just curious, I wonder if the 120 rating was optimistic or marketing, or if they mighr have used a different opt primary impedance. The voltages are the same. might check my dr103 this wkend for max clean output, I never have. It will make your pant legs flap if you hit a hard chord:) can't get that anymore. Reason I ask is that the plate and screen voltages are very close to your amp.. I also have a Matchless sc-120 (4xel34 fixed biAs) and the plate voltage is 540@ 50ma, it puts out its rated power dead clean. The screens are supplied by2kof resistance plus 1k per tube after a big choke, its still pretty tough on tubes.
Try droppin the input ac a bit to 110 to 112 with a variac or bucker, if you get it around 460 on the plates it will smooth it out and you will be so happy. Not that anyone NEEDS 120 watts, but its not that loud:))
SC 120/ Hiwatt Conversion Pics
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