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This explains why Mr. Fujita used 12AU7 x 2 and 12AX7 x 1 instead of 2X 12AT7.
1. To achieve higher stability of NFB, the overall NFB amount is decreased, and instead, cathode feedback is applied to 300B.
(This is why the third winding for cathode NFB is added to the OPT.)
2. Applying cathode NFB requires bigger driving amplitude for 300B.
3. To achieve such large driving amplitude, 12AU7 is more suitable than 12AT7, as its max. rating is higher than 12AU7.
4. Since the gain of 12AU7 is smaller than that of 12AT7, 12AX7 is used for the first stage.
Mr. Fujita conducted the circuit simulation to make sure his theory is correct, and confirmed that bigger amplitude can be achieved when 12AX7x1+12AU7x2 than 2x12AT7.(The driving ability of 12AX7x1+12AU7x2 with TU-8600 circuit is 200V peak-peak without wave clipping)
Edits: 07/18/17This post is made possible by the generous support of people like you and our sponsors:
Topic - Why TU-8600 used 1X 12AX7 + 2X 12AU7 - vkung 20:19:13 07/02/17 (0)