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I'm planning on making a ST-70 clone, based on Kara Chaffee's MKIII mod, like my MKIII + Akido. I was thinking of instead of a 12AT7, using a 12AX7/5751/6AV6/6AT6. What would I have to replace to use one?


I was looking at the DIYTube ST-70. It's very close, except for the LM334 and the capacitor (C2) between the 5751 and the 12BH7.

Hi,

12AX7 will do in there, just...

1) Adjust the 39K resistor on the 12AT7 anode so the DC operating point is the same.

2) More gain will equal more overall NFB. You may need to reduce the 20R resistor inline with the 200R one on the cathode.

Cheers!