I wonder what the difference is between "early 400 EFB" and "any other version"... the much later Fisher 400, like mine, (from my reading) has a output transformer with larger primary impedance, but I did not double check to see if they in turn (no pun intended) changed the operating point of the output tubes, or did they just change the feedback ratio.
UPDATE:
Ok, did just check, they in fact moved the operating point of the output tubes by increasing the plate and screen voltage and making bias slightly more negative to go along with the increase OT impedance. So the EFB would have to be just slightly modified to give a slightly more negative bias (early serial numbers, -15v bias, 395v plate and 345v screen of 7868, while my serial number shows -17v bias, 405v plate, 365v screen of 7868)
Early 400 EFB]
UPDATE:
Ok, did just check, they in fact moved the operating point of the output tubes by increasing the plate and screen voltage and making bias slightly more negative to go along with the increase OT impedance. So the EFB would have to be just slightly modified to give a slightly more negative bias (early serial numbers, -15v bias, 395v plate and 345v screen of 7868, while my serial number shows -17v bias, 405v plate, 365v screen of 7868)
Early 400 EFB]
Last edited:
![Early 400 EFB]](https://audiokarma.org/forums/data/attachments/1714/1714260-ea516a746274b23ec2670fee901005ac.jpg?hash=6lFqdGJ0sj){kind=link}