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RE: Audioromy FU29 amp




Yes the guy I bought it from did the McShane "level 2 plus" power supply mods and also changed all the coupling caps to West Cap PIO's and also increased decoupling capacitors to larger sizes.

I got a schematic from Bob Carver that I would like to try some day, it disconnects the screen grids from the output transformer taps and supplies them with plain DC, which makes the amp "pure pentode" instead of an ultralinear design. Carver says he talked to Stu Hegeman who designed the amp back in the 1950's, and Hegeman wanted a non-ultralinear design but Harman Kardon marketing said it had to be ultrlinear because that was the "flavor of the day" and also allowed them to get more RMS power under test so they could rate the amp higher. Carver says the non-ultralinear design actually provides more current to the load but measures worse. Carver also has a bias mod he calls "DC restoration" which allows a lower idel bias point but increases bias under signal so the amp is actually (effectively) biased higher than stock, but runs cooler. I would like to try the "pure pentode" mod but am not so certain about the "DC restoration" thing. Still, Bob Carver knows way more about amp design than I do so maybe I will try it.

Here's the schematic Carver sent me, fyi I asked him if it was OK to post & share and he said SURE, OK.
Science doesn't care what you believe.



Edits: 03/26/11 03/26/11

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