I look at the Fisher SA-1000 schematic and this one hell of an amp

Vintage Asylum

Classic gear from yesteryear; vintage audio standing the test of time.

The first triode you could probably eliminate being it's a cathode follower and it was probably put there to match various input impedances of the time or if you had rumble in your records you could roll off the bottom end.There is no gain there and being the 12AX7 input is loaded with the typical 470k of the day,a good preamp will be a perfect match.The ECC808 is kind of unique in the sense that the plate couples directly to the B+ source.I wouldn't call the 47k in the power supply a plate load because it's directly feeding both triode sections.

Notice the cathodes of the 8417s are inductively coupled thru the cathode FB windings to ground..You will also notice it is running pure pentode with a 200v potential difference between the plate and G2..I'm almost inclined think you can run KT120s in this puppy but you may have to run an auxiliary filament trafo.Maybe not tho.The grid to ground resistors may have to be raised but maybe not.I would definitely regulate G2 420v if not lower it to the 300v range.This is if you convert to KT88s.It is very possible that the ECC808 is there to keep from over driving the 12AX7 as well because the driver requirements of the 8417s are small.To optimize this amp for running KT88s,you would definitely have to make some changes..Just rewiring the tube bases will make it work but you still need at least 90v of drive,even at those plate voltages.

"
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

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Topic - I look at the Fisher SA-1000 schematic and this one hell of an amp - Michael Samra 00:17:13 08/08/16 (13)

All of the gain by one 12AX7 stage - Scholl 16:24:35 08/11/16 (0)

Not "pure pentode"... - Steve O 09:57:50 08/08/16 (9)

How are they conncted in a form of UL when G2 - Michael Samra 11:31:36 08/08/16 (8)

RE: How are they conncted in a form of UL when G2 - Steve O 12:36:27 08/08/16 (7)

GOT It - Michael Samra 16:14:23 08/08/16 (6)

RE: GOT It - Steve O 19:27:52 08/08/16 (5)

RE: GOT It - Michael Samra 19:49:53 08/08/16 (4)

RE: GOT It - Steve O 21:23:32 08/08/16 (3)

RE: GOT It - Michael Samra 21:55:25 08/08/16 (2)

RE: GOT It - Steve O 06:12:24 08/09/16 (1)

RE: GOT It - Michael Samra 06:24:43 08/09/16 (0)

tubes - FenderLover 06:47:58 08/08/16 (1)

RE: tubes - Eli Duttman 10:04:56 08/08/16 (0)

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