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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
Follow Ups:
MAybe that's good.
...finals are actually connected in a form of UL.
goes directly to a tap in the B+ supply? UL typically doesn't have a 200v differential between G2 and the plate and the G2 winding comes directly out of the output trafo.. Are you maybe figuring the cathode FB windings into the equation? My posted schematic is difficult to read but there is a full schematic on line that I couldn't upload here.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
True pentode mode requires that the voltage betw g2 and k be held constant under all possible conditions. While the the SA1000 circuit holds g2 voltage constant wrt ground, the voltage betw g2 and k varies in step with signal because the ks are tied to a winding on the OPT and are not at AC ground potential. Net effect is that g2-k voltage varies at some proportion of a-k voltage just like conventional UL.
Just to be clear,the cathodes are tied to DC ground thru the cathode windings by virtue of the DC resistance of the each cathode winding,correct? One side of each cathode winding does go to gnd,right?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
That's pretty much it. DC conditions are different than AC and non-pentode operation is an AC condition only.
Steve
Have you had any thoughts on the input cathode follower triode? My thought was it wouldn't be needed if we were running KT88s because we wouldn't be over driving a KT88 as easily like we would an 8417.My guess was it was used to match the gain of different preamps for just that purpose.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Since the input stage is a cathode follower its gain will be somewhat <1 and it's outside the OA feedback loop so it has little effect on the gain structure of the amp. I believe its primary purpose or function is the buffer element in the active subsonic (hi pass) filter. Secondarily, it also provides simple input buffering when the subsonic filter is disengaged. If this were my amp I'd be definitely leaving input stage as is. An active filter in any consumer product of this era was quite rare and the whole thing is worth preserving IMO.
Steve
I would physically leave it in of course but I said what you said in my very first post when referring to the subsonic filter.I figure the tube was used for matching impedance from the preamp to the input voltage amp.Look at the pilot SA-232 and notice the first 12AU7 triode which is a gain stage outside of the loop as well and it's puzzling as to why they didn't use the use the 12AX7 for the input voltage gain amp and phase splitter and then use the 12AU7 as a CF driver.I know EL84s are easy to drive but in UL they are harder to drive.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
I was responding to your statements: "My thought was it wouldn't be needed if we were running KT88s because we wouldn't be over driving a KT88 as easily like we would an 8417.My guess was it was used to match the gain of different preamps for just that purpose."1. The input CF input stage has nothing to do with "overdrive" of the finals. It's an input buffer and has no effect on the output stage drive capabilities of the circuit. IOW, the decision to keep it or not would not be based on the finals in use.
2. The input attenuator was in the circuit to match gain. The CF buffer was a nice but unnecessary feature and was probably a secondary benefit of implementing an active filter.
Edits: 08/09/16
The CF buffer was a nice but unnecessary feature and was probably a secondary benefit of implementing an active filter.
That's what I thought..I knew it had an attenuation function and the high pass filter mainly due to record warp and or rumble of the day.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Mike,
What are an ECC808/6KX8 and ELL80/6HU8 tubes? Can these tubes still be found? Are there good subs?
I wonder why Fisher used them.
Thanks!
Avery Fisher had a German engineer working for him. That fellow was quite familiar with the best W. German tubes available.TFK's ECC808/6KX8 is variation on the ECC803 theme that contains a "6" V. heater and an internal shield. Rewire the sockets to use culled Russian 6n2p-ev stock and you are good to go.
Lorenz made the ELL80/6HU8 twin power pentodes. They are scarce and costly. I know of no easy substitution. FWIW, Fisher also used the ELL80/6HU8 in console amps, with 1 bottle/channel in PP. That's about as upscale as a console amp can get.
Eli D.
Edits: 08/08/16
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