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In Reply to: RE: How are they conncted in a form of UL when G2 posted by Michael Samra on August 08, 2016 at 11:31:36
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.
Follow Ups:
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
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