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In Reply to: RE: Odd hum issue - suggestions ?? posted by rogerh113 on May 31, 2017 at 08:35:42
I'm glad you fixed it but what is a "neutral to ground lug"?
The neutral wire (white wire) connects to one end of the primary of the power transformer and does not connect to earth ground (the green wire) or anything else within the chassis.
Do you mean you ran a wire between "signal ground" and "chassis ground"?
The schematic show such a wire to already exist.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
I cant resist reviving this thread years and years later as got the opportunity to fix an SA12 and I feel the urge to share my findings.The schematic posted there points to a 3.48k R7 which is the cathode R of the 2nd gain stage. It is incorrect. The value of the actual resistor is 1k which makes the stock Vp of gain stage 2 to be about 90V only. The parallel follower (tube #2 labeled gate driver) will be undercurrent and sound brittle and raspy. The original design boosts the output stage bias to the max to create a warmer sound.
I have made some changes as I repaired the unit (bulging caps everywhere)
1.) change R8 to 50k to increase current and push Vp to about 115
2.) that way the Vk at tube 2 increases.
3.) reduce NFB resisor R10 to 18k (36k x2) and parallel it with a 24pF cap to suppress ultrasonics.4.) reduce C2 coupling from 0,47 to 0.1 to avoid any motorboating
5.) reduce R4 to 91k, section off the B+ from R4 to R8 using a 560 Ohm and bypass again at the top of R4 using a 30uF high quality film cap.
6.) most important of all REDUCE the bias of the output tage from the measured 480mA to 200mA (the old biasing was just ASKING FOR TROUBLE IMO)
7) replace R52 with choke so that even with the increased draw of the tube stages ripple is minimized.I cant comment on the SA100 / SA220s cos they both use an SRPP follower vs paralleled cathode follower and I am not a fan of SRPP or whatver white cathode follower. The SA12 tube stage seem to point to a much more natural sound. Voice it warmer and there is no need to run the mosfets to the ground. Obviously they will still blow when they see too low a speaker load but thats another problem.
The Greenstreet upgrade hum can be mitigated by connecting a groundpoint on thr board or the middle link of the 2 output stage main filter caps groud to the chassis connected with a 1uF cap.
Edits: 08/23/20 08/27/20
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