While I was hunting down the source of the hum, I found this cap C22 contributed noticeable hum. It is a old ceramic cap crossing the power switch. The physical size is huge. I guess over 1 inch in diameter. It is located behind the Bass pot. Some of line AC is coupled through the bass pot that causing hum. I replace it and relocated it away from the Bass pot. The hum is reduced.
Also, I replaced the power cord with 3-prong cord.
Also, I replaced the power cord with 3-prong cord.
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Good progress. Thanks for sharing your discoveries. I will have to check my SCA35 for that C22.
Where have you physically connected the ground wire in the new cord?
Where have you physically connected the ground wire in the new cord?
Near the speakers negative terminal. I bolted them (with the speaker negative wires) together on the chassis.
Don’t forget that Dave G had some bad 6bl8 experiences and determined that not all tubes labeled as such were real 6bl8 and wouldn’t work properly.
I have looked into this. Regarding the bias point, 6GH8 is very close to 7199. To use 6BL8, it is not a big deal as I can always mod the circuit to bias it as what I want.
I tried couple 6GH8 and 6U8 before, they don’t play well in SCA-35. They hum.
I might have reached the theoretical limit that how quiet SCA-35 could be. My experiment is to measure the hum level before and after I pull the AC plug. After I pull the plug, the amp is powered by the power filter caps entirely. Yet, I still can see the hum about the same level. The hum is coming from the environment. I believe the hum is picked up by its high impedance tone stack.
Another experiment is short the input right at the amp board. Yep, that did kill all the hum.
Let me know if you manage the hum below 3mVp. I can’t imagine how bad the hum could be in its phono stage, which I haven’t tried yet. It seems unusable for me.
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Update.
The journal of hunting down the hum went to dead end. It clearly indicates the hum is picked up at the tone stack. If I am not wrong, the factory wiring uses the chassis as the signal ground. Unless I redo the front pannel wiring, the hum would stay.
BTW, my speaker is a pair of 92dB sensitive floor standing speaker. Any hum would be heard easily.
I think I am done with SCA-35 now.
I swapped back to my trusty Dynaco ST-70. It sounds so clear and it doesn't hum at all.
The journal of hunting down the hum went to dead end. It clearly indicates the hum is picked up at the tone stack. If I am not wrong, the factory wiring uses the chassis as the signal ground. Unless I redo the front pannel wiring, the hum would stay.
BTW, my speaker is a pair of 92dB sensitive floor standing speaker. Any hum would be heard easily.
I think I am done with SCA-35 now.
I swapped back to my trusty Dynaco ST-70. It sounds so clear and it doesn't hum at all.
Some of the hum is picked up by the encapsulated network *. Moving them slightly changes the hum. Their proximity
to the mains cabling is a hint .
* the prepackaged resistor/vapacitor network encapsulated in vax, probably a thickfilm hybrid.
to the mains cabling is a hint .
* the prepackaged resistor/vapacitor network encapsulated in vax, probably a thickfilm hybrid.
Although retaining the tone controls was a specific requirement in the OP, and just hear me out, but IF they were bypassed, the ST-35 PCBs and circuitry bolts right in, no pentodes. Plenty of sensitivity for the "passive preamp" of switching, volume and balance controls, and ST-35 (plus mods?).
The phono stage of the SCA-35 is challenging. You might want to try something like the PAS circuit with all of the tape replay stuff removed and the whole thing slimmed down to a 12AX7 per channel and RIAA EQ on board. This can be done with the original board and some indelicacy, but Dave Gillespie has a nicer alternative. AC heating, even with nulling trimpots, is absolutely possible (still difficult) to make quiet enough, depending on your definition of enough.
Lurking inside the SCA-35 is an ST-35 with a passive preamp and a decent phono stage. It wasn't matched until the Advent receiver many years later. IMO, of course.
All good fortune,
Chris
The phono stage of the SCA-35 is challenging. You might want to try something like the PAS circuit with all of the tape replay stuff removed and the whole thing slimmed down to a 12AX7 per channel and RIAA EQ on board. This can be done with the original board and some indelicacy, but Dave Gillespie has a nicer alternative. AC heating, even with nulling trimpots, is absolutely possible (still difficult) to make quiet enough, depending on your definition of enough.
Lurking inside the SCA-35 is an ST-35 with a passive preamp and a decent phono stage. It wasn't matched until the Advent receiver many years later. IMO, of course.
All good fortune,
Chris