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scott 222 recap issues

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I recently purchased my first hifi project - a Scott 222a. I have done alot of tweaking of pro audio gear for micpres, but am having an issue with this unit - still somewhat of a newbie on a hifi amp- prob just lucky with past mods.

Upon getting this amp, it sounded ok but very dark - severely rolled off up top. Went about replacing electrolytics as well as swapping orange drops for important caps. After doing some replacement, I had a horrible 120hz hum mostly out of one channel but not dependant on volume. When balance was centered it often was a thumping sound that seemed to only be a tube problem. Saw some blue lightning in a tube and it corresponded to sound when pot was centered. Swapped tube and phase splitter tubes and still same issue

After encountering this, I removed all new electrolytics and resoldered to old parts making sure hookups were identical - took multiple pics before touching anything. The problem still remained. I did not swap out the 716p's.

I've checked over and over the wiring for the caps I replaced - don't know - could changing coupling caps etc result in a 2xAC hum throughout?

I'm normally accustomed to a 2 tube micpre and haven't discovered this issue before.

Many thanks, Nathan
Joined 2004
Paid Member
TubeHead Johnny said:
There is no bias supply in this amp, it's cathode biased. When you swapped the tubes, did the hum go from one channel to another? First thing I would do is check all the voltages against the schematic at this link:


Interesting, and a good catch, the only other one I knew of heretofore was the 99 which used cathode bias and a somewhat similar scheme to power the filaments in the low level stages.
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