scott 222 recap issues

[ihscoutlvr74]

Gents,

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

[whitelabrat]

Sounds like something is shorted. My experience is limited. What tubes were sparking? Preamp or output? Photos?

[SY]

You made a wiring mistake, most likely. The thumping and blue sparks are the result of a low frequency oscillation ("motorboating") caused by coupling through the power supply. The volume-independent buzz is another clue that the power supply is miswired.

[kevinkr]

Watch out for the bias supply capacitor in this amplifier - in many Scott amplifiers the can is positive, not negative as is usually the convention.

[TubeHead Johnny]

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:

http://www.hhscott.com/pdf/fs/222A-C1.JPG

[kevinkr]

Originally posted by TubeHead Johnny
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:

http://www.hhscott.com/pdf/fs/222A-C1.JPG

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.

[TubeHead Johnny]

Other than the 99, the 222A and B are the only Scott amps I can think of that use it. Interesting note; The voltage generated at the cathode is used to power the preamp tube filaments.

[kevinkr]

Originally posted by TubeHead Johnny
Other than the 99, the 222A and B are the only Scott amps I can think of that use it. Interesting note; The voltage generated at the cathode is used to power the preamp tube filaments.

I've done this too, it actually works quite well if the cathode currents and voltages are within a range to permit it.