NONE.
I have a few friends who tolerate 'some' hum. I tell them, something is wrong, find it or fix it.
Years of 'some' hum later, they get it fixed, zero hum.
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That amp says KT150 (you have a different version?)
https://tmraudio.com/components/power-amplifiers/lab-12-suara-tube-power-amplifier/
does maker approve using KT120? (don't assume, I would verify)
old post here:
2,658 posts
The varying opinions of the KT150 tube seems to be primarily dependent on whether the amp was designed to utilize the KT150 in the first place. Since the KT150 needs a more stout transformer and has to run on a higher voltage,"
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Return is work and money, be thorough, MAKE SURE it's the amp. Get your Sherlock Holmes hat out of the closet.
Switch tubes 1 at a time left to right, any changes?
VERIFY it is not something else, i.e. only one device with it's own volume control, directly into amp to speakers (check speaker wires at speaker ends). One pair of previously silent interconnect cables with tight connections. Single device thru volume control (preamp if needed).
Power from your normal (dedicated I presume) outlet. Change power to an outlet on a different circuit.
No matter what: always hum, do you have an alternate amp to put in to verify the hum is definitely caused by the new amp before you pack it up?
Same hum, I would return it!