Tube amp buzz (amp, not speakers)

The most likely cause is DC offset. Many common appliances connected anywhere in your house can be a cause, making it seem random. https://www.pooraudiophile.com/2015/03/how-to-fix-dc-offset-and-transformer.html

AC quality is usually brought up and probably for good reason. Some transformers can tolerate it and some buzz. It would be worse at certain times of the day than others.

Wouldn't hurt though to check the transformer screws and mounting.

Some transformers are rubber mounted to reduce the noise since it also can get the chassis buzzing.

Tube amp buzz (amp, not speakers)

Hello -

Almarro a318b makes buzzing sound at random: sometimes it does, sometimes it doesn’t, completely unpredictable. The buzz comes out of the amp itself, like a transformer buzz; the sound out of speakers is completely clean.

I’ll appreciate any ideas - what causes it, what to do? boriso

I also would say DC offset on the mains power if the amplifier uses a toroidal power transformer in its’ power supply.

Here are some pictures of the bottom side of the amp.
https://www.audiocircle.com/index.php?topic=138713.0


I have no idea what kind of power transformer that is in the picture. You might want to check the supporting bolts and nuts for tightness. Do not over tighten. Unplug the unit from the wall outlet the night before you stick your fingers inside to assure the DC high voltage has completely discharged.
.

Run with the cover off and when buzz starts, touch various parts of the transformer and chassis and see if you can identify what is vibrating. Once you locate that, you can address it. 

Your a318b uses extremely high voltages.

As there is no guarantee that it is not still holding such high voltage, even days after being powered off/unplugged, only knowledgeable people should access the inside for diagnostics/repair.

DeKay