Loud buzz and hum from one monoblock!!!

[victor1]

I have a problem with one my monoblock (Chinese Mingda-Mc845-C) . As I turned them on today I heard a very loud hum and buzz from my speaker. I disconnected the source ,swapped tubes from a good one and turned it on again. The same very loud hum and buzz. I checked the output signal from the amp. At the very beginning it was 10V then it decreased to 3V. What can it be? All tubes are ok.

 

[Jeb-D.]

If it's a 60 or 120 cycle hum, it's most likely caused by a grounding issue(such as a ground loop) or ripple in the power supply. Also make sure it's not located right by something that may cause interference like a TV.

 

[poobah]

You may have had a diode failure in your power supply too. Are they tube rectifiers or SS?

 

[victor1]

I can't be ground hoom because I use cheater-plugs.
The amps have solid state rectifiers.

 

[poobah]

Just a guess... but maybe you popped a rectifier...

Does it make any music at all?

 

[Jeb-D.]

Yeah, you'll just have to check the rectifier, supply caps and inductor(may not have one but most SET amps do). And if it's fixed bias check the DC supply to the grid, to make sure thats clean as well. Check each stage because they may have different power supplies. If you have to replace the rectifier I suggest HEXFRED diodes, they have a soft recovery, wich will reduce any spikes in the supply if there are any.