Mesa Boogie MarkV twenty five hum

[TRA2016]

Dear guys,
I bought a Mark V twenty-five a couple of months ago, not sure this issue was already there or not but I just realised yesterday...
Basically when I switch on the ampli and put stdby switch on the right, even if unlugged and volumes/gains at 0, I can hear a hum when I switch the 10 W settings of both channel 1 and 2. The hum drastically decrease (close to disappear) when I use the 25 W settings.
Changed the room (from basement to first floor) as well as plug and cable but problem remains. Last attempt I put a noise reductor in the fx loop but again hum still there..
Thanks

 

[jmontgomery]

It's probably a tube.

 

[primal]

I have nothing of value to add. But it is very interesting that it actually decreases in 25 watt mode.

Call boogie

 

[TRA2016]

Thanks guys !
Kept in touch with Mesa Boogie and I have been suggested to have it serviced.
I will post what they will find.

 

[scoden]

Any updates ? Thanks

 

[TRA2016]

Yes,
I took the ampli to the service and they sent me back stating it is ok, they quoted they tested for 24 hours ! And compared with a new one no differences..
I think they did not get the issue..
So I went to an experienced repairer who listened carefully to me and to the issue I was complying with..
First of all we connected the ampli to a loudspeaker and he agreed with me that the difference exists and the hum is hearable once switching from 25 to 10 watts , then he connected the ampli to an oscilloscope and we actually saw the difference: in 25 w mode the wawe is sinusoidal, in 10 w you can clearly see two peaks of different magnitude ! So the guy replaced the 2 final tubes with two similar and matched from E.Harmonix and not only the hum disappeared but also once connected to the oscilloscope we saw two very similar sinusoidal wawes !
Now the guess : is it necessary to replace the tubes or should I leave them? apart the hum the amply sound well with its Mesa original boogie, howeveerr I was told that the tube are not covered under the warranty.
Any advices would be appreciated, thanks !

 

[jmontgomery]

If I'm not mistaken, the tubes are covered for the first six months.

 

[TRA2016]

Thanks !
Will check the dealer here in Italy !