Mark V:25 combo noise in 10w mode - normal?

[smurfco]

Hello,

Brand new user here, been using Mesa bass rigs for years. I just got a used Mark V:25 combo, and I am wondering if this is normal and what might be causing it. On both channels, it is pretty much dead quiet in 25w mode, but I get a pretty loud hum in 10w mode. This is with both the gain and the master on each channel turned all the way down.

I shot a quick video to demonstrate:



(Note - the hum is a lot louder in person than it sounds on the iPhone-shot video above...)

What could be causing this? I tried swapping the positions of the two power tubes, but the issue persisted. Is this normal or is there something wrong? It seems pretty loud...

Thanks!

 

[mace]

No, that's not normal. Maybe put in 2 fresh power tubes? They operate in different modes (triode for 10W and pentode for 25W).

do you get any guitar sound out of the amp in 10W mode? Something sounds pretty out of whack

I'm sure some of the more technical posters here will chime in with more helpful advice.

 

[smurfco]

Thank you! I tried a different pair of (known good) EL84s but still got the hum.

Yes, I do get guitar sound in 10w mode. The amp works exactly as expected in 10w mode, all the controls work... except there's that hum.

The hum does not get louder or quieter with the volume or gain controls - it's just constant.

Thanks again!

 

[smurfco]

Any other ideas? Thanks in advance!