Bass hum on the lead channel of my MKII

[breogan]

Hi!

My Mesa Mark IIa and I are getting used to be together. The clean channel is a bit bassy on the "normal" mode. The "pull treble" does wonders for the cleans and the clean tone is amazing with this knob pulled out.

When I switch to lead mode a quite loud bass hum appears even if the gain is similar to the rhythm channel.

Is this a typical characteristic of this model? Or should I look for troubles in the circuit?

 

[Restless Rocks]

Given your amps age most here will rallie for a "cap job". There are several things which can contribute to hum on the high gain channel, including single coil pickups, certain types of preamp tubes (unfortunatly I've learned that many of my NOS long plate type tubes pickup unacceptable levels of hum), mismatched power tubes, dirty pots, cheap guitar cord, ground loops both in AC power and your effects pedal battery eliminators, proximity to TV sets and computers (even from the room next door to you), neon lights, bias too hot (usually evident on clean low gain as well). And the list could go on. Best bet; find a mesa tech and discuss replacing your filter and bias caps as well as a thorough pot and socket cleaning, and tube test. Even humbuckers hum if you're too close to the amp itself try moving around the room and see if there's any change in the hum level. Oh yeah, mark amps are known to be a bit flubby when bass control is set too high. Start with the bass set at 2 and work from there.

 

[breogan]

Thanks Restless Rocks!

A lot of things to be checked! I think I may found the origin of the noise. Today I was playing and a fry pan noise started in the clean channel. I used a pencil to see if there were any microphonic tubes and I found that V1 was one of those.

I am going to replace it to see if the noise disappears.

 

[mrd]

Ah man. If V1 is microphonic, every other tube is gonna amplify whatever noise it creates, especially in the high gain stages. Replacing that tube with a known good one will definitely help matters 8)

 

[dunkonu23]

Even humbuckers hum if you're too close to the amp itself try moving around the room and see if there's any change in the hum level. Oh yeah, mark amps are known to be a bit flubby when bass control is set too high. Start with the bass set at 2 and work from there.

Great advice! I discovered this almost immediately after getting my Mark IVa three weeks ago. When I'm playing I can turn different ways and the hum will either go away or get louder. I can barely hear it when my body is between the guitar and the amp. Never had that experience before. After a retube, I don't have a single problem with flubby bass, though.

Scott