I recently bought a (used) Mark V. It was working fine. Seller said tubes were old, so I swapped in some new ones I had (all genuine Mesa):
4 x 6L6
1 x 5U4
1 x 12AX7 (in V1 position, I left the other pre-amp tubes untouched)
...then I played a gig. Went great! Amp sounded amazing, so far so good.
Then I took it to rehearsal a couple of days later, and noticed an intrusive hum. It's like a flat buzz, very constant. Characteristics:
- The hum is only present when amp is fully powered up (i.e., NOT in standby mode)
The hum is present in all channels, and is gain dependent. It's barely noticeable in low gain channel settings (e.g., Channel 1, Clean, low gain). It's very loud in high gain settings (e.g., Channel 3 pretty much anything!)
It's there (but pretty quiet) if nothing is connected to the amp.
It gets worse when I plug a (known good) guitar cord into the amp input.
It gets even worse when I plug my guitar in.
The hum basically disappears if I turn the volume on the guitar down.
The hum varies a lot as I move the guitar around - pickup orientation seems to matter, and pickup proximity to amp matters a lot... the hum gets really bad when I move the guitar close to the amp.
I'm guessing it's tube related, but I'm wondering if anyone has any tips for diagnosing which tube(s) it's likely to be?
Also, could this be caused by the amp getting jostled in the back of the car, driving too/from gigs and rehearsals?
Thanks in advance for any help!