Buzzing studio preamp!

[FastRedPonyCar]

My studio preamp is quite as a mouse on the clean channel but has a loud hum/buzz on the overdrive channel. The higher the overdrive knob is, the louder it is.

I've swapped all of the preamp tubes except for the reverb tube and the buzz is still there.

I don't have any spare 12AT7's laying around. I have 2 questions.

A) would it be safe for me to test the amp using a 12AX7 in the reverb socket? Or would this damage the amp?

B) Has anyone else experienced this humming sound before and if so, what was the culprit?

Noise from an amp this old is expected but it's very noticable. I have to go to the clean channel when not playing and I can't roll the volume knob back and clean up the overdrive channel without being overpowered by the humming sound


Visaually, nothing is loose or out of the ordinary inside the preamp and the amp makes the exact same sound in both power amps I've used and direct to my mic preamp to the PC so it's definately the preamp causing the noise or more specifically, something up with the overdrive channel.

 

[CudBucket]

Drew, I had the same issue with mine when I got it but I don't remember it being quite as bad as you've described. When I changed my tubes (and maybe after I replaced the reverb tank), it got better. I'll have to check it.

Yesterday I fired it up with my 50/50 and got all kinds of noise from the pair so I'm not sure what's up. I think it might be a grounding issue now that both items are racked. I also shortened the ac cables once they went into the rack so it may be the snaking I did with other cables. I need to troubleshoot.

Dave

 

[FastRedPonyCar]

It could be a grounding issue as it's not in the rack. I ordered a pair of rack mount ears from mesa last week so hopefully they'll be in soon and I can either confirm or deny that as being the source of the noise.

 

[FastRedPonyCar]

I replaced all the tubes and still a very loud hum/buzz on the overdrive channel. :?

I've got a pair of rack mount wings on the way so its a last ditch hope that it's some crazy grounding issue but I highly doubt it. When I flip the ground lift switch to lift the ground on the back it just makes the noise a lot louder.

I'm thinking that SOMETHING inside the amp is not connected. Some ground wire or something isn't connected or something has a bad ground connection and i have zero ideas where to start looking


Is there anything that mesa could do over the phone or do I just need to suck it up and take it to an amp tech?

 

[CudBucket]

Something's definitely up Drew. I had bad *** noise from my Studio on Super Bowl Sunday. Or at least I thought I did. Pull it and the 50/50 from the rack yesterday and realized that the ground isolation switch was in the "lifted" position on the 50/50.

I tested the amp through the cab with no pre attached and found that was the cause of my noise. Put everything back in the rack and the Lead channel was quiet. It's not as quiet as my Road King but still quiet enough. A noise gate would easily silence it.

I really like this preamp. It's a stupid steal at $400 a pop.

I know it sucks to send it in but couldn't you roll with the Valve Master while it got worked on? I'm almost looking for an excuse to send mine in to Mesa for a check and clean bill of health. Maybe even have the C+ mod done to it's Lead channel.

Keep us posted.

Dave

 

[FastRedPonyCar]

tested it through the cab w/no power amp?? Didnt know you could do that.

anyways, I ran it through my mic pre into the computer and the noise was there.

I could get by with the VM if I have to but I don't know how long I could live without the mesa...

Or if I can afford to have mesa fix it right now. I'm scared at what they'd charge.

 

[CudBucket]

tested it through the cab w/no power amp?? Didnt know you could do that.

I meant the 50/50. Ran it into the cab with no preamp connected. Then I'd dime the output and presence and listen for noise. No noise with the controls maxxed. Once I'd hit the ground lift, it'd freak.

 

[FastRedPonyCar]

yeah the T100 is quiet as a mouse as the preamp buzzes just the same in both it and the valvemaster.

 

[BertG]

sounds like you are pushing the gain a tad too hard,trim your gain back, it'll dissapear.

 

[FastRedPonyCar]

The gain isn't really the issue as when the amp is at gig/practice volume with the band, even with the gain at 1, it buzzes and increasing gain increases the humming/buzz.

Is there a ground that's associated with the lead channel? The amp is silent on the rythem channel but it could be that the amp just doesn't have the output on that channel to make the noise noticeable.



worth noting is that when I got the amp, it didn't make nearly this much noise... and transporting it back and fourth out of the rack leads me to believe that SOMETHING could have gotten knocked loose inside or weakened some solder joint (maybe one of the ground solders) enough to create the noise.

My new rack should be here hopefully by the end of the week so I can get it mounted in there to see if it's indeed a ground loop issue. I also ordered a pack of the rubber gound isolater washers for mounting rack equipment with. If none of this works, I guess it's off to Mesa.