Single Rectifier buzzing like crazy

[doyler]

Hello

I have had my single rec for about 5 years and it has always been relatively quiet. A few weeks ago when I was playing through channel 2 (red) it started buzzing like mad. I changed all the tubes with brand new ones, which made no difference. I get a lot of microphonic noise when I tap V1, which made me suspect it as the source of the problem but it doesn't matter what tube I put in there it wont stop buzzing. When I turn my amp on having it only connected to the speaker cabinet it makes the noise. When I plug a cable into the input and wave it around a bit the noise changes which makes me think it may be a grounding problem. The amp still plays, but the buzzing is super loud even at about quarter volume.

I am really stumped here. I have taken it to be fixed and the dude kept it for two weeks and then said he couldn't fix it. He said he checked everything and couldn't isolate the problem. Has anyone else had this kind of problem? I would really appreciate any help as it has been over a month now and I just wanna play some guitar.

Thanks

Matt

 

[domct203]

Welcome to The Board.

V1 will always be very sensitive to the tap test because of it's position in the circuit. Unless it squeals, rings, crackles or howls when tapped it is most likely OK. Change each preamp tube one at a time by swapping in a known good tube and checking for the buzz (it's good habit to power down the amp when swapping tubes). If you go thru all the 12AX7's and the amp still buzzes it could be a power tube issue, especially if they are 5 years old. I would change them anyway. If the buzz goes away then great, if not keep the old pair as spares.

Also, have you tried a different circuit (outlet) in the house? I would think so, as you stated your friend could not isolate the issue. I agree that your description sounds like a ground issue. Did you try a different power cable? Did your friend check continuity of ground to chassis?

If after checking/swapping all the tubes in the amp does not help it is time to take your amp to a tech. I strongly suggest a Mesa Certified tech as they know Mesa Amps the best, and should not charge any more than any other tech in the area.

Good luck,
Dom

 

[RussB]

Welcome 8)


A tech should be able to use an oscilloscope and trace the noise to the point of origin within a half-hour :wink:


But you can a try a few things,

1. plug your guitar into the effects return jack. Does it buzz? If so, the problem is in the power section, if not, the buzz is in the pre-amp.

2. Make sure the input jack is tight

Are you able to work on the amp? Do you have tools and the skills?

 

[doyler]

Hi

Thanks for the responses. I have already checked all the tubes individually, and have replaced them all a few times in the past 5 years. This is not a tube problem.
The amp is buzzing with nothing plugged into it. It makes no difference if I have the effects loop on 0% or 100%. When I plug my guitar (any guitar + any cable) into the effects return it still buzzes. So the problem is in the power section then? I'm not afraid of trying stuff myself if I have advice. I don't have any sort of metering equipment though only a soldering iron.

 

[RussB]

When you plug into the return jack, you are removing the pre-amp from the circuit. You problem lies in the power section.

Solder connections to the tube sockets, a faulty filter cap in the mains filtering or the bias supply, a bad ground connection....just a few wild-assed guesses :lol:


You need to get the amp to a certified Mesa tech

 

[doyler]

Does anyone know where the ground connection is? I have thought it was a ground issue for a few days now as the buzzing changes when i pick up my phone, move my guitar closer to power sockets etc... If I knew where the ground connection was I could check it out. Thanks

 

[RussB]

BE FRIGGIN' CAREFUL! :x

A jolt of DC power can kill you!


The area circled in yellow on the left is the main grounding point. All AC and DC current is grounded through that point. Check that it's soldered well and the screw is tight.

Where the arrow points to on the right is the ground wire from the main board to the power tube board

 

[domct203]

Does anyone know where the ground connection is? I have thought it was a ground issue for a few days now as the buzzing changes when i pick up my phone, move my guitar closer to power sockets etc... If I knew where the ground connection was I could check it out. Thanks

If you have a DMM you could check for continuity between the ground pin at the plug and the amp's chassis without removing the chassis.

No disrespect, but I recommend you DO NOT go aimlessly poking around inside the chassis. There are LETHAL VOLTAGES stored in the power capacitors, even when unplugged. These stored voltages are enough to stop your heart.

Call a Mesa Certified tech.

Dom