Mark V Terrible ground sound

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thebomb

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Help! I bought the amp I've wanted my forever through fb marketplace. It is a used 2014 Mark V 90W Combo. I love this amp. I've owned it only three weeks. I used it for one gig, practice at home, two rehearsals, and last night at a gig it crapped out on me.

It happened at the end of the first set in Channel 3, solo. I had just finished a screaming solo in the high register. I began do do some picking during the breakdown and a terrible hummmm began. Sounds like when a guitar cable is unplugged.

Listen while I try to trouble shoot on stage during the set break: https://youtu.be/V2_dcuM8Nr4

I was using a strat. In the above video, I have already switched to my tele. I'm switching on and off the mute on the foot switch. I get it on all three channels. Then, we turned off the foot switch, channel one works fine. I could switch to channel two and three with the knob on the back.

Here is when I thought I fixed it: https://youtu.be/Z0C_pFcmRtQ You'll hear the ground humm in the beginning, then the fix a t 1:40

I thought it was a bad foot switch cable, so I resolved to switch manually for the rest of the show. WRONG!

I found that when I got the amp singing, really driving the tubes, the loud, awful HuMMM came back. I turned the amp off, missed some of a song, then turned it back on before the next song. Worked again. I switched the amp off in between songs for the rest of the night. I did not get a chance to hook up thee foot switch again.

I'm praying someone here will know what is worn with this! Please help! I paid $2300 for what I thought was the amp of a lifetime. There is only one amp tech in my area- Harrisburg PA. I hear there is a long wait at the shop, although they have a good reputation, and we are a gigging band.
 
Thanks, I've been trying to replicate the problem at home and I can't.

Could this be problem with the power I plugged into? There was not a power conditioner and I was not plugged into a power strip.
 
Possible, but in my experience it's unlikely. I did have a dirty power issue once in 20 years. The other case I had was the club had a huge neon light over the stage, I had to un-plug that.
 
To me, it sounds like it may simply be a power tube red-plating. When that happens it will result in a loud hum. I suggest trying swapping in new power tubes - if the old tubes were red-plating and you left the amp on it may have taken out a resistor. Good luck.
 
Tuna141 said:
To me, it sounds like it may simply be a power tube red-plating. When that happens it will result in a loud hum. I suggest trying swapping in new power tubes - if the old tubes were red-plating and you left the amp on it may have taken out a resistor. Good luck.

Thank you for the tip. I'm not familiar with switching out tubes or repairing resistors. What is "RED-PLATING?" Which tunes are the power tubes? How might I identify the faulty one?

It did occur again during rehearsal, so I can rule out power supply and heat from being in the sun. Funny: It happened during the same song!

Anyway, it seems to happen when the amp has been running a few hours, and when it is screaming from power chords with leads. If I turn the amp off, then on again, it "resets" and the problem goes away.
 
It sounds like you have an intermittent problem which can be very hard to track down. To rule out tube problems you will need a back up set of known good tubes in order to test as below. In the Mark V that will cost you a fair chunk of change but that's part of running tube amps.

Mesa Boogie Amplifier Troubleshooting video:
https://www.youtube.com/watch?v=wSl-l9aWDHw

You will find the same information in text form in the Mark V manual on pages 51-52:
https://mesa-boogie.imgix.net/media/User%20Manuals/070425-MkV_160928.pdf

Basically you are looking to isolate a bad tube be it in the preamp or the poweramp section.
 
Thanks for the help. I've learned a lot. I think it is a bad 5U4GB Rectifier tube. I will update once the replacement comes.
 

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