HELP - MESA BB750 output issue

[gastric]

I have a MESA Big Block 750 I purchased used a month or two ago. It's been working fine, loving the sound, no issues... until practice last night.

• Powered the head on
• Worked fine for a while
• I heard an audible "pop" noise
• Then the output was barely audible and was extremely overdriven sounding

I ended up running through our PA which is a laptop run to a power amp and used Amplitube for the practice which was fine, got me through.

Today I had time to further troubleshoot the issue and came up with the following:

• When powering the amp on, I hear an audible "pop" through the speakers. Sometimes more than one. I can't recall whether or not this happened in the past or not.
• The tuner out works fine.
• The DI out works fine when in PRE. Totally clear sound.
• The DI out sounds heavily overdriven and the DI volume actually fades in and out slowly.
• I tried pulling tubes and switching them around but I have no idea which tubes do what in this head, and the PCB isn't labeled at all to help in that regard.
• The cabinet works fine when powered by my BBE BMax and Peavey IPR. So the issue is with the MESA head specifically.
• I only have a SINGLE Groove Tube 12AX7 I can pull from my working BBE BMax-T to substitute for a single MESA tube for troubleshooting potential tube issues. Which I have not yet done, but will AFTER posting this long thread.

I'm hoping for some general troubleshooting suggestions.

I'm also hoping that someone knows exactly what each tube is doing in this head. Here's a photo in case it's helpful. For clarification reference each tube by it's location "top left, bottom left, top right, bottom right" so there's no confusion as to which tube you're referring to.



You can also download a 32MB ZIP of high resolution photos that'll provide more detailed photos that you can zoom in on.

[Matt Dean]

A similar thing happened to me once with one of my M-Pulse 600s. I live near the Mesa factory, so I took it over... they ended up replacing the power section (MOSFETS) which solved the problem.

So, unfortunately, I think the moral of this story is that you'll need to send it to Mesa. The upside is that they'll fix it.

[gastric]

I swapped the tubes one at a time with the Groove Tube from my BMax-T and it has no effect. In fact, while the DI still provides the undulating, overdriven output with in post mode, the speaker output is now providing nothing other than some popping sounds shortly after power up.

Unfortunately while MESA seems to have great service - always reachable via phone - they're only available M-Th during business hours, and today is Friday.

My unit is still covered under their generous 5 year transferrable warranty, though I'm not looking forward to the shipping bill and length of time I'll be without the head if I have to send it all the way to CA.

[BassmanPaul]

If the unit is still under warranty put the thing back together before you void said warranty. Send it back to GB or at least wait until you contact them for their instructions.

Paul

[gastric]

I'm pretty sure swapping tubes isn't going to void the warranty.

I did cut some wires so I could attach alligator clips and insert my own home made resistors into the layout to see if that helped, but it didn't. Do you think they'll be able to notice??? _{I'm totally joking}.

[gastric]

Follow up:

There's two local authorized MESA warranty shops so I didn't have to ship it anywhere. Apparently the issue was a bad solder point near one of the two internal fuses, caused the fuze to go bad but not blow (I checked them myself and they were visually fine), the amp oscillated to some degree and blew a resistor.

All fixed up, covered fully under warranty, about a week turn around.

Happy camper as long as it continues to work.

[gastric]

Ahhh, the issue was actually a bad FUZE. But not a blown fuze. One of the internal welds in the fuze was bad and causing arcing and providing intermittent issues. Tech showed me the fuze, said it was the first time in his entire life he's ever seen a defective/bad fuze.

Supposedly the audible "pops" I heard were the amp's internal protection kicking in.

Just sharing the remainder of the information I have in case someone else is searching for similar things. In my case simply replacing the fuze probably wouldn't have worked as it killed a resistor on the board in the process.

Back in action and remembering why I loved the MESA BB750 sound so much. So lively and warm, lots of mids available.

[BassmanPaul]

I'm glad it was only a fuSe. Go forth and play!

Paul