Amp problem- help/advice appreciated

[Zman]

I have a 4 month old Mesa single rectifier solo 50 and have had a issue since it arrived. The amp sounds fine for 5 minutes and then starts to sound muddy/fizzy. Then it will start to sound fine again after several minutes then go back to mud/fizz after a few minutes. This goes on the entire time I play (usually 2 hours).

At one point the amp sounded fine for 5 days straight but the problem came back. This was before I tried anything to try to solve the problem (I was waiting for the new tubes to arrive that I had ordered as I thought it would be a simple tube issue).

I have put in all new power and preamp tubes, tried a different boost pedal (I use a clean boost in front to tighten up the low end), different cords, speaker cable, checked the voltage at the outlet with a multi meter (read 121-123 V), I have a power conditioner with a voltage meter on the front of it and the meter reads the same 121-123V and stays consistent as I play, and I tried plugging straight into the wall. Checked the ohms on my speaker cab (which was fine). I have also tried a different guitar but the problem is still there.

Any idea on what could be causing the problem I am having? I want to cover everything before contacting Mesa to send it back for service and I thought having a few ideas about what might be causing this problem would help when talking to them.

Sorry for the long post - any ideas would be really appreciated!

[jmohr58]

I am really not really the voice of experience when it comes to tube tech, but as far as electronics issues in general, an intermitten problem could be caused from a bad solder joint, barely visable crack in a pc board, a capacitor thats on its way out (or wasn't really very good from the start) a part such as a resistor that one lead end was broken or cracked during manufacture or shipping but gets vibrated closer or farther from point of good contact during use or transport, heating and cooling expansion and contraction, or something else along these lines.
With what you have already done to troubleshoot the problem, provided you are confident that the Tubes, Cables etc that were used for testing were Known Good Parts! I fear that if you delve much deeper you may no longer have a valid warranty with Mesa. That is just my opinon and personally, there are people on this site who are very knowledgable with the innerworkings of all parts of the tube circuits. They of course may offer a different oppinion, but I myself would only take the matter into more depth if a warranty weren't at risk.
To find out if any of the issues I suggested above means cracking the case, well, that would mean "Voiding Your Warranty" And this I advise against!
Best wishes and hope for a simple fix to your problem,
Jerry

[selloutrr]

About all you can do safely is test / replace tubes. since that has been done. it's time to send it back or take it to a service tech.

if it's under warranty send it back for repair it's not worth trying to learn amp repair on this amp.. they are PCB and difficult to work on.
If it's not under warranty find a local Mesa Boogie tech and save the shipping.

You aren't going to gain anymore face trying to diagnose the amp then if you were able to tell them how it's acting up.

[jmohr58]

selloutrr is correct, the tech won't care what you've done.
I was a field tech in the PC based Retail, Insurance, Banking environment with a mainly static geographic region for several years which you get to know the customers you deal with regularly. The scenario is:
A: you have a client who really does try to help and over time has learned what to check and gets pretty sharp on troubleshooting the problems before you arrive, and really does do what they said they did.
B: the client that say's he or she did certain steps but you are getting fed BS.
C: the client who thinks they are helping and tries to fix a problem and creates a bigger one in the process.
And even with the A: secnario, eventually that client moves to a different posistion or quits etc. and you wind up with a new B: or C: at which point you hope they don't touch anything before you get there anyway.

[tubeswell]

send it back if its under warranty.

[Zman]

I am gonna send it in for service-no doubt (never had any intention of fixing it myself). Just wondered if I covered everything I could do to try to solve the problem first.

It seems I did from the replys. Thank you

[Super_Reverb]

Does your problem occur at low pre amp drive levels and/or low output levels? The symptom you described sounds like what I have experienced with parasitic oscillation.

Sometimes instability such as this only occurs at high volume or high preamp drive.

Other than trying some lower gain preamp tubes (have any 12au7s or 5751s?)), there not much you can do externally if it is oscillation.

Seems unlikely though, unless the design is just borderline stable.

cheers,

rob

[selloutrr]

assuming all tubes are in fact good tested for noise, microphonics, Mu and transconductance. we can assume they are not the cause.

The amp should be stable when working properly.

For peace of mind just assume....

If the amp has been shipped, traveled in a car on a bumpy road, dropped, set down firmly... etc. It probably has an intermittant contact / cold solder. the amp plays fine heats up the solder pools and as the amp is played (resting on the speaker cabinet) the vibrations cause the amp to trigger the problem. it eventually gets back to making a solid contact and then repeats the process.

Now send it in to be repaired. Mesa are not a fun amp to work on.