Mesa Boogie 400+ Issues

Jables303 · #1 03-01-2011, 12:20 PM

Hey there TB,

I'm a long time reader, but first time poster.

I've been having some serious issues with my 400+ and wanted to see if anyone else out there has hae similar problems. It's a newer model (2002ish), I've read and heard all about how new models are not as reliable/have issues etc, but I thought that was a myth until now. I run a mesa 2x15 road ready cab with McCauley drivers. I also use a Furman power conditioner in between the wall and the amp.

Here's the run down: I bought it used on craigslist from a guy who had bought it straight from Mesa in California. He said he never had any problems with it. I get it back to my space in Denver and everything sounds great.

It works fine for 7 months then bam, one of the 12 output tube blows up. It shorted from the Plate to the Heater (a "catastrophic" failure). There was a blue flash out of the back, the lights in the room flickered, and it was dead. I took it in to the tech who normally works on my gear. He fixed the socket and put in a new tube for me.

I took it back to rehearsal and blows up again within 45 min, taking the brand new socket and tube with it. I take it back to him, he fixes it again and I decide to fork out the money for a brand new set of matched tubes from Mesa (set me back about $250). At this point I'm down close to $450 in repairs and parts. Get everything back and this seems to do it.

The repair and the tubes hold for about 3 months then it happens again. Only this time it's on a different socket, but the same row. I take it back to the same tech and he goes all out on the amp. He found a service note from Mesa saying to do this one repair with the ground bus for the output tubes. I took it back again for rehearsal and it blows up after 45 min of playing. At that point my heart broke. Both, the tech and myself are at a loss. We know that the output tubes are losing their bias for just a second, causing the blow outs.

I checked the wall plug with a AC ground tester to make sure everything is wired correctly and it is. The meter on my furman shows 122VAC coming from the wall, which is close to standard. My cab just has one jack out the back. The drivers are wired in parallel and when I check it with my DMM I get a reading of 4.78ohms, which is pretty damn close to 4.

Any thoughts, suggestions?

I'm not 100% sure what my next move is. I don't want to fork out any more money on repairs, but I can't get over the 400+ sound.

Thanks,

-Jonathan

levis76 · #2 03-01-2011, 02:00 PM

Is it failing while you are playing? If so, it sounds like maybe a cracked solder joint somewhere. It could be breaking contact just long enough to cause it fail. Going thru and retouching every point is a pain, but it can sometimes solve a lot of mysterious problems.

I'm kind of surprised your tech hasn't suggested sending it home to let the Mesa techs have at it. Shipping is a bitch, but Mesa will send you a box to put it in and the turn around is very good, less than a week in most cases. While it's there, they will do any and all updates, clean it, check the bias circuitry, etc. The works, basically.

Good luck, and keep us posted.

crapshack · #3 03-02-2011, 01:00 AM

my money is on a solder joint. arc welding isn't fun when you don't expect it to happen.

lem8r · #4 03-02-2011, 02:00 AM

Tell your tech to solder reversed diodes (something bit more than 1kV) from plates to ground to protect your tubes and thoroughly isolate every pin 3 and pin 2.

Then try to play on a different cab. As I can seen from schematics, 400+ doesn't have negative feedback (is it true??) and may be can't handle speaker's resonances.