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Help! Mesa 400+

B

billhilly66

Aug 25, 2007
732
5
Plano, TX
#1
I got a 400+ from a fellow TB'r a while back. It was a local deal so I met the guy at his house. He played the amp through a Kustom 2x15 cab and it sounded fine. He said it had cheap tubes in it so I bought it sans tubes. I had my tech go through it and he did an adjustable bias mod on it and installed the 12 6L6's and 4 Tungsol 12ax7's I'd got from the tube store. Now it craps out before it gets anywhere near the volume I'd expect from it. He said everything reads ok and said he'd have to try and research it.

Anybody had this problem?
 
#5
Hi.

Any tube amp, especially one with so many output tubes, is impossible to diagnose on-line.

If Your tech is competent, and he's benched the output section to the 250 or so watts before clipping, the head may be all right.

Is the amp/cab impedance matched?
Is the cab able to handle the power?

Regards
Sam
 
#6
And, what kind of bass are you playing, and into what input? My active MTD Kingston makes my 400+ retardedly loud through a junky GK backline 4x10 super quick. Like 3 on the volume is making the band complain (volume 2 was at like 3.5 I think).
 
#7
I have one of these and having done some wrestling with it a bit here are a couple of pointers.

1) There are over 100 tube pin to socket friction connections in this loud bass amp. It is critical to clean and check tension on these with every re-tube or you are one loud open B slap from crap-out.
2) You just loaded up 16 new tubes and its comforting to think they are all hand selected for perfection by celestial nymphs - but in reality the odds are not so long that one of the bunch is a little outlaw. So after a little warm up / break-in session, its a good idea to re-check em all for conformance to spec.

Get a good set of tubes mounted nice and tight in this amp and they will last a long time as the design does not push them [as long as your setting on your new bias mod aint radical and it should not need to be].



If your tech is mystified after another look, send it to Mesa or get a new local tech.

My $0.02
 
#8
It isn't the cab or the bass. I initially thought it was the cab (15/6) but it does the same thing with my Eden 410 and I tried it yesterday with 810e, same thing. Same thing with several basses too. It distorts before it gets any real volume almost like I've got a hot plate on it or something.
The tech says the voltages are fine and said he'd call Mesa Monday and see if a guy he knows there can shed some light on the deal. I threw it up here on the off chance that I might get an "oh yeah, mine did that too. here's what we did to fix it".
 
#11
This may seem silly but its a fairly obvious solution that I have overlooked myslef before. Have you tried swapping out the preamp tubes? Esp V1 and V4 will have a lot of effect on what your headroom sounds like. I use an older RCA 12ax7 on my v1 because it breaks up very smoothly. I use a JJ on the v4 because it is very solid sounding.
 
#13
Yeah, I had the same issue with my F1X. There was a difference when I turned the blend but it didn't get me where I need to be. The tech said he'd research it so I'll see what he comes up with. If he can't figure it out, there's a shop down town that probably can. I can always send it back to Mesa if it comes down to that.
 

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