Mesa Boogie 400+ - Wild Problem!

[whoatherechunk]

Hey guys!

I used to be an avid poster/viewer of this forum but I took a long break haha. As for my issue, I have had my 400+ since 05'. Every thing has gone smoothly minus a power amp section blowout in which I had to retube the entire power section (JJs). That occured about a year ago. A couple days ago I was playing through my friends Carvin 810 and after like 10mins I started getting an intermittent noise and the speakers were pumping in and out violently. So I switched out my speaker and instrument cables and it continued. The practice ended and I just thought it may have been my friends cab as it is old and pretty junky shape. Fast forward two days and I'm at home at another practice. I use the 400+ and an ampeg 410 and a boogie 210. After 10 mins it did the same damn thing as before and I thought the speakers were gonna fly out of the cab! Ive excluded cables, the cab, and the bass as the culprits so I guess it leaves the amp to be the issue. I thought it may have been the bass but as the noise occured I pulled out the cable connected to my bass and the noise continued! Arghh! I still have the stock boogie preamp tubes in my amp. Could this be the culprit? Sure microphonic noise would be indicative of faulty preamp tubes but this is massive chainsaw noise. There is a huge lack of techs in my city so if I had the opportunity I would take it to one but I'm trying to troubleshoot the issue on my own for now. If anyone has previous experience in such problems or advice, I would greatly appreciate it! Thanks!

[RickenBoogie]

Sounds like you need to send it to an amp tech, if you can't drop it off. I wouldn't be poking around in there unless you're experienced, and have all the needed tools and test equipment.

[whoatherechunk]

I definitely want to do that. Thing is the only tech I relied on is retired and not workin anymore so I would probly have to make a trip to Albuquerque. I'm just wondering if anyone may possibly have a clue as to what is going on.

[B25Expert]

Sounds like a rectifier problem to me

[Jerrold Tiers]

Possibly tubes, possibly a power supply capacitor problem, possibly a bad solder joint problem almost anywhere in the signal chain. Maybe even a rectifier...

Try it again, and when you hear the noise, patch cable into the power amp in, assuming that cuts off the preamp signal. If the noise is unchanged, it is power amp related, possibly power supply related. if it goes away, or nearly does, it's probably in preamp.

The 400+ is best worked on by someone who knows about fixing tube amps.........

[mikeldtl]

Originally Posted by Jerrold Tiers

Possibly tubes, possibly a power supply capacitor problem, possibly a bad solder joint problem almost anywhere in the signal chain. Maybe even a rectifier...

Try it again, and when you hear the noise, patch cable into the power amp in, assuming that cuts off the preamp signal. If the noise is unchanged, it is power amp related, possibly power supply related. if it goes away, or nearly does, it's probably in preamp.

The 400+ is best worked on by someone who knows about fixing tube amps.........

Good advice all around. Jerrod knows what he's talking about (more than me or most people FWIW). The 'effects loop return' jack will indeed break the preamp signal from the power amp. EQ can be switched on or off (doesn't matter). This will at least narrow down the area of the amp, possibly, by cutting it in half-ish. I would think that the power tubes themselves would be OK if they are only a year old, but it could be a laundry list of things. I will put my vote in for tech time.
My personal 400+ loved to blow PI tubes and had some shoddy soldering on the IEC mains connection until I reflowed some joints. I also had to replace the mains fuse holder as it was cold soldered and would shut down intermittently. Like during recording and other important things. I believe that the design of the amp is awesome, but not all of the construction - in my example - was bullet proof. Things break. All of them.
It is one of my prized pieces of gear and if you feel the same, I would take it to a tech regardless.

[MAMMOTHvolume]

is it a ticking or a pulsing?

[whoatherechunk]

Hey guys!

Awesome suggestions and I will definitely try to get around to them tomorrow. It really means a lot that you guys give me super rad advice! I played a show on monday (yes I know I shouldn't have played my amp) and I pinpointed more stuff. Like I said in the initial post, the for-sure possibility of getting that popcorn sounding noise occurs when the amp has been on for a little while. What I noticed is that I can play a song fully and once I stop playing/or play a long sustained note I start getting mad crackle and pop haha. Then if I give it a little rest it goes back to normal. I actually played on monday and somebody post a video of my band (please spare me the your band blows comments). There are two songs and at the end of the first song you can hear the crackle from 2:22-2:29 and on the last song from 4:45-4:51.

jerrold: i'm gonna try the suggestions tomorrow!

mike: excuse my lack of education but what's a PI tube? phase inverter?

mammoth: it's kind of like a mix of both. like in the video it pulsates but not like a smooth tremolo haha. i can just describe it as making popcorn in your microwave.

again, i really appreciate it guys!

[gastric]

I'd start with patching a cable into the EFFECTS RETURN to isolate the power section. If patching in removes the noise I'd guess it's a preamp tube. If you had some spares you could swap one tube out, test, repeat until you identify the culprit. Or rule them out.

[whoatherechunk]

when pulling out power amp tubes i'm guessing it would be a lengthy process since theyre so damn hot. i have spares but my tech that retired labeled em with weird codes so i would have to call him. thanks gastric.

[Detuned]

Man I had the same problem, only mine made cracking noises... Turned out to be a faulty power tube.. Take it to a tech that could check each tube individually, that way you won't have to spend a fortune on the whole set...

[whoatherechunk]

my amp tech made a video of my head. he's not a musician but he knows tubes and worked for NASA for ages. i hope it's finally resolved now!

Mesa Boogie Bass 400 + Being Repaired and Under Test.wmv - YouTube

[Caca de Kick]

Love the Dos XX beer bottle on the workbench.

[whoatherechunk]

I really hope my amp is ready to rock. I left it with my original tech but he was like dying or something and never got to it fully. I actually played the head through his gadgets that show all the sin waves and things checked out alright without any harsh abrupt signs of it being faulty tubes. I'm crossing my fingers. Ahhhhh!

[BassmanPaul]

I don't know who this guy is but I agree with just about everything he says. I too would have run a ground bus for the cathodes - I do with every amp I make. I would like to have seen him put a 40 - 50Hz sine wave through the amp to better approximate the working conditions of a bass amp. 1KHz is a fairly easy frequency.

All in all I liked the man.

[whoatherechunk]

Seems like my tech has made progress but is still stumped . He was able to discover the oscillations I was talking about and he said something I gather about the power section feeding back to the input (power transformer anomalies etc). If you can think of any suggestions after watching this video you'd be a life saver. Here's the video:


Mesa Boogie Bass 400+ Problems - Need Help.MOV - YouTube

[BassmanPaul]

Do the grids of the output tubes have stopper resistors? If not they need them.

Is the B+ supply clean or can the oscillations be seen superimposed on it feeding back into the front end?

If the NFB loop is temporarily disconnected does the instability still happen?

[whoatherechunk]

i'm going to send this thread to my tech. thanks a bunch Paul. never thought this amp would be such a pain to deal with. at least it's served me over 7 years without many problems.

[whoatherechunk]

looks like my amp is fixed (i really hope so...or i'm gonna be stressed out hahaha)

Mesa Boogie Bass 400+ Repair Final.wmv - YouTube

[beans-on-toast]

You've got a great tech there! He knows his stuff and stuck with a difficult problem and got it resolved.

When he was demonstrating that the amp could still be driven into oscillation, he was inputing a signal that was way beyond what a bass is capable of.