Mesa 750 Big Block Problem.

rotaryunison · #1 04-12-2011, 10:00 PM

Hi, pretty new to this forum. I've searched quite a bit for this, but I recently bought a used 750, and have a few questions. First, I'm running a Geddy Lee J-Bass into a Mesa PH212 8 ohm cab. It sounds fantastic. However, with all controls hovering around the 2 o'clock range (including master, but the active mid freq. is around 10 o'clock), occasionally the power amp will shut down. Then, it will come back on after a few seconds.

Am I driving the amp too hard? Or does this sound like a real issue? I'm more accustomed to my guitar amps, which are completely different of course.

levis76 · #2 04-12-2011, 10:29 PM

Yes you're driving it too hard and it's cutting out due to thermal shutdown. Use less gain and more master volume if that's what you need.
Does your BB750 have an input pad switch, some do some don't. If your bass is active, and I think the Geddy is, the pad should be activated. If you don't have the input pad you can ease up on the gain or turn down the volume on your bass. Even some hot passive basses will clip thfat preamp if you pound on it when you play.
IIRC, the older style BB750s would start clipping at about a 12 o'clock gain setting if played with a mildly aggresive attack.
Also, if more volume is what your looking for, you could try adding another 8ohm 212 OR sell your 212 and get two 4ohm 212s so you can run the head at 2ohms and get the full 750W out of it.

rotaryunison · #3 04-12-2011, 10:44 PM

Thanks for the response. The J is passive, and I have the BB750 input pad switch in the 'passive' position. I'm still not used to the controls of a Mesa, they are really sensitive. I had an older 50w all-tube Ampeg before, that I could run wide open for a gooey tube tone. This is a completely different animal. I figured on adding another 212 at some point for more volume. I just wanted to make sure the head was running correctly. It's my first day with it, and I haven't practiced in a band situation yet with it to test the volume.

georgestrings · #4 04-13-2011, 06:42 AM

Sounds like there's something wrong with it - I had a BB750 for awhile, and I pushed it pretty hard into a PH610, and never had a shutdown - even during a 4 hr gig...

- georgestrings

rekesbass · #5 04-13-2011, 07:36 AM

Quote:

Originally Posted by georgestrings

Sounds like there's something wrong with it - I had a BB750 for awhile, and I pushed it pretty hard into a PH610, and never had a shutdown - even during a 4 hr gig...

- georgestrings

+1....I currently own one(newer version) and never had this problem. I also had the older version and and never had this issue until (ironically)I ran a 2 ohm load and the smoke was let out..seems that mesa had a bad batch of mosfets or something..they fixed it twice,third time they replaced it with a new one,haven't had a problem since. Is the head vented well?? Are the fans working properly? Are you using a proper speaker cable? I know the wattage is greatly reduced when using a 8 ohm load but,it should not shut down like that..

rotaryunison · #6 04-13-2011, 07:37 AM

Quote:

Originally Posted by georgestrings

Sounds like there's something wrong with it - I had a BB750 for awhile, and I pushed it pretty hard into a PH610, and never had a shutdown - even during a 4 hr gig...

- georgestrings

How high did you set your input gain?

rotaryunison · #7 04-13-2011, 07:41 AM

Quote:

Originally Posted by rekesbass

+1....I currently own one(newer version) and never had this problem. I also had the older version and and never had this issue until (ironically)I ran a 2 ohm load and the smoke was let out..seems that mesa had a bad batch of mosfets or something..they fixed it twice,third time they replaced it with a new one,haven't had a problem since. Is the head vented well?? Are the fans working properly? Are you using a proper speaker cable? I know the wattage is greatly reduced when using a 8 ohm load but,it should not shut down like that..

Yes, I'm using a speaker cable, it's vented, fans work fine, etc.

georgestrings · #8 04-13-2011, 07:47 AM

Quote:

Originally Posted by rotaryunison

How high did you set your input gain?

Around 1 o'clock - but really, that doesn't matter in the least... I pushed mine pretty hard with a 4 ohm load, for plenty of 10-2 club gigs, and never had a shutdown...

In your case, I'd make sure the inside wasn't dirty - remove the top cover, and blow out the amp's insides with compressed air - and if that doesn't do it, there's something wrong with it - I'd send it in to Mesa, then...

- georgestrings

rotaryunison · #9 04-13-2011, 06:39 PM

Has it ever gone into overload protection? Anyone?

rotaryunison · #10 04-13-2011, 06:56 PM

I contacted Mesa, but haven't heard back yet. It's a little tough to see so I marked them, but here's where my settings are, when it just happened. It's easy to reproduce. It doesn't seem like these are extreme settings to me, but...

rayzak · #11 04-13-2011, 07:46 PM

Those settings are not too extreme. Something's not right.

mmbongo · #12 04-13-2011, 08:58 PM

That amp will drive an 8 ohm load at full bore for weeks without getting warm. Something is wrong with it, or the cabinet, or the cable.

georgestrings · #13 04-13-2011, 09:42 PM

Quote:

Originally Posted by rotaryunison

Has it ever gone into overload protection? Anyone?

Mine never did - only reason I don't still have it is that I traded it in on a new M-pulse 600, which fit my tonal goals better...

- georgestrings

georgestrings · #14 04-13-2011, 09:43 PM

Quote:

Originally Posted by rayzak

Those settings are not too extreme. Something's not right.

Agreed - I pushed mine harder than those settings, for hours - with a 4 ohm load...

- georgestrings

georgestrings · #15 04-13-2011, 09:44 PM

Quote:

Originally Posted by mmbongo

That amp will drive an 8 ohm load at full bore for weeks without getting warm. Something is wrong with it, or the cabinet, or the cable.

Yup - I'd make sure the cab or cable aren't causing your problem before sending it in...

- georgestrings

rotaryunison · #16 04-13-2011, 09:53 PM

Thanks for all the info. I tried with another 410 and it's the same. I'm going to check the cable tomorrow to see if that's the issue, then it is off to a Mesa tech...

levis76 · #17 04-13-2011, 09:54 PM

Yeah, with those settings it should never trip a thermal shutdown. If you have another cabinet and speaker cable you can swap out each part one at a time and see if you can determine where the problem lies.

I'm sure you're using a 12AWG speaker cable to connect the amp to the cab, and not a spare instrument or patch cable, right?

rotaryunison · #18 04-13-2011, 10:36 PM

^Yes, of course I'm using speaker cable, but I do completely understand your asking.

I've been doing this stuff for around 20 years, but have somehow managed to avoid bass amp issues for that time. Figured I'd change up my recording chain and try a live amp + Avalon U5 split for recording with this BB750 I picked up last week, and this is where it got me. Amp issues are so frustrating.

I do suspect it's a mosfet issue, judging from the information I've found. Seems to have been an issue with some Titans as well.

Does anyone know the repair cost for something like that? It's driving me crazy not knowing.

rekesbass · #19 04-14-2011, 09:14 AM

I wish I could tell you what it would cost but,I just don't know(mine was covered under warranty) and my response is more for a bump so maybe other Mesa owners could see this and chime in.

BassmanPaul · #20 04-14-2011, 09:59 AM

If it's a fan cooled amplifier the cooling tunnel may be blocked with crud that has been sucked into the amp. It might need to be vacuumed out.