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Help Me Diagnose My Amp Problem (Please)

C

cmhgreg

Oct 26, 2004
394
3
Columbus, OH
www.myspace.com
#1
First off, I DID do a search first and I WILL contact Mesa about this problem. I just figured I would find out if anyone among my TB brethren has had this problem before and what might be causing it.

I play thru a Mesa M-Pulse 360 into an Avatar B210 Neo. During the past two gigs (both in the last 6 weeks), I have experienced a very disconcerting situation where my amp slowly loses output, stays that way for about 30 to 45 seconds and then comes back to life, again very slowly. The most recent instance was last night during a New Year's Eve gig (right in the middle of Secret Agent Man :( ).

I have tried to figure out what the constants/variables were in both of these situations, since it has never happened during practice. In both cases, I was running the gain at about 10 o'clock and the Master at about noon. Nothing drastic going on with the EQ, the compression or anything else. The only pedal in the chain that was being used at the time was a BBE Sonic Maximizer. I have a Sansamp BDDI, but it was not on either time. Both pedals are powered by a Godlyke Powerall, which is plugged into a power strip, that is plugged into a Hum-X hum eliminator. There are no batteries in any of the units.

The fan is working. I am using a proper speaker cable to hook up my cab. Both times I was playing a passive Fender P bass.

One other thing: both times this has happened I was feeding a brand new Mackie powered mixer through the DI on the back of the amp. The send volume was at about 75%. Also, these were the only two times I have used the DI on this amp. Could it have to do with the DI circuit?

Could this be a heat issue? I wasn't pushing things at all, so if it is a heat issue I would think there is something wrong with the amp. I wouldn't think it would overheat anywhere near the settings I was using.

Or could it be the tubes? I bought this amp used on eBay and I have been using it for over six months with no issues. The seller said the tubes were replaced before he sold it, but given the fact that Mesa's are fixed bias, perhaps he didn't match the tubes to the amp. Could that cause this issue?

Thanks for any input on my output problem. And Happy New Year!
 
#2
UPDATE: I pulled the cover off of my 360 to see if there was any dust build-up and to inspect the tubes. The inside of the amp was very clean, i.e., no dust tumbleweeds. However, I am a little concerned about the tubes. They are Groove Tubes GT12AX7R. I don't know much about this stuff, but I have read that Mesa heads are "fixed bias" and that accordingly, the bias of the tubes have to be in a certain range since there is way to adjust that parameter (without a mod).

Can anyone tell me if these tubes are a fit for my M-Pulse 360? Regardless of whether this is causing my problem, should I order a pair from Mesa or are there other matching alternatives? (I am going to post this question in a separate thread as well.)
 
#8
ModestCargo said:
+1 to Psycho.

Anyway, the problem does not sound like it's coming from a preamp tube - I would really just suggest taking it to a tech.
Click to expand...

Yea, like I said, I'm calling Mesa in the AM, but I figured I would throw it out there and see if anyone has any experience with such a problem. The weird thing is it has only happened twice (both during gigs) and both times I was using the DI. It has never happened during any practices (where I don't use the DI) and the amp doesn't really get pushed any harder during the gigs. That makes it somewhat hard to diagnose. Does using the DI create extra heat or put any extra strain on the amp?

Looks like it is going to need bench tested and I'm going to be without it for awhile, which of course really blows since I have another gig coming up in two weeks.
 
#9
My friend has a Mesa all tube guitar amp that he's had for over 10 yrs. The tubes went last year and he called Mesa. They were so impressed that he got 10 yrs of actual use out the original tubes that they sent him a new set for free and wanted him to send them the old ones so that they could check them out.
 
#10
Are these Mesa engineers all Full Sail grads???

Dude, I bet I could get a lifetime supply of Mesa tubes if I sent them my 40 year old 12DW7's in my SVT! Too bad they rebrand crappy tubes or I'd do it.
 
#11
I just thought of something else. So as I wait to get a return call from Mesa, maybe one of you tech gurus can weigh in on this:

When I first inspected the tubes, I noticed that one of them was not seated completely in the socket. There was a gap of about 1 to 1.5 millimeters between one top of the socket and the tube furthest away from the input jack, which I believe is the one that is associated with the power section. All the pins were in their respective sockets, but it just wasn't fully seated.

How relevant might this be?
 
#12
Update: I just spoke with Rich at Mesa. He was extremely friendly and helpful. He indicated the unseated tube could have been the culprit. He also mentioned three other possibilities: 1) a capacitor might have been knocked loose; 2) a failed mosfet transistor; and 3) the tube in the V2 position should be a Chinese tube. That last one is somewhat interesting; I've never read that on TB (or anywhere else), so Mesa M-Pulse 360 owners take note. The tubes in there right now (installed by previous owner) are GT12AX7R, which are Russian. Rich indicated that this only applied to the V2 tube, not the other one.

He also gave me the name of an authorized tech in the area who I am going to call tomorrow.
 
#14
cmhgreg said:
3) the tube in the V2 position should be a Chinese tube. That last one is somewhat interesting; I've never read that on TB (or anywhere else), so Mesa M-Pulse 360 owners take note. The tubes in there right now (installed by previous owner) are GT12AX7R, which are Russian. Rich indicated that this only applied to the V2 tube, not the other one.

He also gave me the name of an authorized tech in the area who I am going to call tomorrow.
Click to expand...
that seems a little weird to me...I wonder why it is that you're supposed to use a Chinese made 12AX7 in the V2 position...it just seems like a random and arbitrary issue.
 
#15
thesteve said:
that seems a little weird to me...I wonder why it is that you're supposed to use a Chinese made 12AX7 in the V2 position...it just seems like a random and arbitrary issue.
Click to expand...

Very weird. It didn't seem to me that he thought this was necessarily the cause of my problem. He was going through his database of known issues and this was the last thing he mentioned. He wanted to ask his tech a question about it, but the tech was gone for the day.

I guess it begs the question: why would it make a difference if it was a Russian or a Chines tube. I will let some of you who know much more about tube design speculate on that.
 
#16
cmhgreg said:
the tube in the V2 position should be a Chinese tube. That last one is somewhat interesting; I've never read that on TB (or anywhere else)
Click to expand...

Its about a different Mesa model, but here's a quote on this very subject:

Psycho Bass Guy said:
I don't think the problem is necessarily just the JJ's. I think it's a combination of the amp's voicing and the tubes' inherent tone. I remember theorizing a long time ago that Mesa voiced its amps around the cheap Chinese 12AX7's they sold. I have bunch of Ruby 12AX7s from 10-15 years ago, the same ones that Mesa used to sell, so I put a full set of them in the amp. I was right The amp's preamp was nothing like it was with the JJ's. It was a little noisier, but the tone knobs became much more responsive, and even with a 12AX7 as a driver, the amp was much cleaner than it had been with the JJ preamp tubes. What's really funny is that those same Chinese 12AX7's are icepick treble machines in every other amp I've tried them in, but in the Mesa, they sound like RCA's in a Fender, just noisier. It's funny that JJ's are dark in other amps but too bright in the Mesa and that the Chinese 12AX7's that are too bright in other amps sound smooth in it. A plus is that those Chinese tubes are very cheap and abundant.
Click to expand...

Thread: http://www.talkbass.com/forum/showthread.php?t=387202
 
#17
It is with some embarrassment that I return to this thread with an epilogue of sorts. I believe I have found the source of my problem, and it isn't my Mesa (thankfully). It turns out that it was a stinking $5 patch cable on my pedal board. (Edit: it wasn't the Big Shot I/O, but the patch cable coming out of it). One of the solder connections was completely separated and was probably loose for awhile. Man do I feel like an idiot. :hiding:


Two things I learned in all of this: check all of your cables first. And if you still haven't found the problem, check all of your cables again.

Many thanks to all of you who offered help (and sympathy).
 
#18
Just a little add about replacement tubes I just came from my authorized mesa tech for news pream tubes for my M-Pulse 600 and he gave to me for better results he told, a chinese tube 12AX7B for the section close to the input jack and a russian Sovtech
12AX7LPS for the second one
 
#19
vince20100 said:
Just a little add about replacement tubes I just came from my authorized mesa tech for news pream tubes for my M-Pulse 600 and he gave to me for better results he told, a chinese tube 12AX7B for the section close to the input jack and a russian Sovtech
12AX7LPS for the second one
Click to expand...

That's good to hear. I now have something similar going on in my 360. Based on what the Mesa tech told me, I bought a Chinese 12AX7 for the preamp socket and left the Russian 12AX7 in the power socket (both are Groove Tubes). It's seems to have added a tad bit of upper harmonics and overall is slightly cleaner sounding when the gain is down. With the gain up, not a lot of difference.

By the way, the guys at the shop were very skeptical that there was any technical reason to use one tube over the other, but I just figured WTH. It's certainly no worse than the tube that was in there.
 
#20
You know, I like Mesa stuff fine, but that is just plain lazy engineering. An amp should never be dependent on using a certain brand of tube. It should have been designed to sound good with any tube. It's putting a band-aid on a stab wound. Mesa just went down a level of respect in my book. Again.
 

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