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Mesa Boogie M-2000 Basis bias setting etc.

Kartofell

Kartofell

May 1, 2017
158
137
#1
Hey!

My tech said that bias might have "escaped" in my M-2000. Does anybody know what is the correct bias setting for M-2000 amp?

The hi-gain (tube) mode on my amp is really quiet compared to the normal tube mode. When I switch from tube mode to hi-gain the volume drops for like 20dB I would say (so I can't use hi-gain mode). What could be the reason for this?

When I turn the power on this M-2000 on, the volume goes up slowly and it needs some time to get to the full power. Why so? Is this normal? Thank you so much for all the answers!!! I really appreciate it!

I'll have some more questions about M-2000. I hope someone is still using this nice amp.
 
#2
aside from other TB members who have direct experience with this Mesa amp, you can also try calling Mesa directly. They offer support for all their amps, even discontinued models, going all the way back to the very first D180.

As far as bias goes, the M2000 only has a tube preamp, not a tube power section, and I don't think you need to bias the tubes in a pre-amp ... but I am not a tech/engineer, so don't take my advice on this as gospel ;)

AgedHorse is an Mesa design engineer who frequents TB, and the Mesa CS team participate on TB too, so they may be along shortly.
 
#3
M-2000 has an adjustable bias, this is not a question. The question is how?
So I got two answers from MESA:

1) Bias is set by measuring DC drop across any of the .2 ohm source resistors in front of each Mosfet with unit at idle. Target is 6 mV DC and is adjusted via small blue trim pot on Mosfet pcb.

2) The M-2000 bias setting changed over time, and should fall between 6 and 10 mV depending on the era of production.

So, now I need some more specific answers about bias setting for those M-2000 serial numbers:

B2K-000066: made in June 1995
B2K-000193: made in August 1995
B2K-001228: made in May 2001

Hello M-2000 people? Such an awesome amp, and no users?
Anyone at Mesa Boogie? Thanks!
 
#4
Be very, very careful adjusting the bias of a MOSFET amplifier. A twitch too much can switch the whole bank hard on destroying the amplifier. You would then need to get replacement devices that are matched to each other so that the work load is evenly spread across them all.

How does the amp sound? If it sounds good leave the thing alone.

if the tube pre-amp section is quieter than the SS pre-amp changing out the tube for a new one is something that an owner can safely do.

The bias setting of the output stage has nothing whatsoever to do with your symptoms.
 
#6
Kartofell said:
It sounds good, but the FET channel is much quieter than tube. Also hi-gain tube is much quieter than the normal tube mode. My tech would do any bias adjustments if needed. Thanks!
Click to expand...
Your symptoms have nothing at all to do with the output stage bias. Leave it alone. If your tech thinks this is the cause of your problems, you need to find a new, qualified tech, right now.

Your technician needs to use standard troubleshooting methods to trace the signal through the stages. Guessing is not going to work well, and could end up creating costly new problems.
 
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#7
agedhorse said:
Your symptoms have nothing at all to do with the output stage bias. Leave it alone. If your tech thinks this is the cause of your problems, you need to find a new, qualified tech, right now.

Your technician needs to use standard troubleshooting methods to trace the signal through the stages. Guessing is not going to work well, and could end up creating costly new problems.
Click to expand...

It's not just this... the story is much more complicated... trafo failure... burned whole power section (changed resistors, capacitors...) etc. Lots of issues with this amp. Now the sound is different that it was before the failure. Changed everything including trafo and can't get the same sound. My tech is very well qualified and I trust him completely.
 
#8
The answer you received is correct, anywhere between 6 and 10mV across the source resistor, I typically would use 6mV but be sure to check it both warm and cold. Be careful, a mistake made could cause major damage.

Why would changing the power transformer change the sound? Did you use a non-factory transformer?

If you trust your tech completely, then why are you asking your questions here? Ask your tech, he's the one working on your amp and should know the answers to your questions.
 
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#9
I trust my tech, but he doesn't know the correct bias setting for this amp. This doesn't mean that he's not qualified.
Yes, it's a custom made non-factory trafo. Wired to exactly 230V (as written on the back of the amp) but with thicker wire.

Original export trafo is just too expensive for me. I can't even afford to pay the shipping. I got two different answers (from Mesa Boogie customer service and Mesa Boogie FB) about this also.

Mesa FB: The transformer is $324, shipping is $435. Total is $759 (that's insane... I can buy the whole amp for this price).

Mesa customer service: $324 trafo + $64.60USD shipping, so the total is: $388.60USD ... (still can't afford even if could get it for this price, but the guy on FB said that I can't get it for this price, and that his total is correct (I'm so confused): "Sorry, but Trent doesn't understand that we cannot ship USPS to your location. Trent doesn't work in our office, but does answer emails. Thanks"

I think we need Masa Boogie here in Slovenija.
 
#10
Shipping is a big variable, and often USPS (postal) looks like it's less expensive but when you actually book the service it bills out completely differently than the USPS quick calculator. Trent does not have access to anything but the quick calculator as he does not operate out of the factory building (nor do I). We do not have any control over the cost of shipping, or the policies of the shipping companies.

So, what you are saying is that your amp is modified, non-factory repairs, non-factory parts. I'm afraid you are going to have to rely on your tech for additional support. He already has the necessary bias information BUT there's no way to tell if this is now correct because I do not know what MOSFETs you are using, and what the Vgs to Ids curve is. This is why factory authorized service is important for most products, not just ours. We can't be expected to reinvent the wheel and come up with new spec's for your now custom amp.

Because of your location, this may not be the best product for you as we are unable to support a non-authorized tech or service center.
 
#11
agedhorse said:
This is why factory authorized service is important for most products, not just ours. We can't be expected to reinvent the wheel and come up with new spec's for your now custom amp.

Because of your location, this may not be the best product for you as we are unable to support a non-authorized tech or service center.
Click to expand...

ok, so after 15 years of using mesa, I'll switch to some other company then.. or what should I say.. idk. thanks.
 
#13
Kartofell said:
ok, so after 15 years of using mesa, I'll switch to some other company then.. or what should I say.. idk. thanks.
Click to expand...
I'm sorry, but I can't possibly know if the symptoms you are experiencing are due to a typical failure or something that was caused by the the repair attempt. Trying to help has become an excercise in futility because there is no way I/we can acquire the necessary information to give you accurate information back. I've tried and failed to help.
 
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#14
Kartofell said:
ok, so after 15 years of using mesa, I'll switch to some other company then.. or what should I say.. idk. thanks.
Click to expand...

@agedhorse has a long history on this site for going above and beyond to help posters with problems. He does deserve your thanks even though he can’t help you. Threats are not welcome here which you would know if you’d been a member longer than a few days!! :rollno:
 
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#15
agedhorse said:
I'm sorry, but I can't possibly know if the symptoms you are experiencing are due to a typical failure or something that was caused by the the repair attempt. Trying to help has become an excercise in futility because there is no way I/we can acquire the necessary information to give you accurate information back. I've tried and failed to help.
Click to expand...

No one said you failed to help. I had a lot of questions. And you were really helpful. Thanks!

BassmanPaul said:
@agedhorse has a long history on this site for going above and beyond to help posters with problems. He does deserve you thanks even though he can’t help you. Threats are not welcome here which you would know if you’d been a member longer than a few days!! :rollno:
Click to expand...

Haha, threats, what threats? You are funny. :D
 
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#16
BassmanPaul said:
@agedhorse has a long history on this site for going above and beyond to help posters with problems. He does deserve you thanks even though he can’t help you. Threats are not welcome here which you would know if you’d been a member longer than a few days!! :rollno:
Click to expand...

If you'd read carefully, you'd see that agedhorse suggested himself, that M-2000 (or any Mesa product for that matter) may not be the best product for me as they are unable to support a non-authorized tech or service center. We don't have Mesa Boogie authorized service or distribution here in Slovenija, so obviously I can't get any serious support (as they're unable to support a non-authorized tech... but they also wouldn't authorize a tech in Slovenija, because we're too small). That means I'm left as a fish out of water. But still... I was sarcastic, and even if I'd be serious, that's NOT a threat, that's a free will and an advice taken into consideration. Mesa won't get rid of me so easily! ;)
 

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