# Fuse Blown: Mesa Boogie Dual Rectifier - Multi-Watt (Should I be worried?)



## Mike_Blaszk (Sep 16, 2021)

Hey guys,

I purchased my Mesa Boogie Dual Rectifier Multi-Watt from a fellow forum member here a little over a year ago and its been a great amp for me since. However, I figured that the thing needed a full tube change as the existing set had been over a year or two old prior to my purchase and, tonewise, things had recently started to take a change for the negative; particularly after the amp had been on for over 45-60 minutes. So, I replaced all of the preamp and power amp tubes with Mesa glass (STR440's, 12ax7's and 5u4gb's). This had everything running back better than ever today except for the fact that, at about the 1 hour mark, the amp suddenly got very noisey (lots of feedback), the volume suddenly dropped and the amp shut off. In fact, it wouldn't turn back on despite the fact that all of my other electronics were still in working order. Admittedly, this had never happened to me before and I was a little freaked out, but I eventually put 2 and 2 together and replaced the amps main fuse. Afterwards, the amp turned back on and I monitored the inside of it for a minute prior to flipping the standby switch and watched things for another minute without issue. Should I be concerned? Is this something that I should get checked out?

Appreciate any help/suggestions. Thanks, 
Mike B


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## tdotrob (Feb 24, 2019)

I can relate this to when I’m fixing a customers furnace. The customer says I read on the internet that it’s the high limit switch so I changed it. SOMETHING caused the high limit to open and break connection and most often not just a faulty switch that decided to open and a sign of a bigger issue.

I would say the same applies to the amp and if your not an amp fixer guy I would say time to see a tech to be safe and protect your amp from more serious results.


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## Mark Brown (Jan 4, 2022)

tdotrob said:


> SOMETHING caused the high limit to open and break connection and most often not just a faulty switch that decided to open and a sign of a bigger issue.


110% if we change the word "switch" for fuse 

There is always a possibility some one off blew it, but in my experience and pile of dozens of blown automotive fuses.... im gonna go ahead and say it will happen again.


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## nonreverb (Sep 19, 2006)

If I were to take a wild guess, one of your replacement tubes has an intermittent failure.


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## tdotrob (Feb 24, 2019)

Mark Brown said:


> 110% if we change the word "switch" for fuse
> 
> There is always a possibility some one off blew it, but in my experience and pile of dozens of blown automotive fuses.... im gonna go ahead and say it will happen again.


Haha same with sewer’s. When I offer a camera inspection with a drain cleaning 98% of the time I hear back well it’s probably just something got in there. So ask the questions, do you flush wipes or hygiene products? No… ok well it probably wasn’t just a 4” round turd that’s clogging your main sewer line, something is causing stuff to get stuck.


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## Latole (Aug 11, 2020)

Put exactly the right fuse, if you don't you may arm power transformer and more.
On second amp, I always check if previous owner change the fuse for a wrong one.
Put bias switch for the right tubes ; see picture. 6L6
Use the right impedance speaker
If new fuse blow ; you may have a bad Power Tube (s) or defective bias circuit or both.
Or power supply issue.

Mesa Boogie amps are a nightmare to work on.


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## Paul Running (Apr 12, 2020)

Mike_Blaszk said:


> I monitored the inside of it for a minute


If it was my amp, I would monitor the B+ line to the center-tap of the output transformer...place a milliamp meter (not a digital meter as most do not have a fast enough refresh time), in series with that line and monitor the current.








As an extra measure I would give her a burn-in.
Also make sure that those screen-stoppers are actually 1KΩ and in circuit.


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## Latole (Aug 11, 2020)

Paul Running said:


> If it was my amp, I would monitor the B+ line to the center-tap of the output transformer...place a milliamp meter (not a digital meter as most do not have a fast enough refresh time), in series with that line and monitor the current.
> View attachment 440423
> 
> As an extra measure I would give her a burn-in.
> Also make sure that those screen-stoppers are actually 1KΩ and in circuit.


There are lethal voltage there, not for unqualified person.


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## Mike_Blaszk (Sep 16, 2021)

Latole said:


> There are lethal voltage there, not for unqualified person.


I'm definitely not a qualified person lol I can do the most basic of maintenance and troubleshooting. That's about it. I guess my inquiry is more along these line:

Whats the probability that the issue in this case was just actually a fuse that went bad (whether just with regular wear and tear or some ongoing issue with the old tubes that happened to come to a head after installing new tubes) and is there much harm in seeing if the fuse will blow again? My gut tells me to play it today and see if the same thing happens. If it does, take it to a mesa-certified tech. However, I'm also not an amp tech and don't know what I'm talking about.

I appreciate all of the help guys. Thank you.


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## Mark Brown (Jan 4, 2022)

Mike_Blaszk said:


> I'm definitely not a qualified person lol I can do the most basic of maintenance and troubleshooting. That's about it. I guess my inquiry is more along these line:
> 
> Whats the probability that the issue in this case was just actually a fuse that went bad (whether just with regular wear and tear or some ongoing issue with the old tubes that happened to come to a head after installing new tubes) and is there much harm in seeing if the fuse will blow again? My gut tells me to play it today and see if the same thing happens. If it does, take it to a mesa-certified tech. However, I'm also not an amp tech and don't know what I'm talking about.


Not a lot of harm in replacing the fuse and going again. That is why its there, to prevent damage from momentary failure. If the failed is still present, the wee little fuse will just blow again.

It's one of those "cant hurt to try"

It isnt like we rewire our houses every time a breaker trips 

....do expect it to blow. Who knows maybe you get lucky and it was just some inexplicable one off.


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## Latole (Aug 11, 2020)

You put the right new tubes; mesa brand as you say
If bias tube sector at the rear is for 6L6 AND you put the right fuse, amp should be safe


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## Mike_Blaszk (Sep 16, 2021)

Latole said:


> You put the right new tubes; mesa brand as you say
> If bias tube sector at the rear is for 6L6 AND you put the right fuse, amp should be safe


Yes. Mesa brand STR440's, 5U4GB's and 12AX7's, tube selector on 6L6 and fuse replaced with this:



https://www.solomusicgear.com/product/solo-slow-blow-fuses/











Fuse - Slo-Blo, 4 Amp, package of 5, Fuses & Holders - Amazon Canada


Fuse - Slo-Blo, 4 Amp, package of 5 in Fuses & Holders.



www.amazon.ca


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## Latole (Aug 11, 2020)

You amp need a slow blow ? You could be right , I don' know
How many amps ?

This is the A.C.’s (Alternating Current) main fuse and provides protection from outside A .C. fluctuations as well as power tube failure damage. Should the Fuse blow, replace it with the same rating in a Slo-Blo type package. The domestic U.S. version requires a 4 amp Slo-Blo fuse. A power tube short or failure is often the cause of a blown fuse...Follow the cold start procedure mentioned in the switch section and watch the power tubes as you flip the STANDBY to the ON position. If a power tube is going bad or is arcing you will see it! Flip the STANDBY switch to
Standby immediately and replace the faulty power tube and the fuse if necessary.
If you see nothing abnormal as you lift the STANDBY switch, it is possible that a power tube shorted tempo- rarilyandblewtheFuse.Ifthisisthecaseitmayworkagainnormally.Tobesafe,youmightwanttoreplace just the adjacent tube or all power tubes in the “shotgun” troubleshooting tradition and save the replaced
REAR PANEL: (Continued)

















*120 V~ 60 Hz 4A
4 A SLO BLO*
set as spares. Spare fuses are a must for the fabled cord bag along with your spare tubes. Always carry both for they could be worth their weight in gold someday.


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## nonreverb (Sep 19, 2006)

Mark Brown said:


> Not a lot of harm in replacing the fuse and going again. That is why its there, to prevent damage from momentary failure. If the failed is still present, the wee little fuse will just blow again.
> 
> It's one of those "cant hurt to try"
> 
> ...


Try this, put back all the preamp tubes first as you know the issue didn't happen with the OEM stuff. Run it and see if it happens again. If it does, remove two of the new power tubes you have installed and see if it does it again. If not, try the other two power tubes. Don't worry, the impedance mismatch won't hurt it. If you're really concerned, just connect the speaker to the 4 ohm jack if it's 8 ohms. Also, if it does happen, flip the amp around so you can see which tube is failing as there will probably be an arc or the tube will start to glow brighter or actually red plate.
I should add if it does NOT happen after the preamp tubes have been replaced, you probably have a preamp tube that is experiencing an intermittent short. That will require a little more detective work.


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## Mike_Blaszk (Sep 16, 2021)

Latole said:


> You amp need a slow blow ? You could be right , I don' know
> How many amps ?


Manual says 4 amp slo-blo









Fuses - 250 Volt Slo-Blo - 1/4" x 1 1/4" (6.3 mm x 32 mm)


The official Mesa Boogie online store for factory replacement parts, custom parts, hardware, shirts, hats and accessories.




store.mesaboogie.com


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## Latole (Aug 11, 2020)

Right !


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## Mike_Blaszk (Sep 16, 2021)

Started the amp up fine without blowing another fuse and played for 30 minutes. However, the amps volume has dropped significantly and theres quite a bit more interference (Im picking up some kind of Punjabi radio station). Any ideas?


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## Latole (Aug 11, 2020)

Power supply issue.

Burn resistor feeding one power tube or other power supply issue


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## Mike_Blaszk (Sep 16, 2021)

Atleast for the time being, the issues seem to have been resolved by replacing two of the power tubes (only one appeared to show signs of burning/black marks and functioning quite dimly upon closer inspection, but I replaced the set). However, I'm also going to have to look over the preamp tubes at some point because the amp sounds quite different with the FX loop engaged now (could be a cable or something else too). This is such a bummer to deal with when it happens lol; especially randomly losing a power tube during a time of shortages/price hikes. And theres still the possibility of an underlying issue that causes things to blow again down the road.


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## Latole (Aug 11, 2020)

I thought you recently replaced those power tubes with new ones?

There is a good chance that a defective part is the cause of the damage to this tube. It seems to have overheated.
The bias of this tube may not be appropriate or a resistor should be replaced.

This amp definitely needs to see a technician with the many problems you mention.

As for the sound being different, there is very little chance that the preamp tubes are the cause. These tubes would be new and preamp tubes do not wear out. They last forever or almost.


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## Paul Running (Apr 12, 2020)

Mike_Blaszk said:


> Im picking up some kind of Punjabi radio station


That would indicate that there is improper termination at the control-grid side of most likely a preamp tube. All components in an amplifier should be properly terminated. Improper terminations lead to an antenna effect.
Without a scope, I would inspect the circuitry with magnification for fractured solder joints or components, concentrating in the grid circuit areas.


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## Mike_Blaszk (Sep 16, 2021)

Alright. I'm driving it over to Rich Gowman at Amplifiers Plus (certified Mesa tech) to give it a once over to negate any potential issues. I'll let everyone know what the verdict is when I hear back.


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## Mike_Blaszk (Sep 16, 2021)

Everything's fine with the amp. Looks like a tube failure blew the fuse. Once the fuse and blown tube were replaced, everything appears to be back to normal.


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## Paul Running (Apr 12, 2020)

No more short-wave radio reception?


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## Mike_Blaszk (Sep 16, 2021)

Paul Running said:


> No more short-wave radio reception?


Lol I will let you know. Just got the amp back today and haven't had a chance to check anything out myself, but the tech says all is now good.


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## Latole (Aug 11, 2020)

Mike_Blaszk said:


> Everything's fine with the amp. Looks like a tube failure blew the fuse. Once the fuse and blown tube were replaced, everything appears to be back to normal.



Are bias are good ? It is a must to check each time you replace a tube.


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## nonreverb (Sep 19, 2006)

Latole said:


> Are bias are good ? It is a must to check each time you replace a tube.


Mesa amps don't have adjustable bias.


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## Latole (Aug 11, 2020)

nonreverb said:


> Mesas don't have adjustable bias.


You mean a pot.
A tech don't need a pot, just replace resistors values. These amps need a bias check and adjustment if needed.

It is this type of work that separates amateurs and professionals


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## Mike_Blaszk (Sep 16, 2021)

Mesa amps are fixed bias. Not that its completely necessary, but even Mesa stipulates that you don't need to bias tubes ever if you use Mesa-branded tubes (realistically any matching tubes within a particular range will do).


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## Latole (Aug 11, 2020)

Mike_Blaszk said:


> Mesa amps are fixed bias. Not that its completely necessary, but even Mesa stipulates that you don't need to bias tubes ever if you use Mesa-branded tubes (realistically any matching tubes within a particular range will do).


You don't seem to know what a fixed bias is.
In case a tube becomes defective, I always measure the bias because the bias circuit can be defective and damage the new tube. For a technician it is very easy to do.


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## nonreverb (Sep 19, 2006)

Latole said:


> You mean a pot.
> A tech don't need a pot, just replace resistors values. These amps need a bias check and adjustment if needed.
> 
> It is this type of work that separates amateurs and professionals
> ...


A little background on Mesa amps Latole. As the OP mentioned, his tech replaced one tube. If I were a betting man, I'd guess with a Mesa branded tube which coincidentally, are colour coded so they can be installed without rebiasing as the OP pointed out. It's Mesa's way of trying to get you to stick with their tubes long after the warranty runs out.
Secondly, I don't believe you carefully read the post. His tech replaced ONE tube. If someone was adamant about rebiasing a whole set with one new replacement tube, one would need separate bias pots or bias setting resistors for each tube....not going to happen.


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## Mike_Blaszk (Sep 16, 2021)

Regardless, a mesa-certified tech looked at it and gave it the all-clear. When I get the chance, I'll put it through its paces and confirm this, but I really have no reason to have any concerns at this point.


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## nonreverb (Sep 19, 2006)

Mike_Blaszk said:


> Regardless, a mesa-certified tech looked at it and gave it the all-clear. When I get the chance, I'll put it through its paces and confirm this, but I really have no reason to have any concerns at this point.


You shouldn't... he did as per Mesa's guidelines even though it's out of warranty.


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