Mesa Boogie Bass 400+

[Huw10]

I've had my Mesa for over a year now and had it serviced around 3 months ago: 4 new valves, rebias etc.

At practice a few nights ago I switched it on a played about with it for around 4-5 minutes (first time in about 4 weeks - holidays!) and then it kind of just peetered out. The LED had gone out so I switched everything off. I figured it was just a fuse had blown so i replaced it and tried to switch it on to no avail.

There was a kind of burning sort of smell and a lot of heat coming off the 4 new tubes. I checked the old fuse and although it looked in a bad way it didn't look like it had been blown.

My tech lives about a 3.5 - 4 hour round trip away so I was just looking to get some advice before I take it down there. I have a horrible feeling that it might be some sort of transformer issue.

If anybody has any sort of advice they could lend it would be MUCH appreciated!

Thanks in advance for any help.

[umph]

did you use the correct value of fuse? The new valves may have been pulling far to much current and taken the fuse.

[woodyratm]

You should just buy mine Huw

[Huw10]

[quote name='umph' post='959435' date='Sep 17 2010, 01:47 PM']did you use the correct value of fuse? The new valves may have been pulling far to much current and taken the fuse.[/quote]

Yeah, according to the back of the amp. 3.15A

Right a little update. I've opened the bastard up. I had a wee tap on all the tubes and most of them seemed pretty wacked, but I took the two worst ones out and switched it on. Fuse blows, but it comes on. Progress.

So I figured the four new tubes my tech put in should be working fine. I pulled out the 8 old ones and left the four new ones. I switch it on and HUZZAH! It's working, BUT after about 5 mins one of the 4 tubes is glowing super hot, so I switch it off. I figure that tube is prolly pretty much dead now.

Any reason why this could happen? I noticed that the tube directly to the right of it of it was not quite in the socket, could this be why?

Also, say this is the lay out of the tubes in the amp:

t t t t t t t t
t t t t t t t t

how do the pairs work - like this?

1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8

or like this?

7 5 3 1 1 3 5 7
8 6 4 2 2 4 6 8

Thanks again!

[Mr. Foxen]

Whacking the tubes only tells you that whacking the tubes makes noise. Tubes can do that, they are only dead from microphonic if they are howling or something, or you are in the habit of whacking your tubes whilst playing.

Also, running it with missing tubes without knowing the pairings and not adjusting the impedance appropriately is very bad.

[Huw10]

[quote name='Mr. Foxen' post='959836' date='Sep 17 2010, 07:43 PM']Whacking the tubes only tells you that whacking the tubes makes noise. Tubes can do that, they are only dead from microphonic if they are howling or something, or you are in the habit of whacking your tubes whilst playing.

Also, running it with missing tubes without knowing the pairings and not adjusting the impedance appropriately is very bad.[/quote]


I'm aware that I don't know a lot about amps, this is why I am posting and seeking advice.

I do know that the tubes remaining in the amp are paired. I haven't adjusted the impedence, but it was a simple test to see if it was the bad tubes blowing the fuse or another problem.

[Geddys nose]

The 400+ is fixed bias so you have to be careful which valves you put in so they are within tolerances with the fixed bias so I doubt you tec would have biased the amp, also after 4 weeks the caps might have discharged all their stored electric so when you turn your amp on it draws too much current first which produced heat & stress on the amp and can lead to problems, be worth seeking help Asap.

[umph]

[quote name='Huw10' post='959485' date='Sep 17 2010, 02:22 PM']Yeah, according to the back of the amp. 3.15A

Right a little update. I've opened the bastard up. I had a wee tap on all the tubes and most of them seemed pretty wacked, but I took the two worst ones out and switched it on. Fuse blows, but it comes on. Progress.

So I figured the four new tubes my tech put in should be working fine. I pulled out the 8 old ones and left the four new ones. I switch it on and HUZZAH! It's working, BUT after about 5 mins one of the 4 tubes is glowing super hot, so I switch it off. I figure that tube is prolly pretty much dead now.

Any reason why this could happen? I noticed that the tube directly to the right of it of it was not quite in the socket, could this be why?

Also, say this is the lay out of the tubes in the amp:

t t t t t t t t
t t t t t t t t

how do the pairs work - like this?

1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8

or like this?

7 5 3 1 1 3 5 7
8 6 4 2 2 4 6 8

Thanks again![/quote]

Are the plates glowing red? What year is your one i've seen the mesa 400+'s wired both ways. Does it blow fuses without ANY valves in? I think it may be time to take it back to your tech.

[JTUK]

The only thing I can contribute..and this is YEARS old..so may be totally useless, is that my transformer going down did all this.

I think you have gone far enough with basic checks and it is time for someone who really knows what they doing with valve amps, to look at this.
I don't think you are going to be able to rescue it so stop before any more damage is done.

I know fuses going sounds the least of all evils...but IME, they go for a reason and with valves that is dodgy ground.

Historical perspective here of course, but I never really got over my trials and tribulations of valve amps when I first started so this colours everything now..even though they are probably light years on in technology and reliability.. plus I only had old-been-round-the-block valve amps.

Edited September 19, 2010 by JTUK

[thebassman]

[quote name='umph' post='960870' date='Sep 19 2010, 01:56 AM']Are the plates glowing red? What year is your one i've seen the mesa 400+'s wired both ways. Does it blow fuses without ANY valves in? I think it may be time to take it back to your tech.[/quote]

Modern transformers, like in the 400+, are far more reliable, so unlikely to be this.
On my 400+, my tech narrowed it down to the standby switch. It had been blowing fuses for a while.
The switch was letting too much current through, he checked it with a multimeter in line. I strongly recommend you don't do this, unless you know what you are doing. Lots of voltage and current!!
Anyway a replacement was only £15 from westside.

[Subthumper]

[quote name='thebassman' post='961042' date='Sep 19 2010, 11:39 AM']Modern transformers, like in the 400+, are far more reliable, so unlikely to be this.
On my 400+, my tech narrowed it down to the standby switch. It had been blowing fuses for a while.
The switch was letting too much current through, he checked it with a multimeter in line. I strongly recommend you don't do this, unless you know what you are doing. Lots of voltage and current!!
Anyway a replacement was only £15 from westside.[/quote]


Hi I dont wish to sound like your tech did'nt fix your amp but how does a switch pass too much current? Is'nt it more likely that the switch had become intermitant and that was blowing fuses?

[umph]

[quote name='thebassman' post='961042' date='Sep 19 2010, 11:39 AM']Modern transformers, like in the 400+, are far more reliable, so unlikely to be this.
On my 400+, my tech narrowed it down to the standby switch. It had been blowing fuses for a while.
The switch was letting too much current through, he checked it with a multimeter in line. I strongly recommend you don't do this, unless you know what you are doing. Lots of voltage and current!!
Anyway a replacement was only £15 from westside.[/quote]

If anything the transformers they use in modern valve amps are nowhere near as good as the old ones they used in marshalls / hiwatts / soundcity / laney / carlsbro.

find me a modern guitar amp that has transformers as good as partridge!

[Mr. Foxen]

[quote name='umph' post='961666' date='Sep 19 2010, 10:38 PM']find me a modern guitar amp that has transformers as good as partridge![/quote]

You already know Chambo's do.

[umph]

[quote name='Mr. Foxen' post='961669' date='Sep 19 2010, 10:43 PM']You already know Chambo's do.[/quote]
not exactly a production low end amp though is it like the soundcitys / carlsbros

[cocco]

Maybe I drempt this but don't the tubes in the 400+ work in 4s not pairs?

[Mr. Foxen]

[quote name='cocco' post='962945' date='Sep 21 2010, 08:17 AM']Maybe I drempt this but don't the tubes in the 400+ work in 4s not pairs?[/quote]

There's only two directions a tube needs to work in, push and pull, so 4 would be two pairs.

[Huw10]

My tech hasn't answered my texts for weeks. Going to call him today.

Thanks for all the help.

Just as an aside, my mesa is actually variable bias. The guy before me, or possibly before him replaced the fixed bias circuit. It was biased ridiculously low, so I thought that would be the end of all my problems, but apparently not!