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Mesa Boogie 400+ - Wild Problem!
- Thread starter whoatherechunk
- Start date
Well, at around the 4:50 mark, I guess we finally got a definite answer on the output of the 400+.
BurningSkies
CRAZY BALDHEAD
- Feb 20, 2005
- 27,956
- 20,350
- Disclosures
- Endorsing artist: Dingwall Guitars
Well, at around the 4:50 mark, I guess we finally got a definite answer on the output of the 400+.
At ten percent distortion.
tested my amp today in practice. it's still jacked up. i'll call my tech tomorrow and see what's up. one thing i've noticed after trying to pinpoint problems myself is that when i switch the fan from off the high to low, or vice versa it makes a loud crackle. not sure if this is contributing to the problem i have but damn i'm stressed out. 
Hello.
I am now working on this amp. The original poster got in touch with me through anderbass so it is now on my bench in Phoenix (technically Chandler but that's not important).
I was able to make the noise he speaks of right away but even before that noticed a lot of the power tubes are very micro-phonic.
I narrowed the noise down to a single tube that is intermittently shorting internally and literally sounds like fireworks or popcorn.
I also found that this set of tubes wasn't matched very well and biased extremely cold.
I put up 4 youtube videos to show the issue and power output testing etc....
These are my first videos I have posted on youtube so please be kind.
MVI_8431.AVI - YouTube
MVI_8432.AVI - YouTube
MVI_8434.AVI - YouTube
MVI_8437.AVI - YouTube
I am waiting for the owner to respond with what he wants to do. I may add an adjustable bias pot or just swap the resistor to warm this thing up and then retest the output power.
I've now seen 4 different 400+'s from different years, 6 eq's to 7 eq's. Board mounted pots to flying leads. Voltage supply B using 2.7K resistors @ the tubes to using a zener diode to 680 ohm resistors.
Quite a few changes....
Feel free to ask any questions.
I am now working on this amp. The original poster got in touch with me through anderbass so it is now on my bench in Phoenix (technically Chandler but that's not important).
I was able to make the noise he speaks of right away but even before that noticed a lot of the power tubes are very micro-phonic.
I narrowed the noise down to a single tube that is intermittently shorting internally and literally sounds like fireworks or popcorn.
I also found that this set of tubes wasn't matched very well and biased extremely cold.
I put up 4 youtube videos to show the issue and power output testing etc....
These are my first videos I have posted on youtube so please be kind.
MVI_8431.AVI - YouTube
MVI_8432.AVI - YouTube
MVI_8434.AVI - YouTube
MVI_8437.AVI - YouTube
I am waiting for the owner to respond with what he wants to do. I may add an adjustable bias pot or just swap the resistor to warm this thing up and then retest the output power.
I've now seen 4 different 400+'s from different years, 6 eq's to 7 eq's. Board mounted pots to flying leads. Voltage supply B using 2.7K resistors @ the tubes to using a zener diode to 680 ohm resistors.
Quite a few changes....
Feel free to ask any questions.
It is great that you solved the problem Tim. The videos were interesting to watch.
This is a good lesson to people that:
1) vendors that sell matched sets don't always match their tubes as well as they should, a 24 hour burn-in before matching is a good practice, and
2) even if a set is properly matched, they could be matched to bias hotter or colder than average which affects the performance of the amp.
The 400+ amps have oscillation issues that keep coming up. A voltage spike in the power amp can kick the amp into oscillation. Both Marshall and Ampeg address this by adding a set of series diodes across the plates of the power tubes as shown below. Perhaps this safety feature would help with these amps.
This is a good lesson to people that:
1) vendors that sell matched sets don't always match their tubes as well as they should, a 24 hour burn-in before matching is a good practice, and
2) even if a set is properly matched, they could be matched to bias hotter or colder than average which affects the performance of the amp.
The 400+ amps have oscillation issues that keep coming up. A voltage spike in the power amp can kick the amp into oscillation. Both Marshall and Ampeg address this by adding a set of series diodes across the plates of the power tubes as shown below. Perhaps this safety feature would help with these amps.
indeed, very interesting. I enjoyed watching the demonstrations. It's always helpful to discover the resolutions to problems - helps others in the future. Thanks Tim.
P.S. : for a newbie the videos were excellent.
P.S. : for a newbie the videos were excellent.
just wanted to report back and say that the amp is working fantastically. nothing but top notch recommendations for Tim. this was a stressful experience, but a very educational one nonetheless.
HMMMM my Mesa Buster did this at a show recently (of course I was not the one playing at the time). Not the crackly bad tube noise but went into full blown oscillation... Oddly I used it for about 5-6 hours DIMED (to record some gnarly bass tone for a metal album) and it never had a single problem. I wonder if it got some voltage spike from the power at the venue because it definitely went into oscillation like in the 1st guy's videos.
Regardless, thanks to both of those techs for posting some extremely informative videos! I wish I could go to tube amp repair school, or that I at least had more fun EE tools.
Regardless, thanks to both of those techs for posting some extremely informative videos! I wish I could go to tube amp repair school, or that I at least had more fun EE tools.
Awesomeness!
Tim, I am stymied with adding bias pots to my 400 non-plus. Thought i would do this while changing out the filter caps (all 10 of the big ones). I have done this with a Mesa 50/50 in the past, with no problem.
Changing the value of the bias resistors makes no difference on the bias current... Seems completely bizarre. Is there something different about this 400 series?
Thank you!
Tim, I am stymied with adding bias pots to my 400 non-plus. Thought i would do this while changing out the filter caps (all 10 of the big ones). I have done this with a Mesa 50/50 in the past, with no problem.
Changing the value of the bias resistors makes no difference on the bias current... Seems completely bizarre. Is there something different about this 400 series?
Thank you!
There is only one resistor to change to adjust the bias.
Not sure which pair you are tweaking.
On the schematic it is labeled factory select and starred. In parallel with the final bias supply filter cap before and then it splits through the two 100K resistors to each side.
Not sure which pair you are tweaking.
On the schematic it is labeled factory select and starred. In parallel with the final bias supply filter cap before and then it splits through the two 100K resistors to each side.
