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PostPosted: Tue Sep 17, 2013 11:08 am 
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Bottle Rocket

Joined: Tue Sep 17, 2013 10:58 am
Posts: 1
I have a Mesa Bass 400+ amp that is experiencing a problem any help would be appreciated. The problem is when it is turned on the power tubes all 10 of them almost instantly become very hot, and it is making the classic motorboating sound thru the cabinet. I believe the bias has been compromised, but I cannot seem to locate which resistor or resistors control the bias, can someone offer an opinion and guide me to the correct solution. Thanks!

Rad


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PostPosted: Mon Sep 30, 2013 12:24 pm 
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Mark II

Joined: Sat Sep 28, 2013 10:42 am
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Location: Canada
Got it fixed mate ! Free bump if not .


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PostPosted: Fri Jan 10, 2014 6:28 pm 
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Mark II

Joined: Tue Jan 07, 2014 10:54 pm
Posts: 70
radmaximus wrote:
I have a Mesa Bass 400+ amp that is experiencing a problem any help would be appreciated. The problem is when it is turned on the power tubes all 10 of them almost instantly become very hot, and it is making the classic motorboating sound thru the cabinet. I believe the bias has been compromised, but I cannot seem to locate which resistor or resistors control the bias, can someone offer an opinion and guide me to the correct solution. Thanks!

Rad

It has nothing to do with the bias.
The amp is breaking into high frequency oscillation.
When this occurs, many times you can't hear the oscillation.
The Oscillation destroys the output tubes.

I have seen this on several 400+ amps...you are not alone on this.

A normal "bench test" does not detect the problem. It will test normal.
Plug a bass in. Play some notes, lower the volume on the bass guitar volume control...or raise it, then play a note, and suddenly stop playing.
then it oscillates.
The oscillation is intermittent. It only seems to occur when actually playing a bass through the amp.

Which is WHY most repair efforts will fail. The problem comes back again.

Install protection diodes in the output section. These diodes are referred to as: "flyback diodes."
These diodes are standard equipment on many amplifiers, but not THIS amplifier.

After installing the protection diodes, the problem stopped entirely.
All further tests indicated that the problem was permanently solved, by installing these diodes.

soundguruman


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PostPosted: Sat Oct 04, 2014 11:15 am 
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Mark II

Joined: Tue Jan 07, 2014 10:54 pm
Posts: 70
radmaximus wrote:
I have a Mesa Bass 400+ amp that is experiencing a problem any help would be appreciated. The problem is when it is turned on the power tubes all 10 of them almost instantly become very hot, and it is making the classic motorboating sound thru the cabinet. I believe the bias has been compromised, but I cannot seem to locate which resistor or resistors control the bias, can someone offer an opinion and guide me to the correct solution. Thanks!

Rad


I have seen this happen many times.

The problem is caused by a design defect that triggers high frequency oscillation and motor boating in the power amp.
This oscillation burns up the power tubes.

When techs claim that this problem is "fixed," typically the real problem is NOT fixed.

Installing "flyback" protection diodes in the power amp seems to stop the oscillation problem from occurring.


IMPORTANT:
The oscillation does not show up under normal test conditions.
It only happens when the amp is played with a Bass Guitar. That's WHY most techs can't detect it, or fix it.


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PostPosted: Sat Oct 04, 2014 11:31 am 
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Mark II

Joined: Tue Jan 07, 2014 10:54 pm
Posts: 70
sprocket wrote:
Got it fixed mate ! Free bump if not .

It's probably not really fixed.


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