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Old 30th June 2009, 02:35 PM   #1
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Default Mesa Boogie bass amp quiet

Hello!

Im not a native english speaker so please try not to laugh OR laugh as much as u like but try to answer my question as well.

So, the problem is with a MB bass amp m-pulse 360. It was on a 4Ohm load cabinet and it wasnt cranked up when it just got like quieter. Not dead silence, but half as loud as it was before. I dont have a schematic, so its a bit harder to find the fault but im 90% sure that the end on mosfets is ok. Tried to change the tubes in preamp but nothing. The tubes are 2 ecc83. The strange thing is that
the voltage in kinda wierd on the valves.

According to this image http://en.wikipedia.org/wiki/File:EIA-9A.png
the volts are on the first tube starting from one 305, 0 , 0, 9, 0, 250, 0, 1 ,4
and the other ones are 350, 210, 210,4, 0, 211 ,0, 1, 4

Seems like the second tube grid and cathode are getting too much volts or am i wrong? Where to look, what and how to mesure?

Im not home with tubes but if someone shows me the way, maybe im able to find the faulty component and change it.

Thanks
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Old 30th June 2009, 02:52 PM   #2
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It seems that in 2-nd tube-1-st triode, cathode resistor burned /cathode is open/. Check resistor. Check and anode resistor.
And than check cathode cap....Maybe it burned.
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Old 30th June 2009, 07:23 PM   #3
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Resistor is fine on cathode. And the only cap there is from cathode is going to the mosfets input. Out of ideas.
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Old 6th July 2009, 04:51 PM   #4
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No one knows heh?
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Old 6th July 2009, 05:42 PM   #5
ChrisA is offline ChrisA  United States
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Default Re: Mesa Boogie bass amp quiet

So the gain drops a lot but the voltages all still look reasonable?

I think a cap might be "open". Could be a cathode bypass cap. These are typically parallel with a resister. Its a likely to fail part too, electrolytic cap. If you remove one of those the DC voltages don't change but the gain on the stage is reduced. If a coupling cap opens you'd have no sound at all.

You are going to have to get a schematic, put a signal on the input and trace the signal level through the amp.

With the given information we are guessing. But my bet is on a cathode bypass cap.
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Old 6th July 2009, 11:20 PM   #6
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Thanks!

Will check the cap.

I know its a guessing, its a guessing to me too, cos i cannnot find the schematic either.

But thanks anyway.
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Old 6th July 2009, 11:20 PM   #7
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http://criz666.planet.ee/mesaboogie/ Some pictures may be helpful.:P
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Old 7th July 2009, 07:16 AM   #8
Enzo is offline Enzo  United States
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Are BOTH heaters glowing in that 12Ax7 with the 300,0,0 voltages?
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Old 7th July 2009, 08:35 AM   #9
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Seems so yes, as much as it is visable from the top.
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Old 7th July 2009, 03:34 PM   #10
Gabdx1 is offline Gabdx1  Canada
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Try another pre-amp / power amp, try isolate which component has weak gain. Because now it can be anything.
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