Mesa Boogie bass amp quiet

mekaiver · 30th June 2009, 01:35 PM

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

azazello · 30th June 2009, 01:52 PM

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.

mekaiver · 30th June 2009, 06:23 PM

Resistor is fine on cathode. And the only cap there is from cathode is going to the mosfets input. Out of ideas.

mekaiver · 6th July 2009, 03:51 PM

No one knows heh?

ChrisA · 6th July 2009, 04:42 PM

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.

mekaiver · 6th July 2009, 10:20 PM

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.

mekaiver · 6th July 2009, 10:20 PM

http://criz666.planet.ee/mesaboogie/ Some pictures may be helpful.:P

Enzo · 7th July 2009, 06:16 AM

Are BOTH heaters glowing in that 12Ax7 with the 300,0,0 voltages?

mekaiver · 7th July 2009, 07:35 AM

Seems so yes, as much as it is visable from the top.

Gabdx1 · 7th July 2009, 02:34 PM

Try another pre-amp / power amp, try isolate which component has weak gain. Because now it can be anything.

30th June 2009, 01:35 PM	#1
mekaiver diyAudio Member Join Date: Nov 2008	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

6th July 2009, 04:42 PM	#5
ChrisA diyAudio Member Join Date: Jan 2008	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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30th June 2009, 01:52 PM	#2
azazello diyAudio Member Join Date: Aug 2008 Location: Bulgaria	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.

30th June 2009, 06:23 PM	#3
mekaiver diyAudio Member Join Date: Nov 2008	Resistor is fine on cathode. And the only cap there is from cathode is going to the mosfets input. Out of ideas.

6th July 2009, 03:51 PM	#4
mekaiver diyAudio Member Join Date: Nov 2008	No one knows heh?

6th July 2009, 10:20 PM	#6
mekaiver diyAudio Member Join Date: Nov 2008	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.

7th July 2009, 06:16 AM	#8
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	Are BOTH heaters glowing in that 12Ax7 with the 300,0,0 voltages?

7th July 2009, 07:35 AM	#9
mekaiver diyAudio Member Join Date: Nov 2008	Seems so yes, as much as it is visable from the top.

7th July 2009, 02:34 PM	#10
Gabdx1 diyAudio Member Join Date: Jan 2009	Try another pre-amp / power amp, try isolate which component has weak gain. Because now it can be anything.

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