Mesa Bass 400 preamp voltages WAY too high!?

the great wazoo · 10-23-2009, 10:39 PM

Hi Folks,

Have a Mesa Bass 400 with an issue. It worked fine then it seemed to go dead, no sound but lights up fine.
Checked it against the schematics:
http://www.tubefreak.com/2bass400.gif

http://www.tubefreak.com/1bass400.gif

The Preamp node (C) is only a few volts less then the B node. 500VDC+!
This is the case after the plate resistors for the preamp too. There seems to be little if any voltage drop in the preamp.

I first thought perhaps the 10k/2w was pretending to be a much smaller resistor but replacement did nothing.

I am assuming the high voltages are the cause of the preamp to not function but I cannot seem to figure out what would cause this problem.

I have swapped with known good tubes and also removed the power tubes from the circuit. No change in either scenario.

Any ideas about how to troubleshoot this one properly much appreciated!!!

TIA,
TGW

52 Bill · 10-23-2009, 10:47 PM

The preamp is not drawing any current so there is no voltage drop across the resistors.

Are the heaters lighting up? Are the cathode circuits grounded? etc. etc.

the great wazoo · 10-24-2009, 12:29 AM

Hi!

Yes all heaters are lit. Did check all cathodes grounds and they also check out, though the v1B measures really low 330 ohm or so. Intend to lift one end and measure again as well as check other preamp grounds to see if I can something there.
Will report after I poke around again for a few minutes.

the great wazoo · 10-24-2009, 01:32 AM

Wow. I swear I checked the heaters!!! I did really, honest!
But sure enough checking them again revealed pins 4 and 5 were cold.
Power tubes were fine. Found one of the wires from the power tube sockets to the pcb was broken.
Thanks kindly for the shove back in the right direction!! She is working fine and dandy now.

10-23-2009, 10:39 PM	#1
the great wazoo Junior Member Join Date: Jul 2009 Posts: 3	Mesa Bass 400 preamp voltages WAY too high!? Hi Folks, Have a Mesa Bass 400 with an issue. It worked fine then it seemed to go dead, no sound but lights up fine. Checked it against the schematics: http://www.tubefreak.com/2bass400.gif http://www.tubefreak.com/1bass400.gif The Preamp node (C) is only a few volts less then the B node. 500VDC+! This is the case after the plate resistors for the preamp too. There seems to be little if any voltage drop in the preamp. I first thought perhaps the 10k/2w was pretending to be a much smaller resistor but replacement did nothing. I am assuming the high voltages are the cause of the preamp to not function but I cannot seem to figure out what would cause this problem. I have swapped with known good tubes and also removed the power tubes from the circuit. No change in either scenario. Any ideas about how to troubleshoot this one properly much appreciated!!! TIA, TGW

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10-23-2009, 10:47 PM	#2
52 Bill Old Timer Join Date: Jun 2007 Posts: 1,236	The preamp is not drawing any current so there is no voltage drop across the resistors. Are the heaters lighting up? Are the cathode circuits grounded? etc. etc.

10-24-2009, 12:29 AM	#3
the great wazoo Junior Member Join Date: Jul 2009 Posts: 3	Hi! Yes all heaters are lit. Did check all cathodes grounds and they also check out, though the v1B measures really low 330 ohm or so. Intend to lift one end and measure again as well as check other preamp grounds to see if I can something there. Will report after I poke around again for a few minutes.

10-24-2009, 01:32 AM	#4
the great wazoo Junior Member Join Date: Jul 2009 Posts: 3	Wow. I swear I checked the heaters!!! I did really, honest! But sure enough checking them again revealed pins 4 and 5 were cold. Power tubes were fine. Found one of the wires from the power tube sockets to the pcb was broken. Thanks kindly for the shove back in the right direction!! She is working fine and dandy now.

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