Hi all,
I have an old Marshall 9005 guitar amp, it sounds nice but has issues. It keeps losing the bias and cooking the output tubes.
I have been over it with a fine toothcomb but can't find anything wrong. The bias sets up nicely, the balance is perfect between the tubes, it gets hot and then it kills itself.
Any suggestions will be greatfully received.
The schematic is available at Dr.tube http://www.drtube.com/marshall.htm#M9000
I have an old Marshall 9005 guitar amp, it sounds nice but has issues. It keeps losing the bias and cooking the output tubes.
I have been over it with a fine toothcomb but can't find anything wrong. The bias sets up nicely, the balance is perfect between the tubes, it gets hot and then it kills itself.
Any suggestions will be greatfully received.
The schematic is available at Dr.tube http://www.drtube.com/marshall.htm#M9000
Last edited:
exurbia,
I too have one of those amps, rack mount 50W stereo amp, correct? Since there are two separate channels, only thing common is the power transformer. Does the bias act up in both channels or just one? I would really be surprised if it is both since the there is NO bias tap on the power transformer. Bias is derived from the high voltage windings and are separate for each channel. So if you loose bias because of no HV the amp won't work so I doubt that your power transformer is bad, that's good! So the problem must be on one of the PCBs, first to check is C16, .047uf, big blue square job in my amp. Throw some junk tubes in it and monitor on down the bias circuit until you find it fading away. Does your bias balance actually work? Mine does not because there is no DC current flowing in that circuit so there is nothing to adjust. I think there should be a couple of resistors on either side of the pot. I think Marshall goofed on this one, unless someone knows different.
Craig
I too have one of those amps, rack mount 50W stereo amp, correct? Since there are two separate channels, only thing common is the power transformer. Does the bias act up in both channels or just one? I would really be surprised if it is both since the there is NO bias tap on the power transformer. Bias is derived from the high voltage windings and are separate for each channel. So if you loose bias because of no HV the amp won't work so I doubt that your power transformer is bad, that's good! So the problem must be on one of the PCBs, first to check is C16, .047uf, big blue square job in my amp. Throw some junk tubes in it and monitor on down the bias circuit until you find it fading away. Does your bias balance actually work? Mine does not because there is no DC current flowing in that circuit so there is nothing to adjust. I think there should be a couple of resistors on either side of the pot. I think Marshall goofed on this one, unless someone knows different.
Craig
Check the input and output connector grounds.
If the ground connections to the 1/4" (or whatever) phono-jacks rely on mechanical connections, add ground wires.
I'm working on a Traynor amp that went "Red Plate" because of bad grounds.
Got some new EL34s that I won't install till I'm sure the circuits are sound.
If the ground connections to the 1/4" (or whatever) phono-jacks rely on mechanical connections, add ground wires.
I'm working on a Traynor amp that went "Red Plate" because of bad grounds.
Got some new EL34s that I won't install till I'm sure the circuits are sound.
Thanks Craig and TheGimp , i have gone through the bias supply. it is only the left side that went wobbly. The balance on mine does work. Your board should have R20 and R9 installed for the balance.
I cannot find anything wrong, all the values are ok and nothing is open or over/under spec.
This has me beaten, it is looking good for parts at the moment.
TTFN, Peter.
I cannot find anything wrong, all the values are ok and nothing is open or over/under spec.
This has me beaten, it is looking good for parts at the moment.
TTFN, Peter.
marshall 9005
HI I have one of those 50/50 power amps in loft, seems ok apart from crackly pots just dont know much about them , picked it up at a sale for 40 pounds uk, I play bass thru a trace mk 6 old school pre-amp and crown slave, so dont know why i bought it, but friend o mine says if you change output tubes or was it input tubes it not but circuit for hifi use, is he mad???? any ideas ???
HI I have one of those 50/50 power amps in loft, seems ok apart from crackly pots just dont know much about them , picked it up at a sale for 40 pounds uk, I play bass thru a trace mk 6 old school pre-amp and crown slave, so dont know why i bought it, but friend o mine says if you change output tubes or was it input tubes it not but circuit for hifi use, is he mad???? any ideas ???
There are two ways to deal with it, monitor the bias circuit until it goes bad or replace everything in the circuit, I still think C16 is a prime candidate, been there done that. If I remember correctly those boards need to come out for any type part replacement and you don't want to do that more than once. As far as the bias circuit mine has R9 and R20, what I meant was two resistors that go to ground to get some current flowing in the circuit. If you look at the circuit from the wiper of PR2 everywhere the bias goes is a dead end as far as DC current. You have to have some current flow before the adjustment can do anything. If you do have a leaky coupling cap somewhere that might just be enough current to let the adjustment do something. Does the adjustment work on the good side? At least you have another channel to help troubleshoot.
Craig
Craig
I would be suspecting intermittent bias adjustment pots, either the Balance Pot (PR1) or the Bias pot (PR2). If the wiper connection is intermittent in either pot you loose the bias.
I would replace both (actually I'd probably rewire the bias circuit to something "fail safe" but that is more work).
Cheers,
Ian
I would replace both (actually I'd probably rewire the bias circuit to something "fail safe" but that is more work).
Cheers,
Ian
Hi Craig, and thanks to all that have replied with tips. I have been unable to play with the amp or do very much at all for the past few weeks. It will be quite awhile before I am functional enough to hold an Iron. I will have a look at rewiring the bias circuit when I am back up to speed.
Cheers,
Peter.
Cheers,
Peter.
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