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Old 20th October 2009, 12:41 AM   #1
exurbia is offline exurbia  Australia
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Default Marshall amp loses bias

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
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Last edited by exurbia; 20th October 2009 at 12:51 AM.
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Old 20th October 2009, 02:01 AM   #2
llwhtt is offline llwhtt  United States
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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
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Old 20th October 2009, 02:10 AM   #3
TheGimp is offline TheGimp  United States
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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.
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Old 20th October 2009, 03:11 AM   #4
llwhtt is offline llwhtt  United States
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All of the jacks on this amp are the 1/4" plastic type, doesn't rely on chassis connection at the jacks for ground.

Craig
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Old 20th October 2009, 08:37 AM   #5
exurbia is offline exurbia  Australia
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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.
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Old 20th October 2009, 08:52 AM   #6
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You might want to replace the coupling caps which feed the output tubes.
Perhaps they are becoming "leaky" as the amp warms up.
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Old 20th October 2009, 09:22 AM   #7
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Hia,

Hook a meter to your bias and monitor constantly. Find out when you're losing it, it could tell a lot.

I'd also change that "suicidal" bias setup, it's a bad design. Any dirty wipers and *POOF!*... no tubes.

Cheers!
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Old 20th October 2009, 09:21 PM   #8
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Default 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 ???
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Old 20th October 2009, 09:47 PM   #9
llwhtt is offline llwhtt  United States
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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
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Old 21st October 2009, 02:52 AM   #10
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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
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