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Old 25th July 2011, 09:55 PM   #1
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Default Marshall JCM 800 noise

I have tried different output tubes and replaced screen resistors. Plate volts and bias volts are fine. Here is a clip of the noise:

‪JCM800 noise‬‏ - YouTube
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Old 26th July 2011, 01:01 AM   #2
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Check in this order:
1. Short the plates of the driver tube together (pins #1&6). If the noise goes away or is greatly diminished, the problem is in the power supply feeding the phase inverter stage.

2. Short pin 5 of the output tubes together. If the noise goes away or is greatly diminished, the problem is in the bias supply or coupling caps.

3. If ungrounded, short pin 8 of the output tubes together. If the noise goes away or is greatly diminished, the problem is in the cathode current monitoring resistors.

4. Try different output tubes.

5. Surely hope it is not arcing in the output transformer!

Dave
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Old 27th July 2011, 11:00 PM   #3
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Hello dcgillespie, I shorted pin 5 of the output tubes together and the noise is gone. Thank you so much for helping me but now I have to find the culprit. I am getting negative 45vdc at pin 5 for both tubes. The amp also has a master volume control with blocking caps on both sides. If a blocking cap was passing dc wouldn't the master volume stop it from passing to pin 5? I am guessing that my problem is in the bias supply but not sure. I am going to poke around in there tonight and check back here with more info.
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Old 28th July 2011, 02:29 AM   #4
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Everything looks ok to me but I am not tech so I guess I will just start replacing things in the bias supply.
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Old 28th July 2011, 03:28 AM   #5
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Famous -- leave the bias supply and grid return resistors in place, but disconnect the coupling caps into the output stage on the output stage side. If the noise is still there, it must originate from the bias supply. If it is gone, and the driver tube is removed, then it must be a coupling cap, driver plate resistor, or even the dropping resistor or power supply cap for the driver stage.

I take it this amp is using a dual control to function as a master volume control between the driver and the output stage? Disconnect the coupling caps into the output stage first, as if the noise is still there, then this master volume circuit has been completely eliminated from possibility.

Keep working the problem by a process of elimination, and the fault will usually show itself quickly enough.

Dave
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Old 28th July 2011, 05:31 PM   #6
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Hello Dave, yes the amp is using a dual control for the master volume. If you are interested here is a schematic:

http://www.drtube.com/schematics/marshall/4210.gif

I am going to lift the coupling caps on the output side of the master volume and see if the noise is still there.

Last edited by famousmockingbird; 28th July 2011 at 05:37 PM.
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Old 28th July 2011, 07:46 PM   #7
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caps lifted going to pin 5 of output tubes and the noise is still present.
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Old 28th July 2011, 11:58 PM   #8
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This is very interesting. Any noise injected by the bias system itself is injected common mode fashion, so generally it should be non-audible. Based on what you are telling me then:

1. Lifting the coupling caps into the output stage does not stop the noise.

2. Shorting pins #5 together of the output tubes does stop the noise.

3. Different output tubes do not stop the noise.

The only thing left is the 220K grid resistors. If just one of those became noisy, then the noise created would not be a common mode signal, and would sound in the output. Check the 220K resistors by shorting across them one at a time, or both at once if shorting either one does not completely stop the noise. If shorting out one (or both) of them stops the noise, the one shorted when the noise stops is the culprit.

Dave
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Old 29th July 2011, 02:20 AM   #9
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Hello Dave, reflowed some sketchy solder joints in the bias supply and replaced the 150k resistor just before the diode and the noise is gone. Maybe a cold solder joint arcing was the noise but I am not sure. The 220k resistors you speak of are 150k in the U.S. version that I own (the 220k are for EL34's). I am very interested in how you diagnosed this and also interested in what common mode is why it won't be audible. Any information you have is greatly appreciated as I really like to learn about this stuff.
As for the noise I am going to put in the preamp tubes and plug a guitar in and see if the noise comes back.
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Old 29th July 2011, 02:46 AM   #10
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Ok so the noise is gone with preamp tubes out. The noise comes back when the phase splitter is installed.
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