Marshall EL84 20/20 schematic needed

[pingfloid]

I've just bought a second hand Marshall EL84 20/20 power stage and one of the channels does not work. ¿Does someone have the schematic for this amp?.

It's a 20+20W amp in a single space rack unit.

Thank you

Manuel

 

[Giaime]

I guess it's better to move this thread to the Musical Instruments page...

Have you asked over www.firebottle.com/ampage? there are so many people who work with musical instruments- oriented tube stuff... here you'll find only hi-fi geeks!!!

But like any other new/no p2p product, I suggest you take a look to the solder joints, input jacks, tubes (their age, their state and their bias...), burned resistors etc etc...

 

[Tim Wyatt]

you have one side that works, start comparing.

 

[alejo]

Manuel for Marshall schematics go to:

http://www.drtube.com/

and

http://test.18watt.com/

if you want talk in spanish seendme one email, I build one Marshall18 in this day.

 

[pingfloid]

Thanks for your fast answers!

I've figured that the circuit is a standard EL84 push pull with 12AX7 driver, and there are 2 problems:

A - The left channel is working, but the volume is very low.
B - The right channel sounds normal, but it makes white noise (even with the volume turned down).

I have tried the following:

1 - Change all valves for new ones: no difference here
2 - Check the voltages in all the pins of the valves: they are very simmilar in both channels.
3 - Remove the 12AX7 valves: the noise is still there!. So problem B is not coming from the driver stage; it is some component of the output stage that is noisy (and it's not the valve) any clue here?

Thank you very much

 

[vintagekiki]

Marshall EL84 20/20

Shema za Marshall EL84 20/20 nalazi se na adresi:

http://www.about-guitar-amps.com/guitar_amps_schematics/Marshall/Marshall_2020_60_00_BackBoard.pdf
http://www.about-guitar-amps.com/gu...rshall/Marshall_2020-60-02-iss4_BackBoard.pdf
http://www.about-guitar-amps.com/guitar_amps_schematics/Marshall/Marshall_2020-60-02_BackBoard.pdf
http://www.about-guitar-amps.com/guitar_amps_schematics/Marshall/Marshall_2020_61_00_FrontBoard.pdf
http://www.about-guitar-amps.com/guitar_amps_schematics/Marshall/Marshall_2020_62_02_ValveBoard.pdf

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