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I need help troubleshooting a Marshall EL84 20/20


 
6/24/2005 2:17 AM
Manuel
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I need help troubleshooting a Marshall EL84 20/20
I've just bought a second hand Marshall EL84 20/20 power stage. The circuit is a push pull with EL84 and 12AX7 driver. 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 - Check all connections.  
2 - Change all valves for new ones: no difference here.  
3 - Check the voltages in all the pins of the valves: they are very simmilar in both channels.  
4 - Adjust the bias according to the manufacturer specifications.  
5 - 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?  
żDo you know where can I find the schematic?  
Thank you very much.
 
6/26/2005 7:35 AM
Scotty
You are using it with a preamp, right ?  
Marshall have the manual at their website but no schematic. Maybe you can ask them for a copy.  
Do Voltages at plates look ok ? I guess around 250 volts ?  
How old is it ? Old filter caps ?  
Scotty  
 
http://www.geocities.com/hillbillyswing
 
6/28/2005 2:45 PM
Bob Clear
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Marshall will email you a scan of the schem. It's hard to read, but it'll do the job.  
 
These KILL the output tubes - they run them at over 400VDC, fixed bias, and the factory bias setting (as noted on the internal service sticker) is about 110% dissipation.
 
6/28/2005 2:53 PM
Vince
> These KILL the output tubes - they  
> run them at over 400VDC, fixed bias,  
> and the factory bias setting (as noted  
> on the internal service sticker)  
> is about 110% dissipation.  
 
No problem if you use russian tetrode  
6Pi14Pi EB - close enough and much better  
EL84 substitution, about 14W anode  
dissipation.  
 
Vince
 
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