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Help decyphering Marshall 9005 circuit


 
3/2/2004 8:39 PM
Bob S II
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Help decyphering Marshall 9005 circuit
Hey there,  
 
I just scored a late-80s marshall 9005 power amp (stero 50 watts/side, 12ax7s/EL34s), and am in the process of a full retube and general clean-up.  
 
Opening it up, I was expecting a pair of typical Marshall 50-watt output sections, but what I found was a bit different. Looking at the schematic, there's an additional tube stage between each side of the PI and output tubes, the purpose of which I haven't been able to figure out just yet.  
 
http://marshallschematics.com/gifs/9005.gif  
 
Any ideas?  
 
Thanks,  
-Bob
 
3/2/2004 8:49 PM
anonymous
Long Tail Pair Phase Splitter.
 
3/2/2004 9:13 PM
Bob S II
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quote:
"Long Tail Pair Phase Splitter"
 
 
V1 is the LTP - V2 is the one that I'm trying to figure out.  
 
Thanks,  
-B
 
3/2/2004 9:36 PM
Andy
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it's a pair of gain stages. marshall just decided to put them after the phase inverter instead of before. no big deal.
 
3/2/2004 11:02 PM
Ray Ivers
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Bob,  
 
It looks like they've added another gain stage between each half of the the LTP phase inverter and the output tubes, probably to get more loop gain for NFB purposes. Each extra stage also uses local NFB for AC balancing.  
 
Ray
 
3/3/2004 12:39 AM
Enzo
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V2 is a classic driver stage.
 
3/3/2004 1:57 AM
jeremy
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i had one of those. even after redtubing it wasnt very loud. goodluck with it. i suspect the bias circuit will need some modding, too.  
 
jeremy
 
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