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Re: hot tubes, I mean really hot

8/11/2005 5:24 PM
Mark Black
Re: hot tubes, I mean really hot
If what we are talking about is a JCM800 clone the bias filter caps should both be 10uF electrolytic, probably around 100V rated, and installed with the positive end to ground.  
 
The bias (if true to the original) looks like it is derived from one half of the 300-0-300, referenced to the center tap, and knocked down to a reasonable voltage by a 220K resistor before being rectified to negative DC by a single diode, followed by a filter cap and more reduction via series and shunt resistors (one of which is the bias pot), with a final filter cap after all that.  
 
I don't have the TUT book to see his version of a schematic but I wonder if there may be errors. Check out the originals at http://www.drtube.com/marshall.htm#JCM800 to see how they compare.  
 
Good luck - Mark

 
Replies:
Pete Mark! Thanks for your help! You a... -- 8/11/2005 10:16 PM