Juz Posted February 4, 2009 Posted February 4, 2009 Hi guys. I was wondering whether anyone here has an Epiphone SoCal 50 amp head and could answer some questions regarding the re-biasing of the amplifier. I wasn't too pleased with the sound from the stock Chinese Shuguang EL34's or 12ax7's, so I have swapped all of the preamp valves for Electro Harmonix 12ax7EH's and the EL34's with some used EL-34M's with Marshall logos on them. The new EL34's seem to be running a little hot and I would like to have a go at re-biasing it myself. I own a fairly decent Fluke 111 multimeter and have experience in electronics to a certain extent. What I need to know is what bias setting should be used. I would have guessed around the 34ma mark, but want to be 100% sure. Is it an anode or cathode biased amp? Also, whereabouts on the board will I find the adjustment pots to change the bias voltage? This'll be my first attempt at biasing an amp, so as much info would be extremely welcomed. Once i've finished with the Epi, I then have another project amp - an old 60's ELK Viking-Bass which requires an OT replacement and then a full overhaul. Thanks guys. Juz.
m-theory Posted February 4, 2009 Posted February 4, 2009 What makes you assume 34ma? How can you assign an arbitrary number like this, without knowing what the plate voltage is? No way of knowing whether the amp is fixed or cathode biased, without either tearing it open or finding a schematic. If it's fixed, there will be an adjustable pot somewhere.
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