JCM800 2204 bias problem


Posted on Feb 5, 2014

Alright, my 2204 won`t bias properly. I can`t get the tubes to idle at more than about 14 W (435 V * 33 mA cathode current). I popped a set of old Telefunkens in, but the wouldn`t go much over 10 W (440 V * 23 mA). They bias just fine in the middle of pot range in the Dynacord. I suppose I will have to lessen the negative bias voltage, but what`sthe best way to do it


JCM800 2204 bias problem
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here Reduce R28 to 47k or even 33k Rebuild like the schematics Anyone who knows the reason for this change of circuitry What tube are you using 5881, EL34 Some of these 80`s Marshalls are colder biased because of the tube selection back then being sub par. Companies were going out of Bussiness and EL34`s were almost non existant so they went to 5881`s so your amp might be different because of that. Secondly sometimes you have to swap to a different resistor to get the range you need. I swapped one out in a Bogner Extasy that was burning through tubes because it was so hot, I could not bias low enough. Amps need to be slightly modded from time to time. A professional job might not be noticable to the common eye. So look closely to see if the resistor may have been swapped. Do the math on what you will get by swapping before swaping resistors. Wasting one only to find its not enough or too much might only be a few pennies, trial and error sometimes




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