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Post by Albert on Feb 17, 2013 19:05:03 GMT -7
Being the kinda guy who like taking things apart I decided to take some measurements for future reference on my M12 so I thought I would share ... Enjoy
1: As it's Cathode Biased, measure the Cathode Resistor and write it down as Rk. 180Ohm 2: Locate screen resistor, pin 9 in an EL84. Measure it and write it down as Rs. (In most cases this is 100 Ohm resistor.) 500 Ohm/2 or 250 Ohm
3:Turn amp on and let it warm up. When hot measure the voltage across Rk and write it down as Vk. 11.66vdc
4:Measure voltage across Rs and write it down as Vs. 4vdc
5:Measure voltage at pin 7 AND BE CAREFUL AS IT'S GONNA BE 300V OR ABOVE AND IT COULD BE LETHAL! Write it down as Vp. 300vdc
6:Turn amp off and get out your calculator, pen and paper.
7:Figure Vk / Rx = Ik and write it down. (This is cathode current...). 11.66/180=.065..65ma
8:Figure Vs / Rs = Is and write it down. (This is screen current...) 4/250=.016...16ma
9:Figure Ik - Is = Ip and write it down. (This is plate current...) 65-16=49...49ma
10:Figure Vp X Ip = IPD Idle Plate Dissipation and write it down.
300*49=14.7...14.7 Watts Total both tubes ..
11: IPD for a cathode biased EL84 should not be above Max Plate dissipation which is 12W. If it is you need to change the cathode resistor to a higher value.
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