A15MK2 Difference in bias voltage

  • Hi,

    I have just finished my first amp build. Really educational but nevrotic experience. Have been doublechecking everything about 10 times, so the build it self has taken just about forever, but better safe than sorry when you don't know what you are doing ;)

    All voltages are on the low side, but all checks out well within the 10% margin, but not the bias voltage at R51 and R52. Here I have a +-30% difference. When I have -32mv at one side I will have -22,8mv at the other. If I swap the 6V6's, the difference swaps as well, hence it seems like it is tube related. I have 265v on the socket side of both R55 and R56. I have no previous experience here, but this difference seems very substansial to me. Is this normal, or do I have a faulty tube?



  • Hi and welcome to the forum,
    if the power tubes are not matched, there might be quite a difference between quiescent current of both power tubes. Can you enlarge the bias voltage (less negative) to get about 36mV for one tube (instead of 32mV)? Because of your lower plate voltage (265V) you should enhance the plate current a bit. What voltage do you then measure for the other tube? BTW, "bias voltage" is not what you measure at the 1 Ohm resistors, this voltage represents the cathode current (e.g. 35mV/1R = 35mA), the bias voltage is the negative voltage at the grid (pin5)...

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