Reading references to bias settings on a variety of posts, I've seen references to mV and mA? Which is it? Is bias a voltage or a current? Please excuse my total ignorance about this.
Guido F.
You have asked a very wise question. The confusion has come from a lack of information as usual. I've written on this before, here is the short answer.
In most amplifiers there is a one ohm resistor between the cathode of the output tube and ground. We (manufacturers) measure the voltage drop across this resistor. Since it is one ohm there is a one to one relationship between mV and mA. However it is very important that you set your meter to mV or volts to measure this drop. If you set your meter to mA or amps you will be putting its resistance in parallel with our resistance and the bias will be set high.
In some amps the resistor is not 1 ohm. In our RM-10 it is 10 ohms so the reading is 300 mV which corresponds to 30 mA in the tubes. In all the Dynaco amps Hafler chose a resistor that gave the desired bias at 1.56 volts because that allows you to compare the reading to a fresh battery. This was clever because in the days before digital meters one could not be sure of the accuracy of one's meter. It turns out that a fresh flashlight battery is very accurate at 1.56 volts. He set his Stereo 70 amplifier for 100 MA of idle current (rather high) so the resistor was 15.6 ohms.
To further confuse the matter the term "bias" is currently miss-used in it's traditional meaning. If you told an old timer you set the
bias for some mV he would say you are crazy and will burn up the tubes. "Bias" is actually the much larger
negative voltage we put on the tube's grid and is typically 20-60
volts (not millivolts which are 0.001 volts) depending on the tube and the desired idle current. What we are measuring is actually the
idle current of the tube as that is what is important. We really don't care what negative voltage it requires as long as the bias pot can get us there (which it cant in many amplifiers, but not mine as I have a wide range bias pot).
BTW in older amps there was often a 1/4 inch headphone jack that you connected to a real milliamp meter. However the jack had to break the cathode circuit to insert itself. The main problem was that high bias could damage your meter but more important, the switching on the jack when the plug was out could cause distortion as it oxidized over time. The last company to do this was Quicksilver, we never did.
