Changing Tubes on Vacuum Tube Amplifiers - What is biasing in tube amps?
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Hello. This is Jeff Naylor from Mtroniks Engineering in Mesa, Arizona for Expert Village. Now we're
going to show you how to bias your tubes in this amplifier. But first let me tell you
what bias is. Bias is a DC voltage applied to the same point of the circuit as your AC
audio signal. The two factors we are concerned with while biasing the amplifier are crossover
distortion and standby current in your power tubes. Both these factors work in opposition
of each other. As we increase the bias, our crossover distortion goes away, but the standby
current in our tubes increases which decreases tube life over time. Our goal when setting
bias is to find a happy medium between the two. We must first remove the back plastic
plate to locate the three pinned male connector. All Marshall JCM 2000 amps have this three
pin connector for setting your bias. However, it is not always in the same location. On
a lot of amps it is exposed simply by removing your back cover of the chassis and the connector
will be in this general location. In our next segment, we'll learn how to hook up the volt
meter and see where our bias is currently set. See you in the next section.
