Silver Supporting Member
The quoted comment resurrects a 12-year-old thread, so I feel minimally felonious for doing so 3 years later...and the point remains...I like to use gear page and other forums for research, and ran across this post while looking for information I needed. I found the misinformation disturbing and want to clear up a couple of incorrect things that were said here.
A “Fixed Bias” circuit is one in which we adjust the bias when we change out power tubes via a trimmer pot, that then remains “fixed,” until we change tubes the next time and readjust it.
A “Cathode Bias” circuit is one that is dynamic and does not need to be adjusted when we change power tubes.
USN EE 1977
Working with pro-audio for forty year.
If a fixed-bias tube amplifier, like a Blues Jr., doesn't have a trimmer pot, how do we adjust bias when changing tubes? Do we use a bias probe and swap replacement tubes in and out until we get the right specs? Taking the amp to a tech whenever one wishes to swap tubes seems...expensive.