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Biasing a Blues Junior?

seward

Silver Supporting Member
Messages
1,287
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.
Louis Camblor
USN EE 1977
Working with pro-audio for forty year.
The quoted comment resurrects a 12-year-old thread, so I feel minimally felonious for doing so 3 years later...and the point remains...

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.
 

jlagrassa

Silver Supporting Member
Messages
6,394
The quoted comment resurrects a 12-year-old thread, so I feel minimally felonious for doing so 3 years later...and the point remains...

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.
The simplest thing you can do is get your power tubes from a seller that can select tubes that will run close to spec in a Blues Junior but the tubes still run on the hotter side from my experience, Eurotubes is one that I know will do that i'm sure there are many others as well. The best thing you can do is add a bias pot, I have one I installed in my Blues Junior IV so now I can dial it in to where I like it.
 






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