Tube Bias Adjustment

Bill Duncan · From: Lenoir, North Carolina, USA

I just finished a 50 watt Valveking project that turned the very compact amp into a pleasure to play and transport. The work caused me to wonder why almost all the tube amp manufacturers make it difficult to access and set tube bias. Bias adjustment can take the amp the extra mile to make the sound much more pleasing. It would be no problem for the builders to bring Bias adjustment access points to the rear chasis.
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You can observe a lot just by looking

Bill A. Moore · From: Silver City, New Mexico, USA

For the same reason they put "no user serviceable parts inside"!
To properly set bias, one needs to know the voltage and current at the tubes, someone could get shocked if there were test points accessible.

Bill Duncan · From: Lenoir, North Carolina, USA

Ken Metcalf · From: San Antonio Texas USA

This is what I use.

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MSA 12 String E9th/B6th Universal.
Little Walter PF-89.
Bunch of stomp boxes

Tommy Boswell · From: Virginia, USA

I'm with you Bill. I have two amps that have meter probe inserts on the back panel and accessible adjustment screws. Even then, the owner needs to own a meter and know how to use it and know what meter readings to target. And I have another amp that has the adjustment screw but no meter inserts, the owner's manual says "adjust for minimum hum level".

Bill Duncan · From: Lenoir, North Carolina, USA

I understand about the shock hazards, amperages and voltages and the settings, However, with tube amps, just changing output tubes can change the sound and require a bias adjustment. Couple that with an almost impossible task of finding a tube amp technician, you had better count on doing it yourself. It is a required task that would be much easier and safer with test and setpoint jacks and pots made available in the rear chassis.
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You can observe a lot just by looking

Bill A. Moore · From: Silver City, New Mexico, USA

I have built several amps, and the second built I decided to add current test points. I still need to get voltage from my bias tool, haven't added any since.

Bill Duncan · From: Lenoir, North Carolina, USA

Bill A. Moore · From: Silver City, New Mexico, USA

Thanks, Bill, I didn't build that cab, a friend bought a Weber DR kit, got cold feet, and sold it to me. I offered to build it for him, but he just wanted to sell it. What I did have to do was modify the cab, the chassis holes were drilled too far forward, and there was a space between the chassis and grill I had to deal with, (the stripes cover up the extra holes!)

Bill Duncan · From: Lenoir, North Carolina, USA

Bill, it looks great. I like the name plate.
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You can observe a lot just by looking

Bill A. Moore · From: Silver City, New Mexico, USA

I bought some from a guy on the bay, can't find another to duplicate them!

Steve Rosko · From: Georgetown, Texas

My Little Walter has a bias knob. Phil made a pretty big deal about biasing the amp periodically. In my case it does require a tool, but I can do it from the back of the amp. He tells a story about a customer who came in with an amp that “didn’t sound right”. It hadn’t been biased in years. In a few minutes it was as good as new. So, makes you wonder; if a tube amp hasn’t been biased in years how much better could it be?
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84 Emmons D10 PP, MSA D10, Hilton, Little Walter PF89, PF-350 cabs, blueSky, BJS

Bill A. Moore · From: Silver City, New Mexico, USA

It's important to know the values when setting bias, but the final adjustment is always determined by your ears. Maybe the adjustment allowed is small enough in your amp, that you can't get it too "hot"?

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