Tube Bias Adjustment

[Bill Duncan]

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.

[Bill A. Moore]

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]

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.

Not sure about that. Setting bias is not difficult, getting to the area and making connections is the difficulty.

[Ken Metcalf]

This is what I use.

[Tommy Boswell]

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]

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.

[Bill A. Moore]

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]

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, good looking amp. I am considering building a tube amp from scratch. Just my opinion, but none of the modeling amps I have tried sound as good as a real tube amp.

[Bill A. Moore]

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]

Bill, it looks great. I like the name plate.

[Bill A. Moore]

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

[Steve Rosko]

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?

[Bill A. Moore]

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"?

[Patrick Huey]

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.

Bill,
If you mean a Peavey Valve King there is no bias adjustment on those. Fixed cathode bias I believe it’s called…power tube bias is set via resistors so you just replace them with a good, tightly matched set or quad

[Patrick Huey]

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.

I have a ValveKing 120. There is , however, a “bias mod” is common mod a lot of VK owners do that adds an adjustable bias trimpot to the amp and makes it adjustable.
I have a Marshall JCM2000 that has the bias test points and adjustment on back like you spoke of. it can be a good thing to have on an amplifier, but it all depends upon How well it is designed….some amps that have that the reading can be off quite a bit

[Bill A. Moore]

I set this into favorites years ago when I traded for a ValveKing 212. (It's still at my old friends house, a couple of weeks ago he played a new guitar through it, still sounds fine)!
Looks like I'll never do any mods on it!
https://web.archive.org/web/20151113040 ... lectronics

[Patrick Huey]

I set this into favorites years ago when I traded for a ValveKing 212. (It's still at my old friends house, a couple of weeks ago he played a new guitar through it, still sounds fine)!
Looks like I'll never do any mods on it!
https://web.archive.org/web/20151113040 ... lectronics

Bill, I have not modded mine either. Does fine with an eq pedal in the loop and when it comes time for new power tubes just pop in a matched quad and go. Thanks for sharing the link I will check that out tomorrow

[Larry Hamilton]

Phil Bradbury, builder of the Little Walter Tube Amp, designed his amp to be bias friendly. If you have a bias meter it is a snap. The bias adjuster knob is at the back, pull no. 1 tube, insert adapter put tube in it, make sure there’s is a speaker hooked up, load, and away ya go If I can do it’s anyone can do it Super great amps and super great guy

[Larry Hamilton]

Phil Bradbury, builder of the Little Walter Tube Amp, designed his amp to be bias friendly. If you have a bias meter it is a snap. The bias adjuster knob is at the back, pull no. 1 tube, insert adapter put tube in it, make sure there’s is a speaker hooked up, load, and away ya go If I can do it’s anyone can do it Super great amps and super great guy