What does adjusting tube amp bias mean?

gmorris

Quick question. What is meant by "adjusting the bias of a tube amp"?

gidrah

Unfortunately, all I can say is that it's an adjustment that can be performed on some tube amps that kinda puts the recommended voltage back into spec with the tubes as they wear, are swappped, etc. I also know that running tubes "hot" (ie. higher bias) will shorten their life, but some people prefer it.

I hope some of the more knowledgable toobies will help us both out.:)

madmax

A tube can only have current flow in one direction. An audio signal is 1/2 positive and 1/2 negative. So how does that work? What you do is set the zero of the audio signal midway of the tube. This is called bias. Look at it this way, the audio signal might go from -5 to +5. The tube can only handle -10 to 0. You bias the signal so that -5 is really -10 and +5 is really 0. That way you have the same range but it all stays within the limits of the tube.
madmax

Brianomatic

gmorris

Vaccum-tube amplifiers are classified according to the way in which they are biased and the amount of signal drive that is applied. The basic classes are Class A, Class B and Class C. Also you can have combination of Class A and Class B thus Class AB1 and Class AB2. The major difference of the classes of amplifiers is the selection of the grid-bias operating point around which the input grid signal will vary. For example, if the input grid signal varies around a bias operating point at or near the center of the characteristics curve, the amplifier is biased as Class A.

Each tube is designed by the manufacturer with a certain characteristic curve plotting of the plate current (vertical plane) versus the plate voltage (horizonal plane) with a series of grid voltages curves overlayed onto the plot. By either increasing the plate current and decreasing the plate voltage (or vice versus), you will have a linear operating line sliding across the overlayed grid voltage curves. Now, by selecting a Class A operation, a center point on this operating line is selected that crosses the closest grid voltage curve. As you can see, if any of the tube voltages drift, the operating point will change taking the amplifier away from the desired operating Class, in the ST-70 case, Class A.

Every Class has its advantages, Class A produces low distortion. Hence, with a drifting or off-biased amp, the sound will change.

This site has some decent info:

http://geofex.com/tubeampfaq/taffram.htm

Brian

organ

What kind of multimeter do you have? They're not 100% accurate. Best thing to do is to grab two AA batteries, put them in series, put your meter into DC voltage and measure. You should get 3V DC because the two 1.5V batteries are in series. If your meter is slightly off, calculate the difference in percentage and make sure you make up for the difference when biasing the amp. I have a cheap multimeter. My amp needs to be biased at 1.56V but because my meter is not accurate enough, I have to get a 1.66 reading on the meter which is actually 1.56.

Maurice

bikezappa

Brainomatic

Finally some one who understands vacuum tube technology and takes the time to explain it a bit. Bias is very difficult to explain unless you have an old RCA tube manual with the curves. Remember those.

The other explainations are very little help.

E=IR

W=IE

Amen

Brianomatic

Organ

Good point about voltmeters. Not everyone has calibrated calibration equipment sitting on their workbench. The battery calibration is a good way of going.

bikezappa

You're not kidding. Trying to explain biasing isn't easy. The basic thing that I didn't mention for the non-technical crew out there is that the ST-70 is already designed for Class A amplifier biasing with tubes selected for a certain operating range. By biasing, we are not redesigning the amp but only making sure that the tube is biased (voltage applied to plate, cathrode, and grid) within dynaco's original design parameters. It's like taking your car in for a tune up.

Brian