What is the Bias Point?

(by Eric Barbour, Vacuum Tube Valley senior editor)

You don't need any knowledge of electronics to understand how a vacuum tube works. It's actually easier to understand than a transistor or a chip, so we'll try to explain it in a fairly simple form.

The easiest way to see electrons is as a bunch of energetic little guys, who run around when you put pressure on them. The pressure gives them energy, so they want to move. The pressure is called VOLTAGE. And the number of them moving thru a wire each second is called CURRENT.

And in a vacuum tube, electrons are emitted from a hot thing called a CATHODE. They pop off the cathode and sort of float around in the empty space around the cathode. They have to do this because the cathode is made of a material that emits electrons when it is heated red-hot.

Before we go further, there's one thing about electricity that is basic. And it's different from the way people behave, so you have to keep it in mind. If you have a similar personality to someone, you would be attracted to that other person, right?

Well, electricity is the OPPOSITE way. Electrons have an electric charge. And objects can have an electric charge put on them, by applying VOLTAGE to them. But electrons behave the OPPOSITE way from people: 1) like charge REPELS like, and 2) different charge ATTRACTS.

So, since electrons all have a NEGATIVE charge (this is a standard that was established hundreds of years ago), electrons tend to REPEL each other. So a cloud of them in a tube tends to want to disperse OUTWARD. Okay, now we have the cathode with a cloud of electrons all jostling to get away from each other. But there isn't anyplace for them to go.

So there's another metal thing in the tube. It's called either a PLATE or an ANODE. Now, we put a POSITIVE charge on the plate.

Guess what? The electrons, which are negative, are ATTRACTED to it, because it's OPPOSITE CHARGE.
So now, electrons are popping out of the cathode, and zipping straight to the plate. They hit the plate, penetrate, and become moving electric current in a wire. This current can do useful things.

The tube we have now is a DIODE. It can be used to RECTIFY changing voltages, that is, it can turn AC voltage (which changes back-and-forth in polarity constantly) into DC voltage (which is steady).

What if we want to turn small AC voltages into LARGE AC voltages instead? We add another gadget to the inside of the tube. This is called a GRID, because it IS a little grid--of wires. The grid is wrapped around the cathode, but doesn't touch it.

The grid acts like a sieve. It controls the number of electrons passing through it to the plate.

How? Remember I said that in electricity, like REPELS like? By putting a changing voltage on the grid (changing as in, an audio signal), the electric charge on the grid changes.

As it becomes more and less negative, it changes the current into the plate. This causes a changing voltage to appear at the output. The output changing voltage is usually a lot bigger than the input changing voltage, so the tube causes GAIN. This is how an amplifier works.

OK, but are there a LOT of electrons moving through the tube? Yes, there are lots of them. So many that if we left this tube operating, it would destroy itself! The electrons hitting the plate cause it to heat up, and if there's enough electrons heating it up, it will turn red-hot, melt the tube's glass envelope, and the tube will stop working.

So, we apply a BIAS voltage to the grid. This is a steady NEGATIVE voltage, more negative than the cathode. Since like REPELS like, the negative voltage will REDUCE the number of electrons passing through the grid.

We set this voltage so the current to the plate is a tolerable IDLE current. The current is determined by the tube's design and ability to handle it, plus by the circuit outside the tube. Each kind of tube has its own optimal operating point.

By making this bias voltage adjustable, we can set it as desired. However, only certain amplifiers have an adjustment for bias. And those that have bias adjustments often don't make them accessible to the user.

You have to understand, tubes VARY from sample to sample. Just applying a given bias voltage to a tube is OK, if it's adjusted the tube will idle properly (and work properly). But if you CHANGE your tube or tubes, how do you know they'll bias properly without adjusting the bias? The new tube might be different in bias requirement.

It would be GREAT if tubes were rated by their individual bias voltage. Actually most companies that sell "matched" tubes usually print the bias current the tube operates at, right on the box. Unfortunately, each company has their OWN STANDARDS for measuring bias. Plate voltages, plate currents, and other parameters are DIFFERENT from one dealer to another. So they may or MAY NOT be interchangeable.
Tubes vary according to statistical parameters, just like other devices that are mass-produced. If you plot bias versus the number of tubes having that bias, you'll see the classic Gaussian distribution, or "BELL CURVE". Most of the tubes will be in the center of the distribution, close to the optimal operating point of that tube type.

That's where BIAS POINT comes in. By purchasing a tube from TubeDepot.com with a given BIAS POINT rating, you can be sure that a future tube that you purchase from TubeDepot.com will bias the same way, and have the same operating point. So it will SOUND THE SAME. WithOUT having to adjust the bias of your amplifier (which is often difficult or impossible for the user to access anyway!)

BIAS POINT ratings split up the Gaussian distribution into a series of ZONES. This scheme is very similar to the 1-10 rating scheme used by one of the major sellers of pre-tested tubes. (We won't mention their name, but you have probably seen their tubes for sale in music stores.) Basically, that company says that tubes with a lower number on that 1-10 scale "BREAK UP SOONER" than usual. And tubes on the high end of the scale "BREAK UP LATER".

Even though their rating system is based on the individual bias variances of the tubes, the other company likes to claim their system is "secret" and "proprietary". It really isn't. Their "5" rated tubes are right in the middle of the Gaussian bell curve. That's all it means!

The tubes that are on the LOW side of the bell curve are low BIAS POINT tubes. They are usually prone to "breaking up" or going into distortion at lower than the proper volume (for that kind of tube). Because they idle at LOW current compared to the average tube.

So they have LOWER HEADROOM. For some musical-instrument applications, the user may want this effect. It is usually seen as undesirable for high-end audio, because lower idle current means higher distortion, which most people dislike hearing in their music playback.

Tubes in the MIDDLE zones fit the average profile for that tube type. They are most commonly recommended for the average amplifier user, especially for high-end audio amplifiers. These tubes will give the PROPER amount of headroom for their operating point. So the amplifier will perform as the manufacturer intended.

Tubes with a high BIAS POINT will idle at a higher current than specified, and give more headroom than the amplifier designer intended. With high BIAS POINT tubes, your amplifier volume / gain will need to be turned up much higher to achieve "break up". Some instrument amplifier users might wish for this sound effect.

Hopefully, this will help you to understand how BIAS POINT works, and how it can save you some of the troubles usually associated with tube amplifiers, such as the sound varying with new tubes, or the need to get an amp with new tubes rebiased (usually this has to be done by a service technician, if there even is an adjustment for it). Just stick to a BIAS POINT similar to that of your previous tubes, and you can be certain your amplifier is operating and SOUNDING its best.