Tubes - The Mystery Unraveled (Part Three)

This month we continue with the third part in a series about tubes.Last month we covered amplifier classes and biasing. If you haven't read Tubes parts one and two it would be a good idea to check them out in the Metro Music Mayhem archives.

Let's pick up where we left off discussing biasing and analyze several of its "flavors". Remember, tube bias in general, regardless of flavor, is achieved by controlling how much power a tube dissipates when there is no signal going into it (called idling, just like the idle RPM of your car's engine when you are not stepping on the gas pedal). To do this it is necessary for a tube's control grid voltage to be more negative than its cathode voltage. There are various ways to achieve this but the end result, control of electron flow from cathode to plate, is the same.

The "Flavors " we will discuss are terms you hear tossed around quite a bit. These are cathode bias and fixed bias(negative biasing). There is much confusion over what their differences are.

CATHODE BIAS

In the earliest amps a resistor was placed between cathode and ground on the output tubes. When a positive voltage is applied to the tube's plate it pulls negatively charged electrons surrounding the cathode towards it. Since these electrons originate from the ground of the circuit, they must travel through the cathode resistor first in order to get to the cathode on their journey to the plate. Due to a basic electronic principle known as Ohm's Law, if electrons are flowing through a resistor, the voltage cannot be the same on both sides of the resistor. Therefore if one side of the resistor is connected to ground (0 volts) the other side of this resistor connected to the tube's cathode must be at a higher more positive voltage. Since the tube's control grid is at ground potential, we have created a situation where the grid is more negative than the cathode fitting our definition of biasing above. For those of you who are not schooled in electronics theory but want to understand a little deeper, let's use a water analogy. Think of the water coming into your house as electrons and the spigot on your sink as a resistor. There is a certain amount of pressure put on the water which causes it to flow. This pressure is the same function as voltage in an electrical circuit.

The more voltage or water pressure the more current or water will flow. The valve on your sink is analogous to a resistor. If it is turned off no water will flow, just like a high value resistor restricts the flow of electrons. The pressure(voltage) on one side of the valve(resistor) is high and on the other side low. As the valve is opened, as in lowering the resistance value, more water or current can flow. As you approach the valve fully open or zero resistance, all the water or current can flow out of the valve or resistor at the same pressure or voltage that goes into it. Therefore if a resistance is present in a circuit there cannot be the same voltage on both sides of it if there is current flowing through it.

Now that we have explained how cathode biasing works let's see what its results are.

This method has several advantages. A tube will seek its own bias point in respect to the resistance value of its cathode resistor.This means you can use different types of output tubes in the amp as long as the tube's pin outs match the tube socket wiring and the tubes can handle the applied voltages.

Cathode bias varies as the tube is played louder (draws more current) causing a compression effect. As we said before with our water analogy, when more current flows through a resistor it lowers the difference in voltage between the two ends of the cathode resistor. The voltage is now less of a positive voltage and closer to the zero volt potential of the control grid, causing less current to flow and lowering the output of the tube.This is what compresses the signal.

On the other hand if you are not looking at the compression effect as a plus then it is evident that the tube is not able to put out it's maximum power. The partial solution to this was to add a capacitor in parallel to the cathode resistor. The effect of this is to reduce the compression effect and also according to the capacitor value selected allows control over the lowest frequencies the amp will handle helping eliminate flub in the sound.

FIXED (NEGATIVE) BIASING

In a modern tube output stage the way the tube's control grid is made more negative than its cathode is to put a fixed negative voltage on it and also to connect the tube's cathode directly to ground instead of through a resistor .This makes the tubes potential power dissipation much more efficient than with cathode biasing since the cathode resistor is now gone removing the compression effect it causes.

In the early amps before the development of diodes and transistors it would have been an expensive proposition to create a negative bias supply in comparison to using cathode biasing, which adds only an inexpensive resistor and maybe a cap to the circuit. Plus amps were considered plenty loud at the time so designers weren't concerned about raising the efficiency of the output stage with respect to power.

But as time went on, guitar amps were called on to produce more power. At about this time a diode that cost only pennies became available to create a negative voltage power supply and fixed bias became the way amps were designed the majority of the time So now we have a different set of pros and cons than with cathode biasing. Negative biasing eliminates the compression effect caused by the cathode resistor unless the amp is really pushed past it's power supply limits. You also cannot substitute different types of tubes into the circuit. With this style of biasing the tubes can't seek their own bias point so it is necessary to use the tubes that the amp is designed for. Putting a set of 6l6 tubes when the amp is designed for EL34 equals meltdown. But we do have amp's that play louder and when a bias trim control is included you can dial in the bias point of the tubes for maximum tonality and efficiency.We covered this last month in our discussion of class AB amps. Some amps also include a bias balance circuit allowing the tubes to be matched more closely and I assure you that more difference in tone will be heard from the balance control than the raw bias control in most situations due to having control over the balance of the symmetry of the two halves of the push-pull waveform.

Another point of confusion with bias terminology is fixed vs. adjustable biasing. The difference here is that some amps are biased at a specific negative voltage while some amps include a trimmer to allow adjustment of the bias voltage over a certain range, usually not enough to let you use a different type of tube (EL34 instead of 6L6 ETC), but enough range to accommodate differences between tubes of the same model. When the bias is not adjustable you are subject to use replacement tubes that operate in the range that the amp's bias is fixed at. This means that you must probably buy tubes from the manufacturer of the amp or you have no way of knowing if the new tubes are compatible. Very suspicious to me. Mesa Boogie is the biggest proponent of this type of biasing.

It is possible to change the value of a few resistors in a fixed bias circuit to optimize an amp for a specific set of tubes or even to change the bias range enough to work with a completely different model tube. A trim pot can also be added to a fixed bias circuit so it can be adjustable. I do this for people all the time.

Remember there are only about five or six manufacturers of tubes in the world. And by the time the tubes get to you they have probably been re branded by different vendors. For example, Mesa Boogie does not make tubes. Neither does Groove Tubes. They are buying their tubes from the same factories as all the other companies are. Tube quality nowadays is certainly not as it once was and in addition to this think about the tube that travels from Russia to California and then to you. How much vibration and shock has it received causing its parameters to vary from what it is rated at. It's not a bad idea to have the bias checked anytime you install new tubes. Additionally, performing a periodic re balancing of the output stage, if that control is included, will preserve the sound the tubes had when new for a longer period of time.

The last thing I want to mention this month is to dispel another myth. You often hear people refer to biasing styles as being tied to specific power stage classes as in cathode bias must be class A. This is completely untrue. You can have cathode or fixed bias in either class A or class AB.

Next month I will discuss the amplifier power supply including a discussion on tube rectifiers vs. solid state rectifiers. Dual Rectifier amps- be warned- I am going to expose you!