A concise guide to Valve Power Amplifiers. Part 2.
Geoff Kremer outlines the significance of audio amplifier class categories and correct bias.
Summary of Part One.
- All valves operate in exactly the same fashion, but there are variants that have different characteristics.
- The Triode is the simplest amplifying valve, consisting of a source of electrons called the Cathode, a controlling device called the Grid and finally the Anode, (also called a Plate) which collects the modified stream of electrons.
- The stream of electrons is provided by a specially coated material that emits a cloud of electrons when heated by a filament, much like an incandescent light bulb. If a valve does not light, up it will not work. This may be due to a fault within the valve itself which is a very rare occurrence, a dirty connection within the valve socket or a failed power supply delivering current to the heater.
- The Triode is the purest valve, and in my opinion has the best sonic performance of the group.
- In order to enable the Triode to develop more power, a second “Screen Grid” can be added, and the valve is now called a Tetrode. This second grid does however introduce an unwanted kink in the operating curve.
- To overcome the irregularity in the operating curve, the British GEC company originated the KT series of valves, KT66 and KT88. Called the Kinkless Tetrode. The KT88 in particular is widely used today.
- The third and most commonly used power valve is the Pentode. Now with three grids, this valve was designed to have a higher gain or amplification factor and better linearity than the Tetrode. The better linearity and higher gain is due to the inclusion of this third grid called the suppressor grid.
- The Pentodes which are most commonly used today are the EL34 and EL84.
Part two. A Class Act.
All valves need to operate within the confines of a set of parameters laid down by the valve designer. A valve has a characteristic curve on which is plotted the conditions for optimum performance by using the most linear portion of this curve. This characteristic curve is the relationship between the current flowing in the valve, and the voltage applied to the grid to control it. A graph is drawn showing this relationship, and because nothing is perfect, the resulting graph will have a straight or linear part. At either end, where the valve is operating outside its limits, the graph will bend. This is called the operating curve. In order for a valve to work at its most efficient, the operating conditions are set so the valve operates in the flat or linear part of the curve. To achieve this, a fixed, steady, bias voltage is applied to the first Grid and this voltage is determined with relation to to the valve's Cathode.
Class A, Class B and Class AB.
For audio use there are three classes of operating parameters:
Class A is predominately employed for Triode valves and allows the them to operate in the middle of their operating curve. Class A produces the lowest distortion and the sweetest sound, but comes at the cost of making the valve work flat out all the time, and consuming the most power for a given output.
Class B is used for Tetrodes and Pentodes. In this class the valve is made to operate toward the lower end of the curve. It produces the most power, uses the least energy, and is much easier on the valve, but comes at the price of higher distortion. With very careful design of the output transformer and circuitry, Class B can approach the quality of a Class A amplifier, but lacks its delicacy and finesse.
Class AB is as you would expect, a mixture of A and B. It's intended primarily for Pentode valves operating between the Class A and Class B portions of the curve. A well designed Class AB amplifier is perhaps the best compromise, very nearly equal to a good pure Class A amp in finesse and detail, but may lack low end speed and dynamics.
To complicate matters, there are two ways a Tetrode and Pentode can be configured to function. A Triode can only ever be a Triode but both the Tetrode and the Pentode can be arranged to operate like a Triode by connecting their second grids to the anode. Some amplifiers have a switch to enable it to operate in either Triode or Tetrode/Pentode mode. In Triode mode, the amplifier will deliver a sweeter more refined sound, but at the cost of losing about 20% of the power available as compared to working in the Tetrode/Pentode mode. This more powerful method of connection works by connecting the second grids to special tappings on the output transformer and is called Ultra Linear operation.
Bias. - How it's easy to get it wrong, and why it should be right.
There are two methods of applying bias to a valve.
Grid Bias, sometimes called Fixed Bias and which must often be set-up manually.
Cathode Bias, sometimes called Automatic Bias, which is decided upon during the amplifier's design .
I'm going to concentrate on Grid or Fixed bias first of all. Now, this part is very important…
If a grid biased amplifier is incorrectly set-up, the best that can happen is that there will be distortion ranging from mild to severe. The worst is that the output valves will burn out and damage may occur elsewhere. I've seen valves quite literally melt owing to incorrect bias. A properly setup Grid Biased amplifier will allow the output valve to develop a little more power than with Cathode Bias, but there's not a lot to choose between them.
The main drawback to Grid Biasing is that the Bias must be checked fairly regularly. As a valve ages, so its requirements will change and the Bias must be adjusted accordingly. The Bias voltage is applied directly to the First Grid, but its effect is often seen by measuring the current flowing through the valve. This value should be supplied by the amplifier's manufacturer. You will need a voltmeter to measure the manufacturers setup value, although some amplifiers have built in measuring devices such as a small meter or an array of LED's. And now we hit a snag...
It's all well and good setting the bias voltage for a set of perfectly made and never changing valves, but in the real world valves do change. As mentioned earlier, as valves age so their requirements change, hence the need to check the biasing. But there's more...
The amplifier manufacturer often sets the bias value to the valve specifications, but not necessarily as to how the valve is actually operating in the circuit. This is a mistake. By measuring distortion levels and setting the Bias for minimum distortion with the optimum power output, and ensuring the valve is operating within its limits, it will guarantee that the amplifier will be working at it's peak performance level.
Cathode or Automatic Bias.
This method of Biasing is altogether simpler. Since the Bias voltage on the first Grid is relative to the Cathode, one can achieve the same result as with Grid Biasing by altering the Cathode voltage. By introducing a carefully chosen value of resistor in the Cathode circuit, it ensures the valve operates at its optimum Bias level. The upshot is that the valve will automatically find its own level and no adjustments are necessary.
Although primarily meant for Class A operation , Cathode Bias is often used for all classes of use. The only real downside to Cathode Bias is that the valve will not be driven quite a hard as with Grid Bias, but this is far outweighed by the advantages of stability and sound quality. Most Grid Biased amplifiers can be quite easily converted to Cathode Bias with an improvement in performance, stability and reliability.
After many years of work a valve will start to wear out and loose its emission. Indications are that the output power will drop and the distortion will rise. A rough visual indication is that the silver coating inside the valve envelope, called the gettering, will start to fade and become transparent. Any valve who's gettering has turned white will have completely failed, possibly because the vacuum has been lost within the valve owing to a crack in the glass.