Michael S. McCorquodale ©2005Abstract
The basic principles of vacuum tube device operation are presented including the functional properties of common device topologies. This background is leveraged to casually analyze gain stages that are typically found in vacuum tube audio equipment. The summary is intended to prepare the reader for basic design and analysis of amplifier stages.Introduction
Vacuum tubes are constructed of a glass or metal container that is vacuum sealed. The basic elements inside the tube are: the anode (or plate), the cathode, and the heater. The cathode is not essential for operation. A tube that does not contain a cathode is considered to be directly heated, while a tube contains a cathode is considered to be indirectly heated. The heater is driven by a voltage that allows its temperature to rise to the point at which electrons are sufficiently energetic that they can escape from the cathode and into the vacuum space. These free electrons are then attracted to the plate if it is supplied by a positive voltage relative to the cathode. If the tube is directly heated, then the electrons move directly from the heater to the plate. The movement of electrons from the cathode (or the heater) to the plate is called current. By convention, electrons moving in one direction implies a current directed in the opposite as illustrated in Fig.1.
Current will flow only if the voltage on the plate is positive relative to the cathode. If this is not the case, then current does not flow. This is the fundamental basis of vacuum tube operation and that is that sufficiently energetic electrons from cathode can enter the free vaccum space and be collected at the plate if the plate is set at a positive voltage relative to the cathode.Tube Topologies
There are five standard tube topoligies that are utilized in audio electronic equipment. Fig. 2 illustrates the schematic diagrams for each topology.
The simplest device is the rectifier. The tube rectifier behaves in a manner similar to a solid state diode in the sense that when the voltage at the plate is sufficiently high relative to the heater, the device conducts, thus rectifying the voltage at the heater as shown in Fig. 3.
The heater of the rectifier is typically tapped in an amplifier to provide a full wave rectified output when there are two plates present in the chamber. The two plates are driven by AC voltages that 180 degrees out of phase with each other as shown in Fig. 4. Generally the center tap of the power transformer can be grounded, thus providing two signals that are at one half of the voltage between the two secondary taps and 180 degrees out of phase.
The AC output signal at the heater can be regulated to DC by RC circuits where the DC signal serves as the supply rail for the gain stages in the amplifier. Of course, AC ripple is of concern to the designer and should be minimized to prevent the injection of 60 Hz hum into the audio system. This can be accomplished by selecting appropriate capacitors and resistors for the RC network and by using a choke.
The next most complex device is the triode. The triode is a three terminal device including the plate, the control grid, and the cathode. The voltage at the control grid either enhances or impedes electron flow from the cathode to the plate (or equivalently the current flow from the plate to cathode). It should be noted that there is also a heater below the cathode in this device, which is typically not shown in its schematic representation. The heater is typically connected to a 6.3VAC or 12.6VAC source, depending upon the tube type. However, the heater may also be driven from an equivalent DC source, though such an approach is typically not cost-effective. However, AC heater current may introduce 60 Hz hum into the audio system if the heater wires are not installed properly. The heater is what enables current to flow from the cathode to the plate by exciting electrons into the vacuum space, as described previously. If the control grid voltage is positive relative to the cathode, then current flow is enhanced. Similarly, if the control grid voltage excursion is negative relative to the cathode, then the current flow is impeded. Lastly, if the control grid voltage is sufficiently negative relative to the cathode, then all of the current will be impeded with the exception of residual leakage. Controling (or modulating) the current flow in the device by varying the control grid voltage is the mechanism through which gain is achieved. It should be clear from this discussion that if a voltage signal is applied to the control grid and tapped at the plate, the output voltage at the plate will be 180 degrees out of phase with the input voltage as shown in Fig. 5. It should also be clear from where the colloquial name "valve" originates. Indeed, the triode vacuum tube acts as a valve by either enhancing or impeding current flow.
The tetrode was developed due to the fact that there exists a large internal grid-to-plate capacitance in the triode which severely limits its utility in radio frequency applications. The tetrode introduces another control grid called the screen. The screen effectively splits the grid-to-plate capacitance in half since capacitors in series add like resistors in parallel, thus the total grid to plate capacitance is reduced. Typically the screen is kept at a positive voltage to be most effective. This, in turn, accelerates electrons toward the plate.
Despite the fact that the screen reduces the grid-to-plate capacitance it introduces an undesireable phenomenon. The pentode was developed because the tetrode accelerates electrons toward the plate and causes what is known as secondary emission. The electrons that travel in the vacuum space bombard the plate with sufficient energy such that the electrons on the plate become dislodged, thus entering the vacuum space. Since the screen is positive, these free electrons are attracted to it, thus effectively reducing the plate current or equivalently reducing the gain. Addressing this, the pentode contains another control grid called the suppressor. In general, this grid is internally connected to the cathode, thus it is at a substantially lower voltage than the screen. Since the electrons from secondary emission encounter the negative suppressor, they are repelled by it and therefore not collected by the screen.
The beam power pentode is a specialized configuration of a pentode. A virtual control grid is formed by an electric field that is set up between the screen and the plate. The schematic representation of the device is very explicit in this regard.
An artist's rendition of a beam power pentode is shown in Fig. 6. Here all of the aforementioned components comprising the device are shown.
Lastly, the reader should note that some devices contain more than one device in the vacuum chamber. For example, the 12AX7, which is a common tube type in audio preamplifier circuitry, has two triodes internally. Additionally, there are other tube types with six, seven, or more control grids in the chamber. However, these devices are typically not encountered in audio electronic circuits, thus they are not discussed here.Audio Amplifier Stages
WIth a basic understanding of device operation, standard audio amplifier stages can be explored. Fig. 7 presents a typical system level amplifier topology.
The preamplifier boosts the line-level signal and then allows for frequency filtering such as bass, mid-range, and treble adjustments which is also known as tone control. The phase splitter takes the preamp signal and splits it into equal and opposite portions. The resulting signals are used to drive a class AB power amplifier which in turn drives the output transformer.
Several output stage topoligies are implemeted in vacuum tube audio electronics. Here the class A, class B, and class AB stages are discussed briefly. In class A operation, the active devices drive the output for the entire cycle. This can be better understood by examining Fig. 8 where a single active device is illustrated.
Clearly the output stage is being driven entirely by the single triode in this schematic. It should also be clear that the output device must be biased correctly to allow the tube to operate in the linear amplification region. Class B differs from class A in the sense that each half of the output signal is carried by a dedicated active device, or cascade of active devices as illustrated in Fig. 9.
Observe that this class of operation introduces what is known as crossover distortion and it has been grossly exaggerated in Fig. 9 for illustrative purposes. Crossover distortion is attributed to the finite turn-on voltage of each active element in the crossover region. Class AB operation eliminates this distortion by applying the correct bias so each active element will will share part of the output signal in the crossover region. Except for this modification, classes B and AB are identical.Analyzing Amplifier Stages
The preamp is typically implemeted with 12AX7 triode devices. The most common configuration is the common cathode. The common cathode introduces a 180 degree phase shift. This implies that the output voltage decreases as the input voltage increases and vice versa. It is instructive to recall the operation of the control grid and verify that this is truly the case.
The cathode capacitor, CK, allows a signal path to ground because the impedance for an AC signal is small through the capacitor and infinite for the DC signal. The power supply capacitor, CA, isolates each gain stage. If each stage is not isolated from the power supply than an audio phenomenon termed "motorboating" can be experienced. This name is derived from nature of the sound of the audio distortion and is due to signal modulation from stage to stage in the preamp. The last item to note includes the resistors. The plate and cathode resistances are what set the bias point in the device. The grid resistors set both the bias of the control grid and the bandowdth of the amplifier. These topics will be explored in much greater detail in subsequent papers.
The phase splitter is used to derive equal and opposite signals for the class AB power amplifier stage. Although many phase splitter topologies exist, the one illustrated in Fig. 11 is the most common in vacuum tube audio amplifiers.
By examining the circuit in Fig. 11, something similar to a common cathode stage on each side can be observed. This configuration is also known as a differential pair and it is very common in both tube and solid state designs. The resistor network on the cathodes biases the device appropriately. The current in the cathodes of the differential pair is what sets several properties of the configuration, such as the gain and the bandwidth. The input grids are biased by tapping the cathode network just below RK1. RK3 allows a DC feedback path to ground. Feedback is a technqiue by which the amplifier linearity can be improved. Adjusting the feedback will modify the transfer function, or gain, through the differential pair. This comprises the electrical basis of the "presence" and "resonance" controls in typical amplifiers.
The entire configuration behaves similar to a current switch. The input will allow more or less current to flow in the anode of the driven device while causing the opposite effect in the anode of the other device. The input essentially "steers" the current, while the feedback will change how much is "steered." It should be clear that each anode has a signal that is 180 degrees out of phase with the other. This is exactly what is required to produce the signal input for a class AB output stage, which will be examined next.
Pentodes or beam power pentodes are typically found in power amplifier stages. The circuit shown in Fig. 12 is a generalized version of the common class AB topology. As described previously, one pentode conducts while the other doesn't and vice versa through the signal excursion. Although most commercial amplifier manufacturers use 12AX7 devices for preamp stages and phase splitter, a variety of pentodes can be utilized in the power amp stage. For example, in Marshall amplifiers EL34 devices are used commonly while Fender amplifiers generally use 6L6 or 6V6 devies. The output of the plate on each pentode drives a transformer in order to convert high impedance and high voltage to the low impedance and high current required to drive the speaker. The control grid voltage, VB, may be biased via a variable or fixed mechanism where fixed is shown in Fig. 12. This bias is generally negative and is often considered to be the most important voltage in the amplifier. If it is set incorrectly, severe crossover distortion will occur as class B operation is approached. The screen is driven by a positive voltage as explained previously.
Although this is only a very brief introduction to vacuum tube audio electronics, it is sufficient to begin simple design and analysis. Subsequent papers will describe analysis and design techniques for preamplifier stages. Additional subjects that will be examined include vacuum tube SPICE simulation and small signal analysis.Related References
Louis N. Ridenour et al., Vacuum Tube Amplifiers, 1st ed., Massachusetts Institute of Technology Radiation Laboratory Series, New York: McGraw-Hill Book Company, 1948.
Alfred J. Cote Jr. and J. Barry Oakes, Linear Vacuum-Tube and Transistor Circuits: A Unified Treatment of Linear Active Circuits, New York: McGraw-Hill Book Company, 1961.
Karl T. Compton et al., Magnetic Circuits and Transformers, New York: John Wiley & Sons, 1950.
K. O'Connor, The Ultimate Tone, Canada: London Power Press, 1995.
F. Langford-Smith et al.,The Radiotron Designer's Handbook, 4th ed., Sydney: Wireless Press, 1953.