Audio Amplification (part iii)
The Triode Behavior
After the path breaking invention of Triode by Lee De Forrest, today over 100 years later it is the same triode found in some of the world’s best high fidelity amplifiers and despite dramatic technological innovations and inventions over the century, this piece of vintage technology has no competition whatsoever.
Needless to say, this device indeed needs a deeper study and deserves attention. Today triodes are made in various types and characteristics and some of the most popular, proven and high quality triode tubes which are made today are dual which means 2 triode sections within one tube.
Following is a list of Different Triode Tubes available today and used quite often.
The Image on the left highlights the construction of a typical double triode and the image below shows the pinout diagram.
We will now be taking an example of one type of tube and understand it well. The same learning can be extended to other types of double triodes as only electrical and physical parameters differs among them and operational aspects remains the same.
12AX7 or ECC83 Double Triode
It was developed around 1946 by RCA engineers in Camden, New Jersey & was released for public sale under the 12AX7 identifier on September 15, 1947.
As of 2012 the 12AX7 was made in various versions by two factories in Russia (Winged C, formerly Svetlana, and New Sensor, which produces tubes under the Sovtek, Electro-Harmonix, Svetlana, Tung-Sol, and other brands for which the firm has acquired trademark rights), one in China (Shuguang), and one in Slovakia (JJ Electronic), for a total annual production estimated at two million units. The vast majority are used in new-production guitar amplifiers or for replacements in guitar and audio equipment.
The 12AX7 is a high-gain (typical amplification factor 100), low-plate-current triode best suited for low-level audio voltage amplification. In this role it is widely used for the preamplifier (input and mid-level) stages of audio amplifiers. It has relatively high Miller capacitance, making it unsuitable for radio-frequency use.
We will understand all the underlined and emphasized keywords in systematic manner. Lets us look at the manufacturer’s datasheet of this tube.
The first page of this document lists all standard operating conditions for the intended operation and application of the tube. We ses that the heater voltage required to heat the cathode needs 6.3V AC or DC in parallel and 12.6 if used in series. The terminal 4,5 and 9 can be used to wire the tube hear either in parallel or series. We also know from Ohms law and our lessons on Resistance that resistance reduces when two or more of them are connected in parallel and it adds up in series, this is intuitively obvious as in parallel, the area of the resistive element increases thereby giving more room to the electricity whereas in series the length increases thereby increasing the resistive path and hence increased overall resistance. This explains why the heaters in the tube need less (Half) voltage for heating in parallel configuration. Noticeable fact is that the current in parallel will double up.
Its is also worthwhile to learn the fact that heaters draw significant current, one triode tube draws about 0.3 Ampere. Imagine an equipment with 6-7 tubes or more, they together will draw over 2 ampere current and if we power them up using AC source then this high current will create magnetic field oscillating as the AC frequency and may cause induction in other parts of the circuit. There are two ways to address this: 1. The wiring to the heater terminals should be in such a way that the magnetic field generated by each wire should cancel out the one generated by the other wire in the pair and also these wiring should be laid out in the chassis closer to the grounded body corners so that they get confined to the chassis which is at a lower potential (Ground). 2. The other way is to used DC source, which increases complexity of the circuit because the supply is AC and you need to have a dedicated power section for the conversion of AC to DC. The other drawback is the reliability, as the number of components increase the number of failure points increase and hence the robustness of the equipment decreases. However, powering heaters using DC almost 100% address the problems of induction and HUM. The HUM as we call it is because this sound is of the supply frequency which creeps into the equipment and resembles the sound of humming.
Listen to the 50Hz sine Wave:
Let us now look at the plate characteristics curve at page 3 recreated below.
Consider this point
This point tells the voltage at plate corresponding to the grid voltage the point 1.
This point tells the current at the plate for the grid voltage of the curve for point 1 and corresponding plate voltage as point 2.
The above diagrams shows different curves for different grid voltages. As there are three parameters we are dealing with, what is done here is at a different fixed grid voltages, the plate voltages are varied and the plate current is measured for each voltage point. This is how the different curves are plotted.
Consider a point 1 on the curve, it is on the curve for which grid voltage is 1 Volt and corresponding plate voltage is 200 V. It means that if we apply 200V on Plate and 1V on the grid, the plate current will be 2A.
Let us now understand how a change in grid voltage amplifies at the plate and also find out the extent of amplification, technically speaking the amplification factor.
Look at the diagram below:
We see that, for a 1 V change in the grid voltage, there is about 100V change in the plate voltage, an amplification by 100.
This is how triodes act like amplifiers. Its is also noticeable fact that, if you look near the bottom region, the curves are not linear or straight, this means that their behavior changes in terms of amplification than what they exhibit near the upper region. It is important to choose a region which is relatively linear where the amplification factors and related characteristics are consistent.
Please note that when we say change in grid voltage it essentially happens because we apply the signal which we need to amplify at the grid. The grid is applied a voltage, for example 1 volt and the applied signal oscillates up and down over this grid bias voltage leading to corresponding changes at the Plate.
Understanding the Capabilities & Limitations of Triodes
In one of the paragraphs above, we came across the statement that the triodes are suitable for preamplifier for audio frequencies and not in case of radio frequencies.
We will understand why this so and what limits Triode’s capabilities.
The electrodes of a triode valve are in very close proximity to each other this leads to an inherent behaviour of the various electrode pairs to act like capacitors (we have learned that Capacitors are nothing but 2 parallel plates facing each other & separated).
In triodes the inter-electrode capacitances are quite small and are normally less than some 2.5 pF. These small capacitances have negligible effect on circuit operation in some applications, but in others they are of importance and have to be taken into account.
Since the signal is applied at the Grid, the two capacitances, 1. between Grid & Cathode (Cgk) and 2. Between Grid and Anode come into effect (Cga). However there can be more complicated combination of capacitances due to the presence of heater as well, but we will ignore them due to their practical insignificance.
On June 11 1919, John Milton Miller, got his paper published (Click here to read), highlighting the effect of these inherently present capacitances in triode tubes and subsequently known as Miller Effect or Miller Capacitance.
Take a look at the typical triode connected and configured as amplifier, with grid being fed the voltage signal to be amplified.
We know that the signal which is to be applied for amplification is not DC of course, hence the Grid- Cathode and Grid to Anode capacitors will cause the current to flow from the signal source as these capacitors provide a path, otherwise the grid is not connected in any manner to any other electrode. A remarkable thing which happens because of the amplification happening in the tube. At the anode since the voltage is amplified by the amplification factor, say A (assume 100), and also since a fall in grid voltage attracts more electrons and more flow at anode, and the inrese as well, it is clear that the signal amplified at anode gets inverted. Hence there is a difference of 100+1 (100 – (-1)) i.e., 101, ( for ex., 1v at grid gets amplified to -100V at plate, hence a difference of 101).
This leads to the source experiencing an enhanced capacitance by a factor of (Amplification +1). This effect is known as MILLER EFFECT.
If we take a small detour here and brush up the understanding of how the capacitors and inductors behave with frequency, it will make things pretty clear for the learning to come yet.
If the 12AX7 triode discussed above is employed in an audio amplifier circuit with conventional resistance-capacitance coupling to the grid, this input reactance could result in a loss in overall circuit gain at the higher audio frequencies. Such a loss may be acceptable in, say, a guitar amplifier but it would have to be taken into account in the design of an amplifier intended for high fidelity reproduction and certainly not acceptable at much higher frequencies like radio transmission applications.