Tube amplifiers were the active electronic devices par excellence since the beginning of the century well into the sixties. Then they were superseded by the tiny transistors and solid-state diodes, capable of performing the same functions in much smaller spaces, with a lower weight and with very high operating temperatures inferior to those of the valves. It seemed to be a great relief for the musicians: more power and less weight. What else could you want? But, in the early seventies, new companies began to emerge with a bet on the amplification to transistors and the already consolidated ones extended their catalogues with this type of amps to not lose sales on the market. But the transistor based amplifiers were from their beginnings with a serious problem.
Their linearity and their theoretical performance resulted in very cold sounds with little audio character. This is the basic reason why the bulbs have been maintained since then in amplifiers for musical instrument and professional audio applications for recording studios and high fidelity. Their non-linear behaviour and ‘theoretically imperfection’ are left overcompensated with much more musical and attractive sound results in terms of tonality. A simple circuit with a single bulb can give a great character and colour to the sound. Remember that not even a complex digital circuit is capable of emulating 100% the behaviour of a bulb. That being said, let’s take a look at different types of bulbs that could amplify the magic sound so that you can make your decision according to your needs.
Different Types of Tubes for Different Results
One of the basic requirements for the operation of a tube amplifier is the temperature. They need temperatures above 100 ºC and some of them even reach 250 and 300ºC. The component of the tube that is responsible for ‘warming the environment’ is called filament.
It is comparable to the incandescent filament of domestic light bulbs and is what it causes the valve to light up when it is working. There are five tube topologies commonly used in audio differentiated by the number of internal components (not counting the filament) that make them up: SET (single-ended triode) diodes (two components), triodes, tetrodes and pentodes (composed of five components). A single-ended triode is a tube amp with only one output tube per channel that handles both the + and – sides of the waveform. A triode is also one of the commonly used and simplest tube types. The cathode (filament/heater) emits electrons to be picked up by the plate with a grid in the middle that regulates the flow. Tend to be lower power but highly beloved by their users. The anode and the cathode are the two basic components and common to all topologies commented. A pentode has five elements. A cathode, plate and 3 grids. That gives better control of the tube for certain conditions.
The cathode is just around the heating filament and it’s what releases the electrons that flow towards the anode creating an electric current. To create this electron flow, a minimum operating temperature is necessary for the cathode. At room temperature, this phenomenon could not occur. This means that if the filament of a tube breaks, there is no heat and therefore the valve stops working. To further facilitate the movement of electrons between the cathode and the anode, the vacuum needs to be created inside the valve.
In addition, the working voltages are very high (between 300 and 600 volts) to allow electrons detached from the cathode by temperature to be easily attracted by the anode. The rest of the components used in the triodes, tetrodes and pentodes allow controlling the number of electrons that pass from cathode to anode introducing in this way the concept of the amplification. Also, it’s necessary to add that there is a Class A type of tubes. They are very linear but waste a lot of power so the design tends to be used for lower wattage amps.