A tube amplifier is a type of audio amplifier that employs vacuum tubes for amplifying or increasing the power of the audio signal. Tube amplifiers having low to medium power, which are meant for frequencies below the microwaves, were mostly replaced by solid state amplifiers in the ’60s and ’70s. These amplifiers are mostly used as guitar amplifiers, satellite transponders (Direct TV or GPS), radar, very high power radio, stereo amplifiers etc.
Tubes are a better technology these days as compared to what it was in the yesteryears because they now come equipped with better components. The reasons that are responsible for the gaining popularity of these tube amplifiers are:
Simple Circuitry: Tube amplifiers involve the signal to be passed through lesser number of components, yielding a purer and more immediate sound. Simple circuits with very few components have been found to sound clearer than the complicated circuits having many components in the path of the signal. Simple circuits also come with more reliability as there are lesser parts to fail.
Dynamic handling capacity: There are a number of dynamic swings in music, and in tube amplifiers overload takes place with gradual and low even-order harmonic. This even-order harmonic is subtle and less hateful for the ears than the harsh, odd-order harmonic distortion that is shown by solid-state circuits. The designs that come with zero feedback have high output impedance with the result that they are very reactive to loudspeaker load.
Linearity and feedback: Tube amplifiers are voltage amplifiers whereas solid-state amplifiers are current amplifiers. They can work on less negative feedback in order to make the circuit linear. More negative feedback normally reduces the amplifier power and decreases the overall music quality. The ones that come with high feedback designs usually come with a lifeless and boring sound quality, whereas low feedback designs make the music sound more vibrant and clear.
There are other benefits associated with negative feedback. It helps to lower the output impedance of the amplifier. When there is low output impedance, the loudspeaker can take the load better. A negative feedback of zero to 20dB is normally treated as acceptable.
Power Supply: The higher operating voltages present in the tube amplifiers normally permit better voltage swing ability and much better headroom. This high operating voltage generates higher audible energy storage with a lower value capacitor. You can compare 600 working volts of the tubes with 80 working volts of the transistor amplifiers. This is the most important reason why many listeners feel that tube amplifiers generate more sound.
Availability of latest components: By using the transistors in the region where they are better, solid state rectifier diodes offer faster rectification and are capable of addressing a much higher capacitance than the previous tube rectifiers. Tube rectifiers can handle a maximum of only 50 micro-farads, while solid state rectifiers are capable of handling capacitance higher than 4000 micro-farads. The latest electrolytic capacitors available nowadays come with much higher energy storing capacity. The amplifier that possesses better power supply results in broader frequency response and better control on the bass capacity.
Disadvantages of Tube Amplifiers:
-
Tubes need warm-up time, generally 20 to 40 seconds before we get constructive output and almost twenty minutes before the circuits reach a steady state.
-
Tube audio amplifiers are very sensitive to vibration, and this influences performance and durability.
-
There are high voltages required for tubes and that demands the use of expensive capacitors that results in stressing of winding insulation of power supply as well as the output transformers.
-
Tube amplifiers generate a substantial amount of heat which can reduce the life of other elements in the instrument.
-
Tube amplifiers have a short life. They can only last up to few thousand hours.
-
Tube amplifiers should be used with caution as they can have very high voltages and very high stored energy.
