The Vox AC30 is probably the most famous guitar amplifier, which was founded in 1957 British firm JMI ( Jennings Musical Instruments), later renamed Vox. This combo amp model is one of the most famous stage amplifier. The model is produced in two versions: AC30 and AC30 Top Boost was introduced later version with extended tone control. The AC30 has an output of 30 watts, which he gives to the two deep blue painted Celestion G12 speakers.
The reputation of the amplifier founded by several major rock and pop musicians and bands who have used him, the Shadows, the Beatles, Rory Gallagher, who used the AC30 already with key and thus influenced Brian May of Queen. Also, " The Edge" by the Irish rock band U2 uses the AC30.
The AC30 was developed in 1959 by Tom Jennings from a year earlier appeared on the market AC15, initially the one used in AC15 EF86 pentode in the preamp, which was replaced at the beginning of the 1960s by an ECC83. As Hank Marvin had combined from the Shadows during the recording sessions for the number 1 hit Wonderful country two Vox AC15 amplifier and had found a previously unknown sound depth and softness of the guitar sounds on the other side of the equipped with two EL84 tubes push-pull output stage with its 15-watt power output for the sound of larger rooms was not sufficient Vox developed the AC30 with twice the output power. These four Leistungspentoden were used in the push-pull output stage, with two EL84 were connected in parallel.
The special sound characteristics of the VOX AC30 along with the considerable heat that spiked on four output tubes output stage were mostly attributed to the putative class - A operation of the output stage, in which the position of the operating point is in the middle of the characteristics of the output tubes. More detailed studies, however, show that it is in the circuit design of the output stage is a class AB amplifier is where the operating point, however, is actually shifted toward higher bias currents than comparable amplifiers from other manufacturers. More significant for the sonic result is on the one fact that the AC30 is a pure " straight- amplifier ", ie does not use performance- sapping and corrective feedback, on the other hand then used 12 - inch speaker type Celestion play Bulldog with its high efficiency a crucial role.
Many other guitar amplifiers, from Fender over Marshall Hiwatt up to use the linearizing effect of a moderate negative feedback, in the most cases also the presence control is integrated.
The standard high quiescent current setting of the AC30 can be reduced with a higher resistance value of the built-up common cathode resistor. Some AC30 were alternatively supplied with a cathode resistance of 80 ohms, the characteristic sound characteristics of the amplifier are substantially retained and the output tubes are protected.