The beam tetrode, patented in 1933, was invented in Britain by two EMI engineers, Cabot Bull and Sidney Rodda, as an attempt to circumvent the power pentode, whose patent was owned by Philips. Although the beam-plates (when present) could be counted as a fifth electrode (as in a pentode), this type of tube is nevertheless classified as a tetrode, perhaps to underline the difference in principle from that employed in true pentodes, which rely upon the effect of a suppressor grid. Beam tetrodes were widely used as audio power amplifying tubes in consumer items such as radios and televisions and in industrial electronic equipment until the 1960s when they were replaced by transistors. Their main use now is in high power industrial applications such as radio transmitters. Low power consumer beam tetrodes are still used in a few legacy and specialty vacuum tube audio power amplifier devices such as tube guitar amplifiers; the KT66 and KT88 are popular examples in audio equipment, while QY4-400 is an example having 400W anode dissipation, capable of applications in radio transmitters up to 100 MHz. The 4CX250B, mentioned above can be operated at full 250W anode dissipation up to 500 MHz. Many other types abound.