The beam tetrode design of the 6L6 allowed RCA to circumvent Philips' pentode patent. The vacuum tube is then known as a triode, tetrode, pentode, etc. The pentode is closely related to the beam tetrode. Secondary emission can be undesirable such as in the tetrode thermionic valve (tube). The 6V6 is a beam-power tetrode vacuum tube. The beam tetrode was also developed as a high power radio transmitting tube. A parallel tuned circuit is connected in the plate circuit of the tetrode. The space charge grid tube was the first type of tetrode to appear. During the period 1913 to 1927, three distinct types of tetrode valves appeared. However, the tetrode had one new problem. Activity recorded on a tetrode is seen on only one of the four recording wires. The Neutrodyne was popular until the advent of cheap tetrode tubes around 1930. The "tetrode" description, though accurate, does not replicate the vacuum-tube tetrode.