<P> In the 1930s special amplifying tubes were developed which deliberately "folded" the electron beam, by having it strike a dynode to be reflected into the anode . This had the effect of increasing the plate - grid distance for a given tube size, increasing the transconductance of the tube and reducing its noise figure . A typical such "orbital beam hexode" was the RCA 1630, introduced in 1939 . Because the heavy electron current in such tubes damaged the dynode surface rapidly, their lifetime tended to be very short compared to conventional tubes . </P> <P> The first random access computer memory used a type of cathode ray tube called the Williams tube that used secondary emission to store bits on the tube face . Another random access computer memory tube based on secondary emission was the Selectron tube . Both were made obsolete by the invention of magnetic core memory . </P> <P> Secondary emission can be undesirable such as in the tetrode thermionic valve (tube). In this instance the positively charged screen grid can accelerate the electron stream sufficiently to cause secondary emission at the anode (plate). This can give rise to excessive screen grid current . It is also partly responsible for this type of valve (tube), particularly early types with anodes not treated to reduce secondary emission, exhibiting a' negative resistance' characteristic, which could cause the tube to become unstable . This side effect could be put to use by using some older valves (e.g., type 77 pentode) as dynatron oscillators . This effect was prevented by adding a third grid to the tetrode, called the suppressor grid, to repel the electrons back toward the plate . This tube was called the pentode . </P>

Explain how the kink in the characteristics as due to secondary emission from the anode
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