In 1929 begins the problem of cross modulation in the USA caused by strong signals of similar frequency and nonlinearities in the first stage. Stuart Ballantyne and Harold A. Snow (Boonton Research Corp.) found a first answer with the tube 551 later called 51 (Arcturus, Majestic, Raytheon May 1931) with "gradual cut-off" (vari-mu, gradual cut-off, GE experimented in 1919 in vain). Probably first publ. IRE paper December 1930. Soon after the 551 RCA came with the same tube called UY235 (235 later 35 and later joined as 35/51, in Japan UY-235). An article, June 1931 in "National Radio News" shows how necessary they were. In short: Trade show in Chicago in 1931 73% pentode and 94.5% variable-mu tubes. In the same year Philips brought the E445 and Cossor the 220VSG. This was a "super control feature for AVC" as Silver-Marshall stated for model 726. The problem: a great negative voltage is necessary and therefore a special AVC tube is involved for the RCA solution. See General Motors model 220 S-10B (1931). Philco used the Wheeler invention (released Oct. 1929) with its model 95 Screen Grid (1929) where a UY-227 triode works as a combination: detector and AVC diode.
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