In 1942, the 958A with tightened emission specifications was introduced after it turned out that 958s with excessively high emission kept working after the filament power was turned off, the filament still sufficiently heating on the anode current alone.[2][3] After the introduction of the miniature 7-pin base, the 954, 955 and 956 were made available with this base as 9001, 9002 and 9003. Other acorn tubes include: