> 12AX7-- Twin triode audio tube very rugged construction to minimize
> microphonics
> 12AT7-- Twin triode with reasonable power dissipation.
> Could be used as high as about 600mhz in Ham applications
> 12AU7-- Pentode RF/IF/Mixer applications

Actually they were all twin triodes, the difference being noise level,
microphonics, power dissipation like you mentioned.  See
http://www.embedinc.com/temp/12au7a.tif.


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