Magnavox Type "A" Tube
tubes such as the one shown here to be used in their amplifiers.
Like many early tube manufacturers, they were faced with the choice of
using a grid and paying royalties to RCA or developing some other way to
create a workable amplifier that would not infringe the grid
patent. A third choice (using a grid and not paying royalties)
generally resulted in an expensive lawsuit filed by RCA.
Magnavox opted for an alternate, non-infringing approach, and came up with the structure seen below. The control electrode was an "A" shaped plate with a cutout in which the filament wire was suspended. The control electrode and filament were positioned in the same plane (i.e. the control was not between the filament and plate), a key factor in avoiding the patent infringement issue. The serrations along the edges of the cutout probably had no function other than to impress the customer.
Perhaps because of the shape of the control electrode, they called their tube the "Type A". This tube was introduced circa 1924. The example shown here has a stamped brass base and an evacuation tip on top. There was a later version that had a Bakelite base, and no tip. A box for the later version (dated 1926) appears below, but I do not have the tube for it.