The 7788 as a special quality high slope pentode designed for use in industrial equipment where stability of characteristics and long life are required. It is a wideband RF device, but no frequency range is given. From the information available it appears that 40 - 100 MHz is a probable upper value. Oscilloscope amplifiers and other measuring instruments are likely candidates for this valve.
The anode is formed into a channel that only comes close to the frame grid at the centre.
The screening is placed below the lower mica as well as above the top mica.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins, is 47 mm tall.
References: Data-sheet & 1040. Type 7788 was first introduced in 1968.