The 30F5 is a miniature screened pentode that was designed to operate at the standard TV intermediate frequency of 33 MHz. The IF stages in any superhet receiver are where the main signal voltage amplification takes place.
This valve is also specified by Mazda for operation as a triode, this suggests that it would also provide useful gain at higher frequencies, probably 50 MHz for Band I or 175 MHz for Band 2.
The electrode assembly only occupies about half of the valve length. The wire grids can be seen through the outer screen.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 55 mm tall.
References: Data-sheet & 1040. Type 30F5 was first introduced in 1957. See also 1957 adverts.