Robert Southerland & Allan Wyatt
The descriptions of beam-tetrodes and pentodes within the museum make clear the legal and practical differences between them. See Beam Tetrode and Pentode. However, in many lists valves from the different technologies are rated as interchangeable or 'sensibly equivalent'. Why?
Comparison of the anode characteristics of pentodes and beam tetrodes.
A paragraph from an old textbook explains the performance differences shown in the above image.
The material difference between the Ia vs Va characteristics of a pentode and of a beam tube [sic] is that the transition from a plate current which changes rapidly with Va to one which changes slowly with Va occurs sharper and at a lower plate potential in a beam tube than in a pentode.
And of course there is also the legal distinction, since the beam-tetrode was first developed specifically to bypass the pentode patent (and not least to avoid the royalties that would otherwise be due for it!).
If the valve has 2 physical grids, it's a tetrode or possibly a bi-grid, and if it has 3 physical grids (aligned or not) of which the third is a suppressor, it's a pentode. If some technique such as beam plates or aligned grids is used in a tetrode to shape the electron stream in order to create a low-potential region near the anode to suppress secondary emission, it's a beam-tetrode.
After WWII many valve designs were made by several manufacturers as the equipment makers wanted competition and continuity of supply. See Valve Standardisation. With different production technologies and experience each manufacturer chose an implementation that best suited them and in some cases a design can be found where for a given Type Designation the implementation is both pentode and beam-tetrode according to the make.
Where possible this museum will try to identify the electrode whose function is to reduce secondary emission correctly as g3 or bp where known or visible in the exhibit.
Classic audio valves and specialist devices where the valve is used to the limit appear always to be clearly pentode as in the case of the EL34 or beam-tetrode as in the case of the KT66.