Pentode - Types of Pentodes

Types of Pentodes

• Variable transconductance ("vari-mu", "remote-cutoff" or "super-control") tubes in general are those with a non-uniform grid wire spacing to allow them to handle a wide range of input signal levels without excessive cross-modulation distortion, and so useful in Radio frequency stages where Automatic gain control is applied to the pentode. Examples include: 1T4, 6K7, 6BA6, and the EF83 (while perhaps the EF85/6BY7, and certainly the 6JH6, could be described as "semiremote-cutoff" pentodes).
• Sharp-cutoff ("high slope" or ordinary) pentodes have the more ordinary uniform spacing of grid wires, and so mutual conductance decreases in an essentially uniform manner with increasing negative bias, and has a more abrupt cutoff. These pentodes are more suitable for audio amplifiers. Examples include: EF37A, EF86/6267, 1N5GT, 6AU6A, 6J7GT. Often in the European valve naming scheme for pentodes an even number indicated a sharp-cutoff device while odd indicated remote-cutoff; the EF37 was an exception to this general trend, perhaps due to its history as an update to the EF36 ("The Mullard EF36, EF37 and EF37A" at the National Valve Museum).
• Power output pentodes, also referred to as "Beam Pentodes", include the Sylvania (and possibly GE) 6CA7 version of EL34, 6V6GT and the EL84/6BQ5. Sometimes the word "pentode" was avoided, in names such as "beam power amplifier" or "beam power tube", at a time when the pentode patent was an important issue.

The EL34, 6CL6, 6F6, 6G6, SY4307A and 6K6GT are some examples of true "screen pentodes" used for power amplification, using a screen for the suppressor grid, rather than the higher efficiency, lower distortion beam forming plates used in "Beam Pentodes". These tubes are often preferred as output tubes in electric guitar amplifiers because of their higher distortion characteristics.

• A "triode-pentode" is a single envelope containing both a triode and a pentode, such as an ECF80 or ECL86.