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Small signal audio pentodes



Early valve amplifier designers and constructors in the 1920s and 1930s had only triodes to work with. Some modern audiophile designers also restrict themselves to triodes.  It wasn't always this way, in the golden age of valve audio the small signal pentode was king.  Though even when a triode was the answer these types were often chosen, as when wired as a triode they made a better triode than the real thing.  


AC/SP3 AC/SP3/RH

Designed for radio and video frequency use in the 1930s at the birth of television this pentode came to be used by the BBC as a microphone pre-amplifier. Still available in the late 1950s it was available in grade A and grade B, where the grade A valves were selected for very low hum and microphony. 


Datasheet - Ediswan AC/SP3/RH


SP41 and SP61

The Mazda alternative to the AC/SP3 with similar characteristics but physically smaller.  Again designed for RF and video use in the early days of television but with the Mazda octal base. The SP61 has the American standard 6.3V heater, but retains the British base.
Both SP41 and SP61 saw use in wartime British radar systems and related test equipment. These valves were adopted by early audiophiles for high quality preamplifiers. Here's an article from Wireless World Magazine in 1947 by P J Baxandall (of tone control fame)  Baxandall -Hum in amplifiers- Feb 1947.pdf
  
 
 

Datasheet - Mazda SP41

6J7, 6SJ7, EF37A, EF37A, EF40, EF86 - it's hard to believe but these are pretty much the same valve with different bases.  Note that the Brimar 6J7 is marked made in USA,  no octal metal valves were ever manufactured in the UK.

6J7, 6J7G

An American metal tube first released in 1935.  Later released without the top cap as 6SJ7. Types 57, 6C6 and 1603 have the same characteristics but different bases and in the case of the 57 a 2.5V heater. The RCA 1620 is a 6J7 selected for very low microphonics.  The American 6J7 and British KTZ63 have a 300mA 6.3V heater,  in this respect they do differ from the British EF36/EF37/OM5 and later types EF40 and EF86 with 200mA heaters.   This difference is only significant were series heaters are used.  The KTZ63 being found with KT33C output tetrode, and the EF36 with the Mullard CL33. 

Datasheets - RCA 6J7,  RCA 6SJ7GT, 1620


EF36, EF37, EF37A

Military designations were VR56 and CV358.  These valves have the same characteristics as the 6J7G though unlike the American valve these British valves are always metallised - though probably needlessly as the structure is internally screened.  The 6.3V 200mA heater permitted use in series heater arrangements in AC/DC amplifiers with a CL33 or Pen36C output pentode, or two.
Huge numbers of EF36 were used in the world's first programmable electronic computer - Colossus.
 
Datasheet - Mullard EF37

EF40


A miniature version of the EF37 with B8A base. These three examples show the evolution of the EF40 from the first Mullard EF40 with a metal base to the all glass type which was replaced by the more practical B9A base.  The last example labelled as ITT Lorenz is a very odd valve indeed.  It's actually a 9 pin EF86 fixed to a plastic B8A plug - these must have been made for replacement purposes. 

Datasheet - Philips EF40

EF86

The same valve again (EF37A, EF40) with a B9A base. A 12.6V 100mA heater version, the UF86, and the PF86 with 300mA heater were also produced.  The EF86 is found in amplifiers by QUAD and Leak, it also featured in some electric guitar amplifiers of the 1950s and 60s.  It was the preamplifier valve for all the famous Mullard designs of the 1950s - three-three, five-ten, etc.

Datasheet - Philips EF86

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