Even though modern-manufacture vacuum tube makers have yet to reissue it, does the 6AU6 vacuum tube truly qualify as a “retro preamplifier vacuum tube”?
By: Ringo Bones
The NOS or new old stock versions were quite plentiful back in the 1990s when I first got serious into vacuum tube hi-fi as a hobby and even until today modern-manufacture tube makers like Electro-Harmonix, Svetlana and Sovtek still doesn’t have an economically viable need to manufacture their own version of the 6AU6 low-noise RF pentode vacuum tube. And from what I know so far in this hobby, I think the primary raison d’être of those subminiature vacuum tubes manufactured after World War II is to lower the build cost of the preamplifier and phase-splitter stages of power amplifiers that uses output vacuum tubes that date back from the 1930s.
The 6AU6 low noise radio-frequency pentode vacuum tube was developed by RCA Victor Co. Inc. of New York, New York and it is identical to the EF94. Entered into the Electron Tube Registration List back in October 25, 1945 and entered the Manufacturer’s Literature RCA Datasheet back in October 23, 1945. It has a miniature 7-pin base and was often used as the front end sections of radio and television receivers and it eventually gained widespread use in hi-fi audio towards the end of the 1950s. The 6AU6 is an indirectly heated small signal vacuum tube with a 6.3 volt filament and a 300-milliampere filament heater current. The 6AU6 was described as an RF-IF sharp cutoff amplifier for use in sets with series connected heaters and was eventually superseded by the 6AU6A since November 1956.
It gained popularity back in the 1990s to keep build costs down when used as first stage preamplifiers of power amplifiers that use power output tubes that dates back from the 1930s – like the 6L6 beam power tetrode – which was first manufactured back in 1936. One of the most popular applications of the 6AU6 low noise RF pentode was in the World Audio Design K6L6 integrated amplifier back in 1995. The WAD K6L6 circuit topology is similar to the ubiquitous Mullard 5-20 amplifier but the 6AU6 low noise RF pentode was configured as a high-gain single-ended amplifier stage. But the relative rarity of the use of the 6AU6 was that the original 1959 Mulalrd Tube Circuits For Audio Applications handbook never used the preamplifier tube in its construction guide of its ubiquitous Mullard 5-20 vacuum tube power amplifier.