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use in sets with series connected heaters and was eventually superseded by the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.
1 comment:
Along with the ECF80 triode pentode vacuum tube, the 6AU6 low noise RF pentode are two of the most common ones used in television sets from the 1950s to the 1970s and it seems that every TV repairman who worked with TVs from the 1950s to the 1970s have hundreds of these lying around. Given the sheer abundance of NOS types which explains why Electro-Harmonix or Svetlana never manufactured their own version of the 6AU6 low noise RF pentode.
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