With only the General Electric “Grey Plates” variant
available on e-bay and other online stores, is the 12BH7 small signal vacuum
tube critically endangered and soon be in short supply?
By: Ringo Bones
Eastern European, Mainland Chinese and Russian vacuum tube
manufacturers who were very busy meeting the demand of the 1990s vacuum tube
high fidelity audio amplifier revival seems to be not making it the last time I
checked, neither are the “boutique” Russian vacuum tube manufacturers supplying
for the 21st Century electric guitar vacuum amplifier market. Even
though the supply is still relatively plentiful but perceptively retail prices
are creeping up every year, is the 12BH7 small signal vacuum tube so extremely
endangered as to be virtually commercially extinct and will the hi-fi world
soon be facing a shortage of this tube?
Like all of its related television vertical and horizontal
deflection amplifier and flyback vacuum tubes that were developed and
manufactured during the 1960s, the 12BH7 tube – and the PL509 vacuum tube where
the only cost effective source are junked black and white Radiowealth TV sets
from the late 1960s and 1970s – were deemed obsolete by the major consumer
electronics manufacturers during the late 1960s as soon as viable solid state
replacements became a commercial reality. The 12BH7 small signal vacuum tube
was probably released in 1964 by General Electric because the most plentiful of
its type that can still be bought on line is the 12BH7 General Electric “Grey
Plates” variant. The 12BH7 vacuum tube has the same series / parallel heaters
and biasing / pinout as the 12AT7, 12AU7, and 12AX7 series types of small
signal vacuum tubes. Designed for use as a vertical deflection amplifier in
television sets, the 12BH7 vacuum tube’s main application in audio is as a
high-powered driver vacuum tube.
The 12BH7 vacuum tube looks like a 12AU7 vacuum tube that
had taken an overdose of anabolic steroids and spent too long in the gym. The
advantage of the 12BH7 in comparison to the 12AT7, 12AU7 and 12AX7 series of
tubes is its anode dissipation of 3.5-watts and a maximum anode voltage of over
300-Volts. The 12BH7 can also deliver a lot of current – very essential for TV
deflection amplifiers – at Vg=0 making it a good choice for cathode followers.
To supply the very high voltage drive requirement of their
distributive loading output stage, famed vacuum tube amplifier manufacturer
McIntosh use 12BH7s with bootstrapped anode load resistors. Conrad-Johnson and
Audio Research also use them for similar reasons. The large voltage swing
capability and low impedance make the 12BH7 a contender for driving triode
output stages, although there are better, more linear types of vacuum tubes
much better suited for the job.
As the 12BH7 can be used in such a variety of ways, it is
difficult to pin down its sound. In circuits I have tried it in though it
sounds much like a 12AU7 vacuum tube with boots on – i.e. either sounding akin
to an extremely low negative feedback boutique solid state driver circuit or
one consisting of pentode tube and nuvistor hybrid driver circuit. Though yet
still relatively plentiful, the hi-fi world could soon be facing a 12BH7 vacuum
tube shortage if current vacuum tube manufacturers don’t start making them
again.
