Tuesday, July 30, 2013

The 12BH7: The Extremely Endangered Small Signal Vacuum Tube?

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.


Ady said...

As a kid growing up in the Golden Age of Stereo, I remembered my older bro tweaking a McIntosh tube amp's phase splitter stage where the 12BH7 vacuum tubes are used with bootstrapped anode load resistors. This configuration works rather synergestically with Iron Maiden's Die With Your Boots on in the 1980s.

Ringo said...

Speaking of Iron Maiden's Die With Your Boots On - muscular McIntosh vacuum tube amps manufactured during the height of the Golden Age Of Stereo, like the McIntosh MC-30, managed to soldier on during the 1980s as they were only the relatively commonly available tube amps able to drive 1980s style "Flat Earth" hi-fi loudspeaker type loads. While other classic Golden Age of Stereo era vacuum tube amps - like the Leak TL12 and the Quad II - tend to wheeze while driving a pair of Linn Saras or sound wanting when paired with the iconic 1980s Flat Earth hi-fi loudspeakers - the Linn Kans.