Given that the pentode vacuum tube eventually paved the way for the transistor, does this make the EF86 small signal pentode vacuum tube the last vacuum tube to be developed?
By: Ringo Bones
Tracing the EF86 small signal pentode vacuum tube’s history and development is nebulous at best, some date the tube’s release as “the late 1950s” even though Peter J. Walker released the Quad II amplifier in 1953 and in order for him to develop his paraphase circuit / paraphrase circuit, the EF86 tube probably was already in the market at least six months before he released his Quad II amplifier. Despite a hazy release date, the EF86 vacuum tube is still one of the most fascinating preamplifier pentode vacuum tubes ever devised during the Golden Age of Stereo.
The EF86 is a high transconductance sharp cutoff pentode vacuum tube with a Noval B9A base for audio frequency applications. It was probably introduced a few months Peter J. Walker released his Quad II amplifier and used the EF86 in the paraphase circuit / paraphrase circuit of the Quad II. The EF86 was produced by Philips, Mullard, Telefunken, Valvo and GEC among others. It is very similar electrically to the Octal base EF37A and the Rimlock base EF40. Unlike many pentodes, the EF86 was primarily designed specifically for audio applications. It has low noise and low microphony due to the internal bracing and a helical / internal screen to prevent hum. With such claimed advantages, a rubber mounted vibration resistant base was still recommended. The EF86 has a much higher stage gain than any small signal triode vacuum tube which makes it susceptible to microphony. This was reported to occur and worsen with time. The pins are arranged to minimize leakage from the anode and heater to the input grid.
The EF86 was used in many preamplifier designs during the last decades of vacuum tube hi-fi development. And given pentode vacuum tubes allow appreciable amounts of negative feedback to be used without triggering instability and high-frequency rolloff due to the Miller Effect, they merely paved the way for the development of the solid state transistor power amplifier which used much larger amounts of negative feedback in comparison to their vacuum tube counterparts.
Generally, pentodes or pentode type vacuum tubes generate more noise than triodes because of the partitioning of current between the anode and the screen grid. The EF86 has a special internal structure to minimize this problem, allowing high gain and low noise in a single vacuum tube. The gain available from a single EF86 will usually be in the 100 to 200 range, though going for the 200 mark may present linearity and frequency response problems.
The main use of the EF86 vacuum tube will be in the front ends of both preamplifiers and power amplifiers. There are some “mega” versions of the EF86, namely the Telefunken EF804S and GECZ729 / CV4085 – both of which are expensive. The EF86 was used as the input stage of many Mullard and GEC circuits. I had once a “walkie-talkie” that uses EF86 vacuum tubes – three in fact – one tube as a transmitter, one as a preamplifier for the built in microphone and the external microphone to be connected an the third one was use as a Wien Bridge oscillator to convert the 6-volt lead acid batteries or 4 D-cells to a higher voltage to power the tubes. Financial problems forced me to sell it back in 2005 to a friend who once tested it as a wireless electric guitar transmitter rig since it accepts ¼ -inch electric guitar phone plugs and it transformed the stock sound of my friend’s 1990s era Carsbro solid state electric guitar amplifier into something more akin to a 1965 Fender Twin.