Even though it dates from the Golden Age of Radio, but did you know that despite it is still in current production, the Type 85 vacuum tube has been languishing in obscurity for almost forever?
By: Ringo Bones
Kara Chaffee of deHavilland Electric Amplifier Company has recently become a cause-célèbre in the audiophile world when she designed those keenly packaged and cleverly priced zero negative feedback preamplifiers that became well-loved by both critics and real-world audiophiles. Even though she had been designing zero negative feedback preamplifiers since the mid 1990s as the “gold channel” preamplifier circuit for recording studio and boutique audiophile label mastering establishments, Chaffee had earned the reverence of audiophiles the world over when she used an obscure but still in manufacture small-signal vacuum tube that dates back from the 1930s Golden Age of Radio – the Type 85 vacuum tube - and turned it into an excellent sounding preamplifier, the deHavilland Mercury preamplifier that even managed to regenerate a sense of wonder even to the most jaded high-end audio equipment reviewer. But to those unfamiliar with it, here’s a brief history of the Type 85 vacuum tube.
The Type 85 is a 6-pin dual-diode triode multiunit vacuum tube with its distinctive top metal cap – that is a vacuum tube containing several independently acting vacuum tubes in one envelope – as it is composed of twin triodes and two radio-frequency detector diodes in a single glass envelope. First manufactured during the 1930s, it was primarily used as a radio-frequency detector, automatic voltage controller and first stage audio amplifier in AC line operated AM receivers. It is also used as the phase inverter in several 1930s era public address amplifiers. The Type 85 is electrically identical to the octal based 6V7. The Type 85S is a spray-shield type made by Majestic.
The Type 85 has a maximum plate voltage rating of 250 volts though typical operation as an amplifier the plate voltage is around 135 volts, it has a maximum plate current of 8-milliamperes though in typical operation it is around 3.7-milliamperes, it has a maximum grid voltage rating of -20 volts though in typical use this is around -10.5 volts. Typical in its operation, its heater voltage is 6.3 volts and heater current is 300-milliamperes, amplification factor or mu is 8.3, transconductance or gm is 750 and a plate resistance of 11,000-ohms. By way of comparison, an ECC32 has a plate resistance of 14,500-ohms while the 6SN7 has a plate resistance of 7,300-ohms thus making the Type 85 as having higher output impedance that it’s closest rival preamplifier vacuum tubes. As mentioned previously, the Type 85 vacuum tube contains two diodes which are used as radio-frequency detectors like the 1904 era J. Ambrose Fleming’s radio-frequency detector diode.
Dating back to the 1930s Golden Age of Radio and it is still manufactured in “sufficient” quantities by Russian and Mainland Chinese vacuum tube manufacturing firms and even sold in antique radio hobby suppliers in South-East Asia at around 5 US dollars each, the Type 85 vacuum tube has never received any recognition in high end circles – unlike its audio and radio frequency power transmitter vacuum tube siblings like the Western Electric 300B, the 211, the 845 and the Russian GM70 transmitter vacuum tubes which became famous during the 1990s era hi-fi boom. The Type 85’s humble origins as a 1930s era audio frequency preamplifier tube did not solidify its image as a much coveted exotic vacuum tube back in the 1990s.
Internally, the Type 85 is composed of two R-F diodes and a single triode section housed in a single envelope. Such a “compaction” certainly facilitated the mass production of “affordable” 1930s era AM radio designs by combining the front-end R-F detector, amplifier and the automatic voltage controller into one stage. Sadly, this topology is not the sort of vacuum tube likely to engender a cult following either back in the 1930s or in the 1990s. It should be noted that the triode section is almost completely independent of the R-F detector diodes, the only shared element being a cathode sleeve. The top metal cap of the Type 85 vacuum tube is electrically connected to the ground and therefore does not represent a high voltage shock hazard and we should be thankful to Kara Chaffee of deHavilland for approaching this vacuum tube with an open mind and thus discovering its hidden sonic potential.
Sound wise, the Type 85 vacuum tube has a much more gorgeous and creamier midrange than its nearest competition – the 6SN7 vacuum tube – which Kara Chaffee also used in her famed deHavilland UltraVerve preamplifier. And on a side-by-side comparison, the Type 85 even excels the ability of the other famed 1930s era preamplifier tube – the ECC32 – in making modern over-bright over-equalized multi-track 24-bit 192-Khz pop-rock recordings much more pleasing to the typical hardened audiophile’s ears. Even though it is pricier than the ECC88 vacuum tube equipped Musical Fidelity X-Pre, the Type 85 vacuum tube equipped deHavilland Mercury preamplifier sounds much, much better – though the deHavilland Mercury is around 10 times the price of the 250-US dollar Musical Fidelity X-Pre. High cost be damned – or if you have the spare time and the ability to DIY a Type 85 vacuum tube equipped preamplifier – the deHavilland Mercury and its Type 85 ilk can make any reasonably good sounding solid-state power amplifier the ability to create a soundstage as if it is a zero-feedback single-ended triode vacuum tube power amplifier.
Though most modern vacuum tube reissue manufacturers - like the famed Electro-Harmonix and Svetlana - has yet to manufacture their own "audiophile grade" Type 85 vacuum tubes, the ones I currently have and used in my preliminary DIY work are from National Union (Made in USA) NOS ones that still register "OK" in my audio-buddy's small-signal vacuum tube checker and a newer Mainland Chinese one whose brand is written in Chinese characters. Both managed to sound great from a vacuum tube perspective and I think the tone produced by a preamplifier using the Type 85 vacuum tube will easily please "tone freaks".