Tuesday, July 30, 2013

The 6DJ8 / ECC88 / 6922 Vacuum Tube: The “Can Do” High Fidelity Tube?

Originally developed for electronic bench test measurement equipment, did the ECC88 / 6DJ8 / 6922 small signal vacuum tube slowly managed to become the 21st Century’s “can do” hi-fi vacuum tube? 

By: Ringo Bones 

Believe it or not, the 6DJ8 / ECC88 / 6922 small signal vacuum tube was originally developed for electronic bench test measurement equipment / electronic instruments like oscilloscopes and fast sampling oscilloscopes during the late 1950s at a time when the fastest solid-state transistors were only capable of 5-MHz switching speeds. And inevitably it almost became de rigueur in high end high fidelity audio applications during the height of the thermionic vacuum tube amplifier revival of the 1990s, in fact, almost dethroning the ubiquitous 12AX7 / ECC83 / 7025 small signal vacuum tube in most hi-fi audio and some specialist electric guitar preamplifier applications. It is tantamount to the LM318 high speed high slew rate op-amp oft use in precision function generators replacing the Burr-Brown OP27 or those other hot-rod hi-fi audio op-amps like the AD743 and AD797 from Analog Devices, Burr-Brown and Linear Technology that are often used in the analog output section of high performance budget CD players in the 300 to 500 US dollar price range. But what does make the 6DJ8 / ECC83 / 6922 vacuum tube such a special tube for its newly found high fidelity audio application? 

The 6DJ8 / ECC88 / 6922 vacuum tube are a miniature small signal preamplifier 9-pin medium gain triode vacuum tube on a B9A base with 6.3 volt only heaters and an internal screen. Electrical measurements wise, it is distinguished by its very high transconductance mostly the result of its frame grid construction. This is a special type of vacuum tube called a “Frame Grid Triode” so called because the grid is tightly stretched over a rectangular frame to allow very close spacing of the electrodes and hence the high gm or transconductance. 

Originally developed by Ampex, the American division of Philips in 1958 and then further refined for commercial release to hi-fi DIY hobbyist at the time by Philips in Holland under the European designation ECC88. The then Soviet Union produced their own version by the truckload during the 1960s designated as the 6922 – the variant for export to other countries while the Soviet era domestic use version was the 6N23P where the “N” and “P” were printed in Cyrillic letters - with military grade specifications able to work with higher power supply voltages of around 500 VDC anode voltage for use in their missile, rocket and ICBM telemetry systems under license from Philips. 

The 6DJ8 vacuum tube was originally designed for use as a low noise amplifier in VHF and UHF television tuners as an improved successor to the then de rigueur 6BK7 vacuum tube but due to its high gain, the 6DJ8 / ECC88 / 6922 vacuum tube was used in much high end electronic bench test equipment from 1958 onwards before solid state devices of sufficiently fast switching speeds or performed as good as the tube relegated the 6DJ8 tube to the dustbin of electronic bench test equipment history. In fact most high quality oscilloscopes and fast sampling oscilloscopes from the late 1950s until way into the 1970s have a number of 6DJ8 tubes in them. Sometimes I even wonder if the then Soviet Union used their own version of the 6DJ8 tube – in the guise of the 6922 vacuum tube – on the Sputnk I satellite when they launched it back in October 4, 1957.  

Although not originally designed for the purpose, the 6DJ8 / ECC88 / 6922 vacuum tubes became popular in high fidelity audio amplifiers, audio preamplifiers and other related high fidelity audio gear. An industrial or improved higher ratings version of the vacuum tube was produced in European modeled after the higher specification Soviet version called the E88CC during 1959 to 1961. At around the same time, Siemens introduced its own super version of the 6DJ8 / ECC88 under the CCa type. 

New Old Stock (NOS) 6DJ8 / ECC88 / 6922 vacuum tubes produced in the past – as in Cold War era - by major American or Western European vacuum tube manufacturers such as Philips or Ampex remain extremely popular with, and are highly sought after, by audiophiles. Even older Cold War era Mainland Chinese versions of the 6DJ8 – such as the 6N11 – sounds way better than their Mainland Chinese counterparts produced from 1992 onwards. The last time I checked in 2012, the 6DJ8 vacuum tube was still being produced in commercial quantities in four versions in Eastern Europe – like JJ Electronics formerly known as Tesla in Slovakia, by Ei in Serbia and the most sought after current production 6DJ8 type vacuum tubes are produced in Russia by Sovtek and Electro-Harmonix as the 6922. 

Most commercial grade new old stock 6DJ8 / ECC88 are incredibly microphonic when used in lower power supply voltage high gain audio preamplifier circuits. The very close spacing of its anode-grid-cathode structure also means that the maximum power supply voltage that can be used to power the circuits these tubes are used at is very limited or damage may occur. Standard 6DJ8 / ECC88s are almost exclusively used for preamplifier applications where they – if you know the right ones to use to avoid microphony – are very low noise, even suitable for moving coil head preamplifier applications for high end high fidelity vinyl LP replay. Though in most “contemporary” vacuum tube power amplifier circuits whose power supply voltage is around 500 volts DC or higher – the military spec Sovtek, Svetlana  or Electro-Harmonix 6922 vacuum tube however is useful for driver stage use as it has a higher power supply / anode voltage rating and anode dissipation. 

Sound quality is very variable even those famed new old stock types. Some are horribly grainy, others are stilted and mechanical reminiscent of how cheap solid-state amplifiers used in low-rent boom boxes and those low-rent active self-powered personal computer speakers handle rhythm, but some are crystal clear and smooth – especially those that wind up being used in vintage audio gear. The Telefunken E88CC was famed for its smooth naturalness while the Sovtek 6922 despite being criticized by some as being overtly romantically rolled back excels in lower midrange clarity. Even though the American DIY hi-fi scene of the late 1980s were probably the ones who promoted the 6DJ8 vacuum tube to be able to have almost dethroned the ubiquitous 12AX7 vacuum tube around the late 1990s, it is probably the UK based hi-fi firm Musical Fidelity who used the 6DJ8 / ECC88 / 6922 tubes prolifically in their entry level budget high end products like the Musical Fidelity X-Cans headphone amplifier – as in driving low impedance 32-ohm moving coil headphones with style, the X-Pre, the X-10D Buffer Stage just to name a few. And the American firm Conrad-Johnson was probably the first one to use the 6DJ8 / ECC88 / 6922 small signal vacuum tubes in a very low / zero negative feedback preamplifier in their Conrad-Johnson ART Preamplifier. 

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