AKG The Tube (1983)
A large-diaphragm tube condenser that arrived two decades after tubes supposedly died - and made engineers remember why they missed them.
Overview
AKG introduced The Tube in mid-1983, and Mix-Magazine (September 1984, p.162) noted at the time that it was the only tube-powered microphone then in production. That's a strange distinction for a company that had spent the 1970s convincing the world that transistors were the future. The Tube was, in effect, AKG admitting that something got lost in the transition, and building a new product to get it back.
The design borrows from the C12, AKG's 30-year-old tube condenser. Mix-Magazine (September 1984, p.162) reported that the two primary components carried over were the large double-diaphragm capsule and the preamplifier vacuum tube. But this wasn't a reissue. The capsule was built to the CK12 pattern with what the magazine described as a more modern manufacturing approach - a six-micrometer polycarbonate diaphragm rather than the older materials. DB-Magazine (May 1984, p.29) described the diaphragm as a new, temperature-stable Teflon plastic foil, six micrometers thick and gold-sputtered to 0.03 micrometers, spaced 35 micrometers from the fixed plate. Whether the two accounts describe the same production revision or different stages of development isn't clear from the available documentation. Both agree the capsule was a fresh design rather than a stock C12 part.
The power arrangement was what set The Tube apart in 1983. Phantom power, by then the standard way to feed a condenser, doesn't apply here. The Tube shipped with its own dedicated power supply unit. Studio-Sound (May 1983, p.38) noted that the supply also incorporates a two-position bass cut and remote control of the microphone's internal switching.
The tube circuit itself offers a choice. An internal switch lets the user select the operating mode of the triode, giving roughly a 10 dB increase in level when engaged. Tube microphones have lower output than their solid-state counterparts, so this is a practical concession. Engineers working with quiet sources get the option without patching in an external preamp.
AKG's own manual for The Tube, shared with the C12VR documentation, lists a transmission range of 20 to 20,000 Hz. Sensitivity is rated at 11 mV/Pa at 1 kHz with no load, and the manual notes this can be raised by 10 dB via the switch on the PCB. Output impedance is 250 ohms plus or minus 20 percent, with a nominal load impedance of 1,000 ohms or greater. Equivalent noise level is specified as 32 dB per DIN 45405/CCIR 468-2, or 21 dB-A per DIN 45412. Maximum SPL for 3 percent THD is 50 Pa, which works out to 128 dB SPL. The microphone is rated for operation from -10°C to +60°C and relative humidity up to 90 percent at 20°C.
A 21 dB-A equivalent noise level is not remarkable by modern standards, and plenty of FET condensers beat it. For a tube design it's respectable. Engineers who used The Tube in its heyday weren't reaching for it because it was quiet. They were reaching for it because of what the tube did to the signal.
DB-Magazine (May 1984, p.29) put it plainly: the C12 had remained a favorite with many engineers for its musical sound, and that reputation prompted AKG to take a modern look at the design. The Tube was the result. It wasn't nostalgia for its own sake — it was an attempt to build a new microphone that carried forward the qualities engineers valued while using manufacturing techniques that hadn't existed when the C12 was designed.
The package was complete out of the box. Studio-Sound (May 1983, p.38) noted that the price included a sturdy flight case, windshield, shock mount, and all necessary cables. The shock mount, listed in AKG's parts documentation as the H15/T suspension, is an elastic design that holds the microphone body while isolating it from stand-borne vibration. The windshield is the W42. The microphone cable is a complete assembly with 12-pole connectors at both ends, and AKG's documentation lists both male and female 12-pole connectors as available accessories.
Internally, the component list reads like a tube circuit should. A 200-megohm resistor (R6) sets the capsule polarization. The output transformer is AKG part U66. Capacitors range from 220 picofarads up to 33 microfarads. Three INCA2 switches handle the user-facing functions. This is not a starved-plate design running a tube at low voltage for marketing purposes. It's a proper tube microphone circuit with the high-impedance front end that implies.
Disassembly, should it ever be necessary, is documented in AKG's service literature. Three screws on the lower side of the casing tube turn clockwise to release the body, and the casing tube then pulls away while the grip cap is held. The grid cap attaches to the microphone frame with four screws. AKG's manual includes a pointed warning about reassembly: good contact between the inner grid and the contact spring is essential, and poor contact increases sensitivity to hum and stray fields. A microphone that suddenly becomes noisy after a repair is almost certainly suffering from exactly this problem.
The Tube shares some documentation and presumably some design with the C12VR. It also exists alongside the SolidTube, a later tube microphone that Mix-Magazine (August 1998, p.41) described as a marriage of a glowing glass bubble and a wisp of gold. The C60, a tube condenser from 1960 through 1969, represents the earlier generation. The Tube sits between them: not a vintage original, not a modern reissue, but a new microphone built with old ideas and new methods.
A complete example needs both units and the correct cable, since the Tube requires its dedicated power supply and there's no phantom-power option. The 12-pole connector is not a standard XLR, so replacements aren't off-the-shelf items. The elastic suspension in the H15/T mount will have aged, and the rubber or elastic elements may need attention. The tube itself is a consumable. The circuit is not complex, but servicing it requires understanding the high-impedance section and the polarization voltage.
What The Tube offers, when it's working correctly, is the sound that made engineers keep reaching for C12s long after solid-state microphones became the norm. Mix-Magazine (September 1984, p.162) framed it as incorporating components from yesterday with today's miniaturization and modern assembly techniques — a fair summary. It's not a C12, and it doesn't pretend to be. It's AKG's answer to the question of what a tube microphone should be in 1983, built by a company that knew both the old ways and the new ones.
The microphone was finished in a special brown non-reflective coating, according to Studio-Sound (May 1983, p.38). It's a working tool, not a display piece.
Twenty years after The Tube was introduced, tube microphones were everywhere again. Every major manufacturer had one, and the market was flooded with inexpensive imports claiming tube character. The Tube was the first new tube microphone, back when the idea still seemed eccentric. It's not the most famous tube microphone AKG ever made, and it's not the most collectible.
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