Tuesday, July 30, 2013

The 12AT7 / ECC81 Vacuum Tube: The Electric Guitar Reverb Vacuum Tube?

Known for its “chrome-plated hi-fi sound” when used in non electric guitar reverb applications, is the 12AT7 / ECC81 vacuum tube more suited for electric guitar reverb amps than for hi-fi audio applications? 

By: Ringo Bones 

The 12AT7 small signal / preamplifier vacuum tube – also known in Europe by the Mullard-Philips vacuum tube designation as the ECC81. It is a miniature 9-pin medium gain (60 V/V) dual triode vacuum tube on a B9A base with 12.6V / 6.3V series / parallel heater. It didn’t really catch on as an high fidelity audio vacuum tube despite being indispensible in electric guitar reverb amplifier applications like the Fullerton based circuitry of Tweed era Fender amps – though the older stand alone electric guitar reverb units use 6K6 vacuum tubes instead of the 12AT7. 

On the topic of nichrome wire spring and fine wire wound moving iron reverb assemblies that are de rigueur in post World War II era electric guitar amplifier reverbs can be skillfully cobbled up by those still dexterous enough to be able to wound fine wire into their DIY moving coil cartridge for vinyl LP replay have an inherent high frequency roll-off due to inductive losses and stray capacitance in the reverb tank winding assemblies. The 12AT7 and ECC81 vacuum tubes primarily serve as pre-emphasis high frequency boosting circuits to counter the high frequency losses of spring reverb assemblies in order to obtain a more or less flat frequency response across the audio spectrum. Compared to bucket-brigade device (BBD) integrated circuit assemblies intended to replace the spring reverb during the 1980s -electric guitar spring reverb amplifiers have a much better sound quality and can reach 20,000Hz compared to those Godawful sounding BBD based reverb systems – while I.C. based BBD sound very awful and dull and has a bandwidth of only 5,000 Hz. 

The 12AT7 / ECC81 belongs to a large family of small signal dual triode preamplifier vacuum tubes which share the same pin-out – EIA 9A – including the very commonly used low mu 12AU7 / ECC82 and the ubiquitous high-mu 12AX7 / ECC83  small signal vacuum tube. The 12AT7 has a somewhat lower voltage gain than the 12AX7 but has a higher transconductance and plate current ratings which make it suitable for high frequency applications. 

Even though it didn’t really catch on as a high fidelity audio preamplifier vacuum tube when it was originally released back in May 20, 1947 by the American vacuum tube manufacturer General Electric – the 12AT7 vacuum tube was originally intended for operation in VHF circuits such as TV sets and FM tuners as an oscillator / frequency converter, but it also found wide use in audio as a driver and phase inverter in vacuum tube push pull audio power amplifier circuits. In old vacuum tube data books from the 1950s, the 12AT7 / ECC81 tube is actually listed as a VHF type. Its highish mutual conductance of 5.5mA/V together with its high amplification factor makes it ideal as a cathode follower / unity gain buffer or cathode coupled phase splitter. Other versions of the 12AT7 / ECC81 vacuum tube include the 6201 US DoD military version, 6060 special quality version, the B309 by GEC and the ECC85 which is very similar but has a different pinout. 

The 12AT7 / ECC81 has a much higher gm than the 12AX7 / ECC83 so gains in the region of 40 can be achieved with a single section even though its mu is only 55. The 12AT7 / ECC81 will also work well as a cascode or at low voltages at Va = 75Volts and its noise is low. The lowered anode impedance allows the 12AT7 / ECC81 to have a better high frequency response than the 12AX7 / ECC83 and better drive capability. Linearity isn’t really the 12AT7 vacuum tube’s forte though so most audio engineers won’t recommend it in very high level applications such as driving power triode vacuum tubes.
The sound quality of a typical 12AT7 / ECC81 vacuum tube is like a chrome plated version of the 12AX7 / ECC83 vacuum tube – with a bright, almost metallic upper midrange and much less of the 12AX7 / ECC83’s warmth making the 12AT7 more suited to electric guitar spring reverb preamplifier applications. Some people adore the 12AT7 tube’s inherently bright metallic sound and they are often a staple of vacuum tube power amps by Tube Technology known for their bright-sounding vacuum tube based audio gear and the iconic British “valve amp” manufacturer Beard. 


Kat said...

The "chrome plated" sound quality of a typical 12AT7 vacuum tube will probably work for its favor in electric guitar spring reverb applications due to inherent inductive losses of nickel chrome spring reverbs that make them sound inherently dull when using a 12AX7 vacuum tube.

Ringo said...

Not only in the inherently inductive nickel-chromium spring reverb units used in electric guitar applications which roll-off high frequencies like cray - though I still very much prefer their sound quality over those God-Awful sounding bucket-brigade device / BBD chips / integrated circuit based reverbs that can't play anything above 5,000 Hz - a badly wound interstage transformer transformer with an excess of stray capacitance in its windings also rolloff high frequencies like crazy, thus necessitating the inherently "chrome plated / bright" sound quality of the 12AT7 vacuum tube and its European ECC81 variant.

VaneSSa said...

Interstage transformers with a good square-wave response have virtually zero stray capacitance to maintain good high frequency output. Should one use Nobu Shishito's inverted interstage transformers?

Ringo said...

Transformers - whether interstage transformers or output transformers - are very important in vacuum tube audio preamplifiers and power amplifiers. One whose windings have an excess of stray capacitance could either act as a load - in the case of output transformers - hogging and diverting the output signal intended for the speakers or cause parasitic oscillation that could result in motorboating and other audio circuit related nasties. Having a dull sound is the least of the problems of an improperly wound output or interstage transformer. Having a very good to exemplar square wave response seems to be a prerequisite in 21st Century era vacuum tube audio amplification circuits.