Tuesday, July 30, 2013

The 12BH7: The Extremely Endangered Small Signal Vacuum Tube?

With only the General Electric “Grey Plates” variant available on e-bay and other online stores, is the 12BH7 small signal vacuum tube critically endangered and soon be in short supply? 

By: Ringo Bones 

Eastern European, Mainland Chinese and Russian vacuum tube manufacturers who were very busy meeting the demand of the 1990s vacuum tube high fidelity audio amplifier revival seems to be not making it the last time I checked, neither are the “boutique” Russian vacuum tube manufacturers supplying for the 21st Century electric guitar vacuum amplifier market. Even though the supply is still relatively plentiful but perceptively retail prices are creeping up every year, is the 12BH7 small signal vacuum tube so extremely endangered as to be virtually commercially extinct and will the hi-fi world soon be facing a shortage of this tube? 

Like all of its related television vertical and horizontal deflection amplifier and flyback vacuum tubes that were developed and manufactured during the 1960s, the 12BH7 tube – and the PL509 vacuum tube where the only cost effective source are junked black and white Radiowealth TV sets from the late 1960s and 1970s – were deemed obsolete by the major consumer electronics manufacturers during the late 1960s as soon as viable solid state replacements became a commercial reality. The 12BH7 small signal vacuum tube was probably released in 1964 by General Electric because the most plentiful of its type that can still be bought on line is the 12BH7 General Electric “Grey Plates” variant. The 12BH7 vacuum tube has the same series / parallel heaters and biasing / pinout as the 12AT7, 12AU7, and 12AX7 series types of small signal vacuum tubes. Designed for use as a vertical deflection amplifier in television sets, the 12BH7 vacuum tube’s main application in audio is as a high-powered driver vacuum tube.

The 12BH7 vacuum tube looks like a 12AU7 vacuum tube that had taken an overdose of anabolic steroids and spent too long in the gym. The advantage of the 12BH7 in comparison to the 12AT7, 12AU7 and 12AX7 series of tubes is its anode dissipation of 3.5-watts and a maximum anode voltage of over 300-Volts. The 12BH7 can also deliver a lot of current – very essential for TV deflection amplifiers – at Vg=0 making it a good choice for cathode followers. 

To supply the very high voltage drive requirement of their distributive loading output stage, famed vacuum tube amplifier manufacturer McIntosh use 12BH7s with bootstrapped anode load resistors. Conrad-Johnson and Audio Research also use them for similar reasons. The large voltage swing capability and low impedance make the 12BH7 a contender for driving triode output stages, although there are better, more linear types of vacuum tubes much better suited for the job. 

As the 12BH7 can be used in such a variety of ways, it is difficult to pin down its sound. In circuits I have tried it in though it sounds much like a 12AU7 vacuum tube with boots on – i.e. either sounding akin to an extremely low negative feedback boutique solid state driver circuit or one consisting of pentode tube and nuvistor hybrid driver circuit. Though yet still relatively plentiful, the hi-fi world could soon be facing a 12BH7 vacuum tube shortage if current vacuum tube manufacturers don’t start making them again.

Will There Be A 12AY7 / 6072 Vacuum Tube Shortage?

Given that it is currently not manufactured by Russian, Eastern European and Mainland Chinese vacuum tube manufacturers, will the hi-fi world soon be facing a 12AY7 / 6072 vacuum tube shortage? 

By: Ringo Bones 

Even though the 12AY7 / 6072 vacuum tube is virtually similar to the double triode 12AX7 / ECC83 / 7025 vacuum tube, it is quite a mystery why the worlds currently existing vacuum tube manufacturers haven’t started making the 12AY7 / 6072 small signal vacuum tube anymore which is quite a shame given that it offers a tone that is more often than not never found in a current manufactured 12AX7 or ECC83 vacuum tube. 

The 12AY7 vacuum tube is a miniature dual triode vacuum tube with a 12.6-Volt / 6.3-Volt series/parallel heater, a B9A base. This tube is virtually similar to the 12AX7 / ECC83 and has the same heaters and pin-out configuration. It was probably developed by the same RCA electronic engineering team that developed the 12AX7 back in 1946 in Harrison, New Jersey. Though the 12AY7 tube was released commercially back in December 7, 1948. 

It was intended as a low noise small signal preamplifier tube for audio use and as such it is excellent. Its characteristics are just about for preamplifier applications where a gain of around 30 – half that of a typical 12AX7 preamp tube application – is typical for one stage of a 12AY7 tube and with a typically low output impedance. 

Microphony on a good 12AY7 / 6072 tube is just about nonexistent, noise is low but a 6DJ8 / ECC88 / 6922 tube will win in the end due to its “brute force” transconductance or gm. It would be sacrilege to use a 12AY7 tube as a driver tube because there are tubes which are better at being forceful. When used as intended though, the 12AY7 / 6072 tube has a rich, smooth and open sound quality which is very hard to beat. 

The 6072 version is a Soviet era high reliability / ruggedized military version and in my opinion it wins over its commercial new old stock (NOS) cousins in the sound quality stakes by being slightly clearer and more coherent while the typical new old stock 12AY7 tube is slightly softer and more euphonic. While both 12AY7 and 6072 have virtually similar electrical characteristics in both AC and DC conditions.
The only problem with the 12AY7 and 6072 vacuum tubes is their price. Although not yet reached stratospheric levels, both are more expensive than the plain old 12AX7 / ECC83 vacuum tubes. Stock levels are still relatively high back in the 1990s to as of today, but unless current vacuum tube manufacturers restart producing the 12AY7 and 6072 vacuum tubes soon, there would soon be a 12AY7 / 6072 vacuum tube shortage that would make their current retail prices rise to stratospheric levels faster than a Soviet era supersonic capable strategic bomber. 

Even though the only sources of 12AY7 vacuum tubes are new old stock Sylvania types, the Soviet era 6072 vacuum tubes comes in boxes that show their age despite passing with flying colors in a small signal tube checker. Its as if most 6072 tubes currently available today dates back to when Leonid Brezhnev was still the head of the then Soviet Union. The only commercial amplifier manufacturers I know of currently using the 12AY7 tubes and 6072 tubes are Audio Note and World Audio Design in their 300B Amplifier. 

The 12AU7 / ECC82 Vacuum Tubes: The Most Popular High Fidelity Audio Tube?

Even though the 12AU7 / ECC82 vacuum tube is very popular in hi-fi audio circles, is an audio designer justified in choosing another type in the name of sound quality? 

By: Ringo Bones 

Some audio amplifier designers often chose to use the 6SN7 tube in place of the 12AU7 / ECC82 in almost every application in the name of sound quality does it still make the 12AU7 / ECC82 the most popular tube in high fidelity audio circles? Maybe, but it seems this somewhat controversial practice of replacing a more recently designed vacuum tube with something from a previous generation of vacuum tubes a sound audio engineering practice? 

The 12AU7 is a miniature 9-pin (B9A base) medium gain dual triode vacuum tube. It belongs to a large family of dual triode vacuum tubes which share the same pinout (EIA9A). The 12AU7 vacuum tube was first marketed back in October 18, 1946. Known in Europe under its Mullard-Philips vacuum tube designation ECC82, in fact, the 12AT7, 12AU7 and 12AX7 or the ECC81, ECC82 and ECC83 are all exactly the same vacuum tube but with varying electrode spacing and grid pitch to achieve the different characteristics. 

There are many equivalent vacuum tubes with different names, some identical some designed for ruggedness, long life or other characteristics, examples are the US military 5814A and the European special quality E82CC and E182CC. There is also the 6067 SQ version, B329 GEC version and the 6189W military version. The 12AU7 tube is very popularly in high fidelity vacuum tube audio and is used in low noise line level preamplifier, driver and phase splitter, phase inverter in vacuum tube push pull power amplifier circuits. There are opinions which indicate that this popularity is caused by the audiophile industry / hi-fi review polarization as opposed to its true audio performance. The 12AU7 was originally designed and intended as a low impedance driver vacuum tube. Gains of around 12 are typical for a single stage, but more can usually be squeezed out of it if needed. 

Its special quality versions like the E82CC and 5814A are used in specialized non-audio applications during pre-solid state semiconductor days in early digital computer circuitry. Using the special quality versions outside of the purpose they were designed for – like hi-fi audio use during the 1990s – may not provide the intended optimal results. For example, a version of the 12AU7 vacuum tube intended and optimized for digital computer logic gate use may be designed for long life without succumbing to cathode poisoning when mostly switched to low current mode in switching applications but with little attention paid to parameters of interest intended for linear – as in audio – applications such as linearity of transfer characteristics, matching between two sections in push-pull phase splitter audio amplifier applications, microphony levels, etc. 

In high fidelity audio applications, the 12AU7 / ECC82 vacuum tube can deliver quite large currents at low anode voltages, making it useful as a cathode follower when a small amount of power is needed to drive the output stage. As a simple anode follower, the 12AU7 / ECC82 often finds itself as the last stage in preamplifiers or as a driver for parallel-pair output stages. It is also sometimes used as a phase splitter / driver. If used in a cathode coupled configuration, anode resistors must be adjusted to obtain balance.
Sound quality can vary depending on which type is used and of course how it is used. Generally, a typical 12AU7 has quite a dull sound, but this could be viewed as lack of coloration. The 12AU7 – like the 12AX7 – is used by a lot of hi-fi audio equipment manufacturers like Audio Innovations and Border Patrol just to name a few. 

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. 

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.