Cathode Poisoning: Death Knell Of Thermionic Devices?

 Though largely forgotten when virtually all consumer electronic devices turned solid state at the start of the 1970s, is cathode poisoning still a concern for those who still use thermionic vacuum tube based electronic devices? 

By: Ringo Bones 

Though now virtually forgotten when virtually every consumer electronic devices we have are solid state based – including the now “affordable” organic light emitting diode (OLED) video display monitors, there was a time when cathode poisoning was of grave concern – like back in the days when the first programmable digital computers still use thermionic vacuum tubes. But as thermionic vacuum tubes returned in the high end audiophile scene, could the concept of cathode poisoning – in a strange twist of fate – inspire consumer electronic companies to design longer lasting thermionic vacuum tubes, even ones rivaling the longevity of solid state transistors and integrated circuits? 

Back in the 1950s, when vacuum tube technicians were still concerned with their tubes developing “sleeping sickness” whenever it was kept in soft-start mode for a prolonged period of time in radar and digital computer applications, cathode poisoning was of a grave concern on how to prolong the life of their banks upon banks of vacuum tubes when they are mostly switched to low current mode in switching applications. In short, cathode poisoning is the failure mode of a thermionic vacuum tube where the emissive layers degrade slowly with time and much more quickly when the cathode is overloaded with too high a current – which usually results in weakened emission and diminished power of the vacuum tubes or brightness of the cathode ray tubes – i.e. CRTs. Given what every “thermionic vacuum tube experts” had learned through such first hand events, were there any progress made in prolonging vacuum tube life and making cathode poisoning less of a concern? 

Even though there are various rare earth and halogen compound based cathode coatings that prolong thermionic vacuum tube life that make cathode poisoning much less of a concern, in the rather conservative world of thermionic vacuum tubes primarily designed for audio applications, manufacturers are choosing the tried-and-true from a sound quality perspective but older thoriated tungsten filaments which was discovered in 1914 and made practical by Irving Langmuir in 1923. A small amount of thorium is added to the tungsten filament. The filament is heated white hot at about 2,400 degrees Celsius and thorium atoms migrate to the surface of the filament and form the emissive layer. Heating the filament in a hydrocarbon atmosphere carburizes the surface and stabilizes the emissive layer. Thoriated tungsten filaments can have very long lifetimes and are resistant to high voltages. They are used in nearly all big high power vacuum tubes for radio transmitters and in some tubes for hi-fi audio amplifiers. Their lifetimes tend to be longer than those of oxide cathodes. 

Due to concerns about thorium ionizing radiation emission – i.e. radioactivity – and toxicity, efforts had been made to find alternatives. One of them is zirconated tungsten where zirconium dioxide is used instead of thorium dioxide. Other replacement materials include various rare earth oxides like lanthanum (III) oxide, yttrium (III) oxide, cerium (IV) oxide and other mixtures. 

Various rare earth borides had been used to prolong the life of thermionic vacuum tubes but there’s no news yet on how they affect sound quality of vacuum tubes when used in audio applications. Like boride cathode vacuum tubes that use lanthanum hexaboride and cerium hexaboride as coating of some high-current cathodes. Hexaborides show low work function around 2.5 eV. They are also resistant to cathode poisoning. Cerium hexaboride cathodes show low evaporation rate at 1,700 Kelvin than lanthanum hexaboride, but becomes equal at 8,800 Kelvin and higher. Cerium hexaboride cathodes have one and one half times the lifetime of lanthanum hexaboride cathodes due to its higher resistance to carbon contamination. Hexaboride cathodes are about 10 times as bright as the tungsten ones and have lifetimes up to 10 to 15 times longer. They are used in electron microscopes, microwave vacuum tubes, electron lithography, electron beam welding, X-Ray vacuum tubes and free electron lasers. However, these materials tend to be expensive. Other useful rare earth based hexabordes with long lives are yttrium hexaboride, gadolinium hexaboride and samarium hexaboride. 

Even though rare earth based hexaboride cathode coatings for thermionic vacuum tube devices may not yet be a hit in the hi-fi audio world, but I think these might have contributed in making extra long life CRTs or cathode ray tubes in television sets. Back in 1995, I’ve bought a 14-inch GoldStar color TV manufactured by LG Collins Electronics Manila, Inc. It’s a model CN-14A146 with serial number 60524212 which I bought for around 150 US dollars and it is still running until this very day. I wonder if this particular GoldStar 14-inch color TV uses a rare earth based hexaboride cathode coated CRT? 

The 7591 Vacuum Tube: Return of the Prodigal Vacuum Tube?

It became virtually extinct by the time Ronald Reagan ruled the free world, but did the relatively recent reintroduction of the 7591 vacuum tube manage to save our much loved vintage audio gear?

By: Ringo Bones 

From the1966 Ampeg GS-12R electric guitar amp prized by Jazz guitarists for its clean Jazzy timbres to the famed Fisher and Hermon Hosmer Scott receivers from the Golden Age of Stereo, the reintroduction of the 7591 pentode output vacuum tube by Russian vacuum tube manufacturer Sovtek via their Electro Harmonix arm had made every vintage audio gear enthusiast and Jazz guitarist eternally grateful. The reintroduction even makes me wonder if the 1966 Ampeg GS-12 R guitar amp and the famed Fisher and H.H. Scott receivers were even popular in Soviet era Russia and the rest of the East-Block countries during the height of the Cold War. 

Thanks to the reintroduction of the famed vacuum tube in the form of the Electro Harmonix 7591A EH, every Fisher 500-C and  HH Scott 345 and related model receiver – or the 1966 Ampeg GS-12R guitar amp - using the tube can now be resurrected with ease – which was very a very daunting task back during the Reagan administration half of the 1980s where modern pentode type vacuum tube based circuits became extinct since they were slowly being phased out by the start of the 1970s due to the fact that every electronics engineer can more or less replace them with solid state parts - if sound quality degradation due to increased levels of overall negative feedback was overlooked. 

Historically, the 7591 pentode vacuum tube was introduced in 1958 and registered with the EIA in 1960 after being developed and produced by Westinghouse’s receiving tube factory in Bath, New York. Given that Dr. Harvey C. Rentschler used to be the Director of Research for the Westinghouse Lamp Division, one could wonder if Dr. Rentschler was instrumental in the development of the 7591 vacuum tube. And by the way, the best NOS type of this tube was the famed 1962 Westinghouse 7591 which has a clearer sound than the new Electro Harmonix 7591.      

Monophonic Music Utilitarianism In The 21st Century

It might be for all intents and purposes an obsolete music format in the age of 6-channel digital music downloads, but are mono music recordings nonetheless still a utilitarian tool for discerning audiophiles? 

By: Ringo Bones 

From the perspective of today’s de rigueur discrete 6-channel digital surround sound, it is quite sobering to reflect on the historical origins of loudspeaker positioning for optimum two-channel stereo presentation. Around the dawn of the Golden Age of Stereo in 1956, F.H. Brittain and D.M. Leakey wrote articles in the Wireless World magazine on the subject, entitled Two Channel Stereophonic Sound Systems. Stereo – as in two-channel stereophonic sound – in the home wasn’t a common and practical reality until way into 1958, yet messrs Brittain and Leakey laid down basic principles that are still being followed slavishly until today. Well, back in the mid 1990s, Brittain and Leakey’s method were followed by the then newly recruited generation of hi-fi enthusiasts / audiophiles without them even actually knowing who Brittain and Leakey were or the two loudspeaker positioning method that they developed. 

Know them by name or not, there are still veteran audiophiles out there who are not using the “tried and tested” Brittain and Leakey arrangement. Brittain and Leakey’s tried and tested method does have a number of advantages – principally in the creation of a solid narrow image when both left and right speakers are fed with an identical mono signal. This is of crucial importance when it comes to creating a wide, stable, precise left-center-right two-channel stereo soundstage. 

If your speakers are optimally set up, you should hear a narrow center image when a mono signal – or a monophonic music recording – is played. Generally, this will only be achieved if the speakers are angled – or toed in – so their axes cross at an imaginary point in or in front of the main listening area, thus the importance of the toe-in of hi-fi loudspeakers in your listening room. 

With speakers flat against a wall and pointing straight down the room, an arrangement often employed in British homes – and typical in cramped apartment block type dwellings in Singapore and Hong Kong resident audiophiles – you won’t be able to achieve a narrow center image with mono recordings, but effectively create a “double mono” effect. As a result, stereo soundstaging will be less pinpoint precise than it could be, resulting with a vague center placement and poor “spread” between left / right channels.

Whether it is wholly desirable to achieve a narrow center image with two speakers is a moot point, as live sound is never so precise. But a narrow center image does ensure that you hear exactly how a recording was actually miked and mixed stereophonically, by allowing one’s home hi-fi speaker set up to recreate the subtle shifts of amplitude and phase that our ears use to locate sounds. 

Try this experiment; With a mono music recording or a mono radio broadcast (this can be often found in the AM radio band) – or if your FM only tuner has one – switch it into mono mode to convert what was once stereophonic FM broadcasts into instant mono. Using such methods, listen to how narrow your phantom center image sounds. You should hear a thin narrow central line of sound and not be aware of the two spaced loudspeakers – only an image between them. But be careful though as some recorded musical instruments – and even certain singers’ voices – can sound quite timbrrally different when converted into mono. A good example is Gloria Estefan – her voice sounds wholly different in mono when compared side-by-side with her full-blown stereo recordings.   

If you don’t get a clear impression of a center image, check for correct speaker phasing then angle each enclosure in or out until the center image snaps into strong focus. If possible, alter the distance between your listening seat and the speakers, moving closer or further away to see what happens.

A narrow center image with two-speaker mono will not necessarily result in a more pleasing overall sound, but things should be more precise – imaging wise. And this should allow you to hear the music exactly as it was recorded – in your listening room and in your own home. 

Is Mono Hi-Fi?

Today’s kids who grew up on the Apple i-Pod may balk at the thought, but are monophonic sound recordings still be considered hi-fi? 

By: Ringo Bones 

Though not as exciting as finding an authentic Daguerreotype photograph or even a Tintype photograph of Dr. Jose Rizal having a very heated chess mach with Friedrich Nietzsche, back in the hi-fi boom of the Clinton Economic Expansion era of the 1990s, there are no shortage of jokes about audiophiles who still clung on to the idea that mono is still better than stereo, a case in point are those hilarious cartoons In The Absolute Sound by Golliver and Gaughn involving someone named Mr. Mono who runs the Intergalactic Headquarters of the Singular Ear, but given that there are rather gorgeous sounding recordings made before the advent of two-channel stereo, should we audiophiles be all to dismiss any form of monophonic sound – no matter how tonally gorgeous – as not hi-fi? After all, it wasn’t until well into 1958 that two-channel stereophonic sound in the home became a reality; and why shouldn't really great sounding - as in realistically spooky sounding - monophonic recordings be dismissed as bona fide audiophile demonstration discs?

I still remember Stereophile magazine’s Michael Fremer’s review of the Sutherland PH-2000 Phono Preamplifier while playing a mono recording of Louis Prima’s The Wildest Show At Tahoe in which he quipped “Who needs six channels of shit when you can have one channel that sounds like this?” almost mirrors my reaction when I finally can afford my own proper hi-fi rig around the middle of the 1990s and was curious enough to test out monophonic recordings. My 1980s era heavy metal cassette collection whose ping-pong left –right, left-right bouncing of those distorted excruciatingly loud electric guitars may seem exciting on a boom-box can quickly become tiresome in a proper hi-fi rig, especially with more recent (as in from the late 1980s) fully stereophonic recordings having a less than stellar sound quality. 

And it’s important to forget that quite a number of recordings with a musicological importance – not just ones known for their sound quality – were recorded in mono. Elvis Presley’s more intriguing works before he conscripted by the US Army and shipped off to the Rhineland and the Sudetenland (are there any Elvis in the Rhineland and Elvis in the Sudetenland bootleg albums out there?) are all recoded in mono. And let’s not forget those “big mono” Jazz recording made during the late 1940s and early 1950s – Miles Davis Bags’ Groove is a perfect example – in which the sound comes right between the left and right speakers and seems to have a very expansive sound-stage despite it being a monophonic recording. Ina good hi-fi rig, one has to listen very hard to identify a good “big mono” recording. An audio-buddy of mine had been listening Miles Davis Bags’ Groove for 18 months back in the 1990s before he finally knew the truth that it is a monophonic recording. No audiophile in his or her right mind will ever dismiss Miles Davis Bags' Groove - either the vinyl LP or the JVC XRCD pressing - as a bona fide audiophile demo disc. In short, it seems that sound quality – as in how close it seems it is to the live musical event – is the main determining factor in determining if the recording is truly high fidelity, though one’s personal taste and opinion tends to matter also like I think Count Basie’s best works are recorded in mono. 

EF86 Vacuum Tube: The Last Vacuum Tube?

Given that the pentode vacuum tube eventually paved the way for the transistor, does this make the EF86 small signal pentode vacuum tube the last vacuum tube to be developed?

By: Ringo Bones 

Tracing the EF86 small signal pentode vacuum tube’s history and development is nebulous at best, some date the tube’s release as “the late 1950s” even though Peter J. Walker released the Quad II amplifier in 1953 and in order for him to develop his paraphase circuit / paraphrase circuit, the EF86 tube probably was already in the market at least six months before he released his Quad II amplifier. Despite a hazy release date, the EF86 vacuum tube is still one of the most fascinating preamplifier pentode vacuum tubes ever devised during the Golden Age of Stereo. 

The EF86 is a high transconductance sharp cutoff pentode vacuum tube with a Noval B9A base for audio frequency applications. It was probably introduced a few months Peter J. Walker released his Quad II amplifier and used the EF86 in the paraphase circuit / paraphrase circuit of the Quad II. The EF86 was produced by Philips, Mullard, Telefunken, Valvo and GEC among others. It is very similar electrically to the Octal base EF37A and the Rimlock base EF40. Unlike many pentodes, the EF86 was primarily designed specifically for audio applications. It has low noise and low microphony due to the internal bracing and a helical / internal screen to prevent hum. With such claimed advantages, a rubber mounted vibration resistant base was still recommended. The EF86 has a much higher stage gain than any small signal triode vacuum tube which makes it susceptible to microphony. This was reported to occur and worsen with time. The pins are arranged to minimize leakage from the anode and heater to the input grid.    

The EF86 was used in many preamplifier designs during the last decades of vacuum tube hi-fi development. And given pentode vacuum tubes allow appreciable amounts of negative feedback to be used without triggering instability and high-frequency rolloff due to the Miller Effect, they merely paved the way for the development of the solid state transistor power amplifier which used much larger amounts of negative feedback in comparison to their vacuum tube counterparts. 

Generally, pentodes or pentode type vacuum tubes generate more noise than triodes because of the partitioning of current between the anode and the screen grid. The EF86 has a special internal structure to minimize this problem, allowing high gain and low noise in a single vacuum tube. The gain available from a single EF86 will usually be in the 100 to 200 range, though going for the 200 mark may present linearity and frequency response problems. 

The main use of the EF86 vacuum tube will be in the front ends of both preamplifiers and power amplifiers. There are some “mega” versions of the EF86, namely the Telefunken EF804S and GECZ729 / CV4085 – both of which are expensive. The EF86 was used as the input stage of many Mullard and GEC circuits. I had once a “walkie-talkie” that uses EF86 vacuum tubes – three in fact – one tube as a transmitter, one as a preamplifier for the built in microphone and the external microphone to be connected an the third one was use as a Wien Bridge oscillator to convert the 6-volt lead acid batteries or 4 D-cells to a higher voltage to power the tubes. Financial problems forced me to sell it back in 2005 to a friend who once tested it as a wireless electric guitar transmitter rig since it accepts ¼ -inch electric guitar phone plugs and it transformed the stock sound of my friend’s 1990s era Carsbro solid state electric guitar amplifier into something more akin to a 1965 Fender Twin.