Selenium Rectifiers: The Most Musical Power Supply Rectifiers?

Originally used on Avery Fisher’s Fisher 500-C Receiver, is the selenium rectifier the most musical power supply rectifier ever used for hi-fi applications?

By: Ringo Bones

Sadly, as we headlong into the second decade of the 21st Century, the “failure-prone” selenium rectifier originally used in the Fisher 500-C receiver had now been relegated to the dustbin of history and are the usually the first ones to be replaced with more modern silicon bridge rectifiers whenever a Fisher that dates back to 1964 needs to be brought to present day operating condition. But should everyone wanting their old Fisher 500-C receiver to be brought to present day working condition need to replace the original stock selenium rectifier that still works despite of its reputation of being failure-prone? After all, selenium rectifiers tend to be more musical than their more modern silicon-based counterparts, right?

Given that the thermionic vacuum tube / valve manufacturer Mullard managed to manufacture the most reliable – and most musical sounding – version of the AD 149 PNP germanium output transistor during the 1960s – Mullard doesn’t seem to be able to manufacture their own more reliable, and hopefully a more musical version, of the failure-prone stock selenium rectifiers on the original Fisher 500-C receiver. And if they did, they might be as rare as hen’s teeth this day and age. But given the inherent unreliability of selenium rectifiers, why did Avery Fisher used it as the main rectifier system when he first designed his iconic Fisher 500-C receiver? Is it really more musical sounding in comparison to its more modern silicon counterparts? But first, here’s a primer on what this selenium rectifier business is all about.

Selenium rectifiers belong to the family of metallic or dry-disk rectifiers. A typical selenium rectifier consists of a layer of selenium, a semiconductor, deposited on an iron plate. The selenium acts as a P-region while the iron plate acts as the N-region. And when a voltage is applied in the forward bias direction (positive to the selenium and negative to the iron), current flows readily through the rectifier. When connected in the reverse bias direction, the resistance of the rectifier becomes very high and very little current may flow through it. Several such diodes may be connected in series to increase its voltage-handling ability. This is primarily how the power supply of the original 1964 era Fisher 500-C receiver converts the 110-volt 60-Hz AC of American wall sockets into the various DC voltages required to power it up and play tunes, or of news broadcasts, etc.

Given you are one of the fortunate few to be able to use one as part of the power supply of your experimental DIY hi-fi power amp (tube or solid-state), selenium rectifiers tend to have a “musicality” not normally found in typical run-of-the-mill modern silicon rectifiers that are now de rigueur in mass market audio equipment. Its closest modern equivalent sound-wise and musicality-wise is a high-speed Schottky diode rectifier and a Rubycon Black Gate capacitor equipped power supply. Sadly, high-speed, high current Schottky rectifiers are now a rarity after Mainland China invented a RADAR system heavily dependent on high-speed Schottky rectifiers in its operation that can “see” stealth aircraft.  

The “unreliability” and the “failure-prone” nature of the selenium rectifier may have been due to the still primitive – compared to the standards already prevalent during the 1980s to this day – state of solid-state device mass production of the late 1950s and for much of the1960s. Also, I can only assume that a typical early 1960s selenium rectifier could also be very prone to galvanic corrosion if the selenium and iron plate interface gets exposed to atmospheric moisture – especially in hot, tropical climes. These glaring caveats aside – a selenium rectifier mass produced with high-reliability in mind has a musicality not normally found in today’s mass-market generic silicon-based power supply rectification systems, unless of course you are sold to the still much more musical thermionic vacuum tube rectifier system based on the original Mullard GZ34 or 5AR4.

Putting The Vacuum In Vacuum Tubes

Given that most of our hi-fi components these days have gone solid-state, does the quality level of the vacuum inside our vacuum tubes still matter?

By: Ringo Bones

Even though maintaining a near-perfect vacuum inside a typical thermionic vacuum tube – or valves as they are called in merry old England - that’s still in use in some “purist” hi-fi equipment these days is vital for the tube to do its intended function, there are probably more people more interested to know who’s in the running for this year’s Miss Teen Topanga than the level of quality of the vacuum in the vacuum tubes in current production. Given that most of our hi-fi components these days have already gone solid-state, does the vacuum quality inside a typical vacuum tube in current production still matter?

These days – as in well into the second decade of the 21^st Century – thermionic vacuum tubes are horse and buggy technologically wise compared to other of our home entertainment gear that have since gone the solid-state route. It is primarily their musically and psycho-acoustically consonant to the human hearing sound of vacuum tubes that have still endeared them in purist high-end hi-fi and the electric guitar amplification world that hitherto most solid-state designs still can’t achieve its own version of “musicality” and purity of tone. But given that most vacuum tube manufacturing equipment in current use probably dates back before World War II, manufacturing thermionic vacuum tubes of both high quality and reliability given the near-perfect vacuum required is getting harder and harder as we headlong into the 21^st Century.

Thermionic vacuum tubes need a high vacuum which has to be achieved during manufacture and maintained during its entire service life – typically 2,000 to 10,000 hours. Even if a satisfactory vacuum is achieved initially, through pumping and ignition of the getter, occluded gases in the metal electrodes and glass enter the vacuum over time, especially if the heat treatment to drive them out before the vacuum tube is sealed is perfunctory, or the metal-to-glass seal around the pins leak due to unmatched coefficients of expansion between the glass and pin materials. Vacuum tubes, after all, are still a triumph of 20^th Century materials technology and carefully controlled production processes.

But thermionic vacuum tube design and manufacture can be more than just maintaining a near perfect vacuum inside its glass envelope. In a July 1943 issue of the Scientific American magazine, Dr. Harvey C. Rentschler told in a then recent meeting of the American Physical Society that gases can dissolve in the crystalline structure of metals. In his experiments during the previous eight years back then have led to the conclusion that atoms of gas – like oxygen, hydrogen, or nitrogen  - actually dissolve in the crystalline structure of some metals just as salt dissolves in water. These gas particles then “loosen” the electrons in this structure, causing them to be emitted from the metal more readily when heat is applied. This “explanation”, according to Dr. Rentschler, should result in longer-lasting vacuum tubes and accomplish important savings in the size and the number of electric batteries, generators, and other apparatus needed to supply the filament power. Thanks to Dr. Rentschler’s discovery, there are vacuum tubes designed and manufactured after World War II that can function with an anode voltage or power supply as low as the standard 48-volt phantom power in a typical mixing board or desk. For example, the 12AX7 preamp tube can function when supplied with an anode voltage as low as 45 volts DC and yet it is still perfectly happy in a circuit that runs on 250 volts DC power supply.

Even though a typical high quality vacuum tube has vacuum levels at 0.000001 Torr or millimeters of mercury (a typical atmospheric pressure on planet Earth at sea level is 760 Torr or 760 millimeters of mercury) there are places and conditions elsewhere in the cosmos that would put the levels of vacuum found in a typical high quality thermionic vacuum tube’s glass enclosure to shame. The Horsehead Nebula and related celestial mists are more rarefied than the highest or hardest laboratory vacuum – or manufactured vacuum tubes – scientists had ever created so far here on Earth, but in many interstellar regions of the Milky Way galaxy, these whispy mists are banked so deep, cloud on cloud, that they completely hide the stars and galaxies which lie behind them. And yet on average, they are 50,000 times more rarefied than the vacuum enclosed inside a typical vacuum tube.

Astronomical instruments that were considered state-of-the-art during the 1960s had also found out that the convection currents in the outermost atmosphere of the red supergiant star Betelgeuse are comprised of atoms that are more loosely packed than in the most perfect vacuum scientists has ever been able to create here on Earth. These astronomical instruments had even shown that the region surrounding the Horsehead Nebula and the outermost atmosphere of the red supergiant star Betelgeuse is more rarefied by a factor of 50,000 or more than the “vacuum” inside a typical high quality thermionic vacuum tube!

Germanium Transistor Based Audio Power Amplifiers: High Fidelity’s Undiscovered Country?

Given those lucky few who managed to construct and still enjoy their very own, why are germanium transistor based audio power amplifiers seem like an “undiscovered country” in the hi-fi world?

By: Ringo Bones 

Even though during the mid to late 1990s, hi-fi equipment manufacturers have already managed to produced the “holy grail” of the budget conscious audiophile – i.e. solid state power amplifiers of either silicon transistor or MOSFET based that can rival the sound quality of single ended zero feed back triode audio power amplifiers based on either the 300B or the 2A3 vacuum tube – while priced competitively at between 500 to 1,000 US dollars each. Yet unknown to most audiophiles, a type of power transistor – namely of the germanium type – can even approach closer to the sound of a zero feedback SET audio power amp than either silicon or MOSFET types. But why aren’t hi-fi audio power amps or even integrated amplifiers based on germanium transistors flooding the hi-fi market these days? 

To those electronic enthusiasts fortunate enough to dabble with germanium transistors, these types of transistors are very notorious for their over-the-place variability. Even though they are the first ones to be mass produced for consumer electronics use, germanium transistors are somewhat difficult to manufacture and not very stable. Germanium transistors are very hard to produce with consistent parameter quality on a large scale – as in widely varying gain, leakage, noise and overall tone – even germanium transistors manufactured from the same batch. 

The inherently widely varying parameters in germanium transistors means resistor values selected for AC / DC biasing, Q-point operation, feedback and stability that works for one circuit may not necessarily work in another similarly designed circuit even though both use germanium transistors from the same batch. This means resistor values must be “tweaked” – i.e. slightly varied higher or lower in order for a stereo pair of a germanium transistor based audio power amplifier will achieve the same consistent tonal quality. 

And during much of the 1960s, even then commonly available germanium output power transistors – like the now extremely rare AD 149 PNP germanium output transistor which can produced 10 watts in a single-ended configuration if properly heat sinked – remains under utilized by electronic enthusiasts of the day because back then heat sinking was often inadequately specified in published audio power amplifier designs of this sort. Back then, specification sheets for germanium transistor audio power amplifier designs were not totally reasonable and many marginal designs with inadequately heat sinked germanium output transistors that can only safely handle 500-milliwatts boasted 120-watt peak-to-peak power ratings. 

Assuming if you are lucky enough to find “truthful” specification sheets and application notes for germanium transistors these days, it is safe to conclude that it is a more superior semiconductor in comparison to silicon transistors – as in silicon bipolar junction transistors and MOSFETs. Not just on subjective sound quality terms because germanium transistors conduct better than their silicon counterparts because germanium transistors have inherently higher electron mobility, smaller band-gap and requires lower impurities to dope into P-type. Most of this parameters probably explains why a germanium transistor based audio power amplifier based on the AD 149 PNP output transistor that is properly heat sinked to produce a healthy 10 watts in single-ended configuration can easily perform with a sound quality much closer to that of a zero-feedback single ended triode (SET) vacuum tube power amplifier based on the iconic 300B or 2A3 tube in comparison to its silicon based bipolar junction transistor and / or MOSFET counterpart. Just think how better the 1970s era Naim NAP 250 integrated amplifier could have sound if audio engineers at Naim discovered a way to design a germanium transistor based audio power amplifier able to produce 35-watts RMS.