The ECF80 Triode-Pentode Vacuum Tube: The Integrated Circuit Vacuum Tube?

Given that it contains two different thermionic amplifiers in a single enclosure, does the ECF80 triode-pentode vacuum tube qualify as an “integrated circuit vacuum tube?

By: Ringo Bones 

Multiunit vacuum tubes may not be able to compete in size and component count with their solid-state integrated circuit counterparts, but if your hankering after that good old robust tone vacuum tube sound, post World War II era subminiature vacuum tubes offer those in a more reasonable sized package. The ECF80 triode-pentode vacuum tube is unusual in that this multiunit vacuum tube can run both its triode and pentode sections at about the same current. The ECF80 makes a great Mu-follower with the pentode strapped as a triode on top and it is often described as a medium Mu triode sharp cut-off pentode. 

Even though it is not featured in the 1959 Mullard Tube Circuits For Audio Amplifiers, Mullard says that the ECF80 was designed for operation up to 220-MHz and it was first introduced back in 1954. As 1950s era television receivers tend to use the PCFnn 300-milliampere filament heater vacuum tubes, the production of the ECF80 with a 6.3 volt heater filament suggests that it was aimed at the 1950s era professional VHF communications market. In a typical VHF application of the ECF80, the triode section is often used as a Wien-Bridge oscillator and has to produce a 5-volt peak-to-peak signal which could allow the ECF80 to be used as the active transceiver tube in 1950s era walkie-talkies. The screen pentode is the mixer. The vacuum tube has two cathodes so the two independent amplifiers are enclosed in a single envelope. The twin anode construction is clearly visible with the triode occupying the much smaller anode. The two sections can be used separately if required and thus extending the versatility of this vacuum tube. 

Given that post World War II subminiature vacuum tubes are primarily used as a cost saving preamplifier and phase-splitter sections of power amplifiers that use 1930s era output power tubes, the World Audio Design K6L6 Integrated Amplifier is probably the most famous application of the ECF80 triode-pentode vacuum tube where it was used as a “novel” phase-splitter. The problem with conventional double-triode phase-splitter, like the 12AX7 double-triode phase-splitter section of the ubiquitous Mullard 5-20, is that its high output capacitance caused by the Miller Effect. This causes high frequency loading on the input vacuum tube and reducing bandwidth, making it very difficult to use appreciable amounts of negative feedback without instability due to the phase shifts incurred. A pentode vacuum tube has a very low output capacitance and high gain due to the shielding effect of the screen grid. This means that the loading on the input vacuum tube is greatly reduced, increasing bandwidth and decreasing troublesome phase shifts. And the WAD K6L6 Integrated Amplifier’s use of the ECF80 triode-pentode vacuum tube as a phase-splitter also eliminated the need of an interstage phase-splitting transformer which also kept costs down. 

The 6AU6 Low Noise RF Pentode: The Retro Vacuum Tube?

Even though modern-manufacture vacuum tube makers have yet to reissue it, does the 6AU6 vacuum tube truly qualify as a “retro preamplifier vacuum tube”?  

By: Ringo Bones 

The NOS or new old stock versions were quite plentiful back in the 1990s when I first got serious into vacuum tube hi-fi as a hobby and even until today modern-manufacture tube makers like Electro-Harmonix, Svetlana and Sovtek still doesn’t have an economically viable need to manufacture their own version of the 6AU6 low-noise RF pentode vacuum tube. And from what I know so far in this hobby, I think the primary raison d’être of those subminiature vacuum tubes manufactured after World War II is to lower the build cost of the preamplifier and phase-splitter stages of power amplifiers that uses output vacuum tubes that date back from the 1930s. 

The 6AU6 low noise radio-frequency pentode vacuum tube was developed by RCA Victor Co. Inc. of New York, New York and it is identical to the EF94. Entered into the Electron Tube Registration List back in October 25, 1945 and entered the Manufacturer’s Literature RCA Datasheet back in October 23, 1945. It has a miniature 7-pin base and was often used as the front end sections of radio and television receivers and it eventually gained widespread use in hi-fi audio towards the end of the 1950s. The 6AU6 is an indirectly heated small signal vacuum tube with a 6.3 volt filament and a 300-milliampere filament heater current. The 6AU6 was described as an RF-IF sharp cutoff amplifier for use in sets with series connected heaters and was eventually superseded by the 6AU6A since November 1956. 

It gained popularity back in the 1990s to keep build costs down when used as first stage preamplifiers of power amplifiers that use power output tubes that dates back from the 1930s – like the 6L6 beam power tetrode – which was first manufactured back in 1936. One of the most popular applications of the 6AU6 low noise RF pentode was in the World Audio Design K6L6 integrated amplifier back in 1995. The WAD K6L6 circuit topology is similar to the ubiquitous Mullard 5-20 amplifier but the 6AU6 low noise RF pentode was configured as a high-gain single-ended amplifier stage. But the relative rarity of the use of the 6AU6 was that the original 1959 Mulalrd Tube Circuits For Audio Applications handbook never used the preamplifier tube in its construction guide of its ubiquitous Mullard 5-20 vacuum tube power amplifier.   

The Type 85 Vacuum Tube: The Rescued From Obscurity Vacuum Tube?

Even though it dates from the Golden Age of Radio, but did you know that despite it is still in current production, the Type 85 vacuum tube has been languishing in obscurity for almost forever?

By: Ringo Bones 

Kara Chaffee of deHavilland Electric Amplifier Company has recently become a cause-célèbre in the audiophile world when she designed those keenly packaged and cleverly priced zero negative feedback preamplifiers that became well-loved by both critics and real-world audiophiles. Even though she had been designing zero negative feedback preamplifiers since the mid 1990s as the “gold channel” preamplifier circuit for recording studio and boutique audiophile label mastering establishments, Chaffee had earned the reverence of audiophiles the world over when she used an obscure but still in manufacture small-signal vacuum tube that dates back from the 1930s Golden Age of Radio – the Type 85 vacuum tube - and turned it into an excellent sounding preamplifier, the deHavilland Mercury preamplifier that even managed to regenerate a sense of wonder even to the most jaded high-end audio equipment reviewer. But to those unfamiliar with it, here’s a brief history of the Type 85 vacuum tube. 

The Type 85 is a 6-pin dual-diode triode multiunit vacuum tube with its distinctive top metal cap – that is a vacuum tube containing several independently acting vacuum tubes in one envelope – as it is composed of twin triodes and two radio-frequency detector diodes in a single glass envelope. First manufactured during the 1930s, it was primarily used as a radio-frequency detector, automatic voltage controller and first stage audio amplifier in AC line operated AM receivers. It is also used as the phase inverter in several 1930s era public address amplifiers. The Type 85 is electrically identical to the octal based 6V7. The Type 85S is a spray-shield type made by Majestic. 

The Type 85 has a maximum plate voltage rating of 250 volts though typical operation as an amplifier the plate voltage is around 135 volts, it has a maximum plate current of 8-milliamperes though in typical operation it is around 3.7-milliamperes, it has a maximum grid voltage rating of -20 volts though in typical use this is around -10.5 volts. Typical in its operation, its heater voltage is 6.3 volts and heater current is 300-milliamperes, amplification factor or mu is 8.3, transconductance or gm is 750 and a plate resistance of 11,000-ohms. By way of comparison, an ECC32 has a plate resistance of 14,500-ohms while the 6SN7 has a plate resistance of 7,300-ohms thus making the Type 85 as having higher output impedance that it’s closest rival preamplifier vacuum tubes. As mentioned previously, the Type 85 vacuum tube contains two diodes which are used as radio-frequency detectors like the 1904 era J. Ambrose Fleming’s radio-frequency detector diode. 

Dating back to the 1930s Golden Age of Radio and it is still manufactured in “sufficient” quantities by Russian and Mainland Chinese vacuum tube manufacturing firms and even sold in antique radio hobby suppliers in South-East Asia at around 5 US dollars each, the Type 85 vacuum tube has never received any recognition in high end circles – unlike its audio and radio frequency power transmitter vacuum tube siblings like the Western Electric 300B, the 211, the 845 and the Russian GM70 transmitter vacuum tubes which became famous during the 1990s era hi-fi boom. The Type 85’s humble origins as a 1930s era audio frequency preamplifier tube did not solidify its image as a much coveted exotic vacuum tube back in the 1990s. 

Internally, the Type 85 is composed of two R-F diodes and a single triode section housed in a single envelope. Such a “compaction” certainly facilitated the mass production of “affordable” 1930s era AM radio designs by combining the front-end R-F detector, amplifier and the automatic voltage controller into one stage. Sadly, this topology is not the sort of vacuum tube likely to engender a cult following either back in the 1930s or in the 1990s. It should be noted that the triode section is almost completely independent of the R-F detector diodes, the only shared element being a cathode sleeve. The top metal cap of the Type 85 vacuum tube is electrically connected to the ground and therefore does not represent a high voltage shock hazard and we should be thankful to Kara Chaffee of deHavilland for approaching this vacuum tube with an open mind and thus discovering its hidden sonic potential.    

     

Sound wise, the Type 85 vacuum tube has a much more gorgeous and creamier midrange than its nearest competition – the 6SN7 vacuum tube – which Kara Chaffee also used in her famed deHavilland UltraVerve preamplifier. And on a side-by-side comparison, the Type 85 even excels the ability of the other famed 1930s era preamplifier tube – the ECC32 – in making modern over-bright over-equalized multi-track 24-bit 192-Khz pop-rock recordings much more pleasing to the typical hardened audiophile’s ears. Even though it is pricier than the ECC88 vacuum tube equipped Musical Fidelity X-Pre, the Type 85 vacuum tube equipped deHavilland Mercury preamplifier sounds much, much better – though the deHavilland Mercury is around 10 times the price of the 250-US dollar Musical Fidelity X-Pre. High cost be damned – or if you have the spare time and the ability to DIY a Type 85 vacuum tube equipped preamplifier – the deHavilland Mercury and its Type 85 ilk can make any reasonably good sounding solid-state power amplifier the ability to create a soundstage as if it is a zero-feedback single-ended triode vacuum tube power amplifier. 

Though most modern vacuum tube reissue manufacturers - like the famed Electro-Harmonix and Svetlana -  has yet to manufacture their own "audiophile grade" Type 85 vacuum tubes, the ones I currently have and used in my preliminary DIY work are from National Union (Made in USA) NOS ones that still register "OK" in my audio-buddy's small-signal vacuum tube checker and a newer Mainland Chinese one whose brand is written in Chinese characters. Both managed to sound great from a vacuum tube perspective and I think the tone produced by a preamplifier using the Type 85 vacuum tube will easily please "tone freaks".

Zero Negative Feedback Preamplifiers: An Audiophile Must Have?

Given the deleterious effect of negative feedback circuits on sound reproduction, would a working zero negative feedback preamplifier soon become an audiophile must have?

By: Ringo Bones

Either by first-hand listening or by reading them in hi-fi oriented publications, negative feedback circuits “suspected” deleterious effect on music signals – like loss of natural timbre, a duller less expressive performance, increased aural fatigue and missing life and energy in reproduced sound – may be the primary consequences of the application of negative feedback. And even though modern electronic industry and the availability of modern solid-state amplification would be impossible without the application of negative feedback, many seasoned audiophiles have deplored its existence since its widespread use near the end of the 1920s. And some seasoned audiophiles even reasoned that the primary raison d’être of audiophile recordings is making solid-state hi-fi gear sound like they are made of vacuum tubes. 

It is no secret that single-ended triode audio amplifiers with zero negative feed back have that magical quality of making less-than-pristine recordings – especially mainstream rock recordings – sounds as if they are made by boutique audiophile labels, literally!!! And it is also no secret that making zero negative feedback single-ended triode vacuum tube audio amplifiers as user-friendly as a typical packaged Bose Lifestyle Systems also makes them prohibitively expensive for the typical audiophile, never mind a first time one. So what would one do, would zero negative feedback vacuum tube preamplifiers provide a more economically viable option? 

Ever since the Musical Fidelity X-Pre vacuum tube preamplifier during the latter half of the 1990s, many “working-class audiophiles” saw it as a godsend of making their real-world priced audiophile solid state amplifiers  - like the Pioneer A-400 Power Amplifier – sound as if it is a single-ended vacuum tube audio amplifier, at least up to a point. But unknown to most audiophiles at the time, the vacuum tubes used on the 250 US dollar Musical Fidelity X-Pre are the same ones used on the 15,000 US dollar Conrad-Johnson ART preamplifier – the 6922 dual triode – the high-voltage Russian equivalent of the ECC88 / 6DJ8 dual triode preamplifier vacuum tube.  

While the circuit on the Musical Fidelity X-Pre is not a zero feedback preamplifier, the negative feedback used is much lower than that typically used on most mass-market solid-state audio gear that, sound quality wise, the Musical Fidelity X-Pre managed to fly rings around comparably-priced solid-state audio gear in its price range at the time. Not to mention that it also has the ability to improve a bit the duff sound quality of most mainstream pop and rock recordings of the time – albeit just a hint of what a full-blown zero negative feedback vacuum tube gear can do. 

Comprising just one stage: a true zero-feedback common cathode or anode follower preamplifier circuit employing paralleled 6922 / ECC88 / 6DJ8 dual triode vacuum tubes. And even though tube purists might argue that the choice of Conrad-Johnson to use 6922 / ECC88 / 6DJ8 dual triode vacuum tubes on their 15,000 US dollar ART preamplifier isn’t optimal, the non use of negative feedback made the Conrad-Johnson ART sound way better than the 250 US dollar Musical Fidelity X-Pre. But is there a potential to make a zero negative feedback amplifier that’s potentially better sounding that the ART and the X-Pre? 

A few months ago, I managed to DIY a common cathode / anode follower zero feedback preamplifier that can use ECC32 / CV181 or other 6SN7 type dual triode vacuum tube provided the requisite biasing voltage adjustments are made. To make such a preamplifier that uses gigantic pre World War II vacuum tubes made before Dr. Harvey C. Rentschler managed to perfect how to manufacture acorn-sized miniature vacuum tubes like the 12AX7 run as noiselessly as a modern solid-state preamplifier necessitated an über-designed power supply with big choke filters and high-voltage capacitors. And just like the 15,000 US dollar Conrad-Johnson ART, it ended up as a two-box preamplifier. Sound wise, it has the ability to make duff recordings sound more audiophile up to a point and it can be connected to a real-world solid-state amplifier like the Pioneer A400 and make it sound really gorgeous. Though a 6SN7 / ECC32 based zero negative feedback preamplifier can never be manufactured and sold as inexpensively as the 250-US dollar Musical Fidelity X-Pre. Well, at least it sounded like a scaled-down ART. 

DSP Based Digital Loudspeakers: Caveat Emptor?

With the vinyl LP revival still incrementally on the rise since the 1990s, are DSP based digital loudspeakers still represent a good buy?

By: Ringo Bones

The favorable review of the DEQX PreMate – a DAC, preamp, room correction and speaker correction all in one box - by Stereophile magazine back in December 2014 had got me thinking and checking out new DSP based digital loudspeaker systems even though my chances of ditching my current system and going “all digital” is just about zero. But given that I’ve heard one being demoed in Hong Kong recently, I wonder if DSP based digital loudspeaker systems, like the one which the DEQX PreMate can be used to retrofit any analog based system given it provided wonderful results with even an analog LP input and it can also handle 24-bit 192-KHz PCM digital / DVD audio and even SACD – that DSP based digital loudspeaker systems has finally come of age? 

The first time that I got the time and money to go shopping and buy a serious high-end audio kit was during the early 1990s. At that time, vinyl LP playback and vacuum tube amplifiers – especially single-ended triodes – where the proverbial bees knees. Back then, I had the good fortune to closely audition a number of DSP based digital loudspeaker systems that offered digital domain equalization and room correction abilities –although not long enough to make me conclude that they are better than an all-analogue system built around vinyl playback. 

One of these was the DGX Audio’s Digital Deconvolution Audio System consisting of the DDA-1 Digital Deconvolution Power Amplifier and the DDL-1A Loudspeakers. It was sold part-exchange to a hi-fi shop I frequent in my end of the woods back in 1995 as the owner upgraded it to a single-ended triode based system. The DGX Audio’s Digital Deconvolution Audio System, to my ears at least, managed to sound like one of those powerful single-ended triode vacuum tube amps employing Nobu Shishito type inverted interstage transformers but it was limited to playing back Redbook spec 16-bit 44.1-KHz sampled PCM material, which I think the owner, sold it because of the impending 24-Bit 92-KHz DVD Audio which was slowly creeping in during that time and it did sound better than CD.  And until this day, the DGX Audio Digital Deconvolution Audio System unit still remains unsold in that particular hi-fi shop despite of the oohs and ahs of everyone who heard it being played.     

During the mid to late 1990s, I was really tempted to buy a “digital equalizer” in the guise of the Z-Systems’ RDP-1 Reference Digital Preamplifier, which is an all digital preamp with tone controls. Despite some favorable reviews and the product did managed to generate a good impression on me after hearing one being demoed in a local hi-fi convention, at the time, I just thought that my system really needed one given that it was already sounding okay to my ears at least and it priced at 5,000 US dollars each at the time (I wonder how much these units are going today second-hand?). And inexplicably until this day, I’ve never heard or read in hi-fi magazine reviews of the Z-Systems RDP-1 being compared to an all vacuum tube based preamplifier. 

Another DSP based digital speaker I had the good fortune of hearing first hand was the Quadrature Model DSP 5 Loudspeaker. Dubbed as the first American DSP based digital loudspeaker back in 1996 because at that time, the only other high-end audio manufacturing firm making one was the UK based Meridian. The Quadrature Model DSP 5 offered digital domain time domain and phase correction that easily made its soundstage presentation more natural than ordinary similar sized loudspeakers on the market at that time and not to mention a midrange creaminess that’s comparable with single-ended triode vacuum tube amplifiers sporting over-sized output transformers. Inexplicably, nobody mentions this product anymore this day and age. Despite of its already long history of relative success, are DSP based digital loudspeaker systems really a good buy or should we exercise “caveat emptor”? 

Jeff Joseph of Joseph Audio commented on the March 1998 issue of Stereophile magazine that: “Digital is still revolving at a rapid pace, with little real consensus in sight. Buying digital speakers involves a significant investment, with the risk that such technology will be rendered obsolete in the coming year. The highway to high-end heaven is littered with yesterday’s state-of-the-art digital products at fire-sale prices.” Fast forward 17 years later and Jeff Joseph’s nugget of wisdom at the time still holds true, some hi-fi stores even have some DSP based digital loudspeaker systems that still remain unsold when they bought them back in 1995 because prospective buyers of such products, despite being wholly impressed, immediately backed out after learning that such products an only “play” 16-Bit 44.1-KHz sampled CDs or Redbook spec CDs despite such products being offered at one-tenth its original price when it was still new back in the early to mid 1990s! Are newer systems that offer room corrections but offer full compatibility – i.e. to analog vinyl and Super Audio CD playback – a better buy for the first time audiophile?