On Vacuum Tube Power Amps…

By Bryan Geyer

As commonly credited to Mark Twain*…
“It’s easier to fool people than to convince them that they have been fooled.”
“The trouble with the world is not that people know too little; it's that they know so many things that aren't so.”

Were this not the case, the production, sale, and use of receiving-type vacuum tubes would have permanently perished throughout the world. Instead, some tubes still cling to life in the modest audiophile and amplified guitar markets**.

The continued use of vacuum tubes in guitar amps is easy to comprehend. That’s because guitar buffs treasure the sound of 2nd harmonic distortion, and tube amplifiers present high levels of that noxious stuff. Plus it’s very easy to create more whenever you overdrive (clip) a vacuum tube amplifier’s output stage.

The on-going acceptance of vacuum tube design in high-end audio power amplifiers is more puzzling, especially given the premiums paid for power amplifiers that always…
            …operate at high temperatures, at all output levels,
            …utilize hazardous (potentially deadly) high voltages,
            …perform poorly when compared to solid-state equivalent***,
            …require endless costly maintenance†,
            …and violate every precept of the “go green” environmental initiative††.

Yes, it’s a puzzle. Science seemingly gets smothered by expectation bias and confirmation bias; refer Audioholics: “Do Our Expectations Determine Our Experience of Sound More Than We Realize?” at https://www.audioholics.com/room-acoustics/mind-over-music.

The audiophiles of today that use tube-type power amplifiers tend to be < age 65. This tells me that they’re not likely to have owned a power amplifier during the era when all electronic equipment was vacuum tube dependent. The genial glow of those orange filaments is not always reminiscent of happy hassles for those of us that serviced such stuff. We recall the hours devoted to trouble tracing and repair; maybe charred salvage.

A significant number of audiophiles say that they prefer the “warm” sound of a tube-type power amplifier. What’s generally overlooked is that the source of this warm coloration traces to a design limitation that’s implicit with all traditional tube-type power amplifiers. Because the output stage of a vacuum tube power amplifier operates at high source impedance (several kΩ), it’s not optimum to directly drive a low impedance load (e.g.: a loudspeaker). A load that presents low impedance is best driven by a source that exhibits still lower impedance. Indeed, a value of zero source impedance would be ideal. The classic means of curing this circuit-to-load impedance disparity is to introduce an output transformer between the tube circuit’s final stage and the external load. This (big, heavy, and expensive) magnetic device will then, by virtue of its differing internal winding ratios, transform the high impedance state into a low impedance source, so that the signal can better mate with its intended low impedance load. Audio engineers conclude that it’s probably this processing path through the windings of the output transformer that create the perception of warmer sound, so “tube sound” likely doesn’t trace directly to the presence of vacuum tubes—it’s the consequence of coupling the circuit’s output stage to the load by means of a transformer. What you’re hearing is “transformer sound.”

Tung-Sol Ad, 1955
As always, this beneficial design fix (add transformer) is not without restrictive limitation. An audio output transformer is a non-linear device by nature, with low end frequency response that’s largely dependent on the mass of its magnetic core, and high frequency response that’s subject to the vagaries of leakage inductance and stray capacitance. It’s also prone to waveform saturation at high signal amplitudes, and that tends to create undesired harmonic distortion due to soft clipping. Further, design constraints generally limit the output source impedance to some 3 or 4 Ohms. Lower Zout is impractical, and higher Zout options must sometimes be applied (when increased load impedance permits) in order to linearize a tube amplifier’s voltage gain, as the latter will often vary (by several dB) with changes in the loading impedance (refer “Special Footnote,” at end).

Modern solid-state power amp circuits don’t have to contend with any of these messy generic issues that plague tube-type power amplifiers. A solid-state power amplifier’s natural internal source impedance will be very close to zero; normally somewhat < 0.1 Ohm, and it will be stable. That near zero value is more than an order of magnitude below the source impedance of any transformer aided tube-type power amplifier, so there’s negligible undesired source/load interactive variance. In addition, the solid-state power amplifier can provide direct coupling to the load, so there’s no transformer interface imposed. This assures better transient damping, with wider, flatter frequency coverage; also less distortion, firmly fixed voltage gain, lighter weight, and (potentially) lower cost. As a result, the signal that gets delivered to the load will be a highly accurate representation of the input, and the sound that’s perceived will be determined purely by the input source and by the load, not by the compound interaction of a higher driving impedance in tandem with the load impedance. This is why a well designed solid-state power amplifier has no intrinsic sonic signature, it’s functionally transparent.

Vacuum tube users sometimes recommend a particular power amplifier + speaker with a preferred connecting cable, with choice of cable based on listening tests. Beneficial cable effect is potentially possible when a tandem source/load termination happens to form a euphonious (aurally pleasing) filter. However, do realize that any termination so marginally sensitive that the niggling impedance variance conveyed by a few feet of cable can cause audible change must be highly unstable to start. Merely moving such cable might provoke further change. This shaky state is precisely why audio engineers extoll the load invariant advantage assured by driving the speaker from a near-zero (≤ 0.1Ω) source impedance, something that’s naturally inherent with solid-state power amplifier design.

Despite all of the noted technical and environmental deficiencies, there’s no ethical deceit implicit in the promotion and sale of high-end vacuum tube power amplifiers. Natural acceptance and approval of such product reflects the innocence of human trust, just as with faith in a deity or a belief in astrology. Blind trust defies rational explanation, but many regard trust as noble—no reasoning required. Others are more realistic—they want to see the data. How you side in this issue is your choice, but don’t let the inexorable tide of audiophile groupthink††† swamp straight thinking and good science.

BG (February 25, 2020)

*A popular attribution; probably apocryphal.

**All of the U.S., British, Dutch, and German producers of vacuum tubes are now either defunct or ceased making tubes some four decades ago. The entire world market for new tubes is presently served solely by obscure sources in China, Russia, and Slovakia. New tubes that get labeled with the names of long deceased and respected sources (like Tung-Sol, Mullard, et al) exist simply because a Russian entrepreneur bought the right to reuse dormant copyrights. (Tung-Sol died in the early 1960s.)

***Ruler-flat power response, near-zero (<  0.1%) total harmonic distortion (THD) at full rated power, and ultra-low output impedance (less than 1/10th the Zout of a typical tube amp) is now routine in the case of solid-state power amplifiers. Identical measurements made on the very best vacuum tube models show that they can’t approach those values. For example, typical tube-type power amplifier THD limits are ~ 16X to 50X worse (1% to 3% THD instead of 0.06% max.) than as specified for a popular “mid-market” solid-state power amplifier (Parasound Halo A23+).

†A matched pair of vintage “NOS” Tung-Sol 6550 output pentodes = $220. Refer…http://vintagetubeservices.com/pentodes/. Back in the era (1963-1976) when I used a Marantz model 8B stereo power amplifier (four EL34 output tubes, rated 35 Watts/channel), I had to replace the output stage pairs on the order of every 30 months to maintain optimum “in spec” operation. I also readjusted the bias settings 2-3 times/year to minimize IM distortion (SMPTE) at full output, using a Heathkit AA-1 analyzer and a ’scope.

††A typical 150 Watt/channel stereo tube-type power amplifier consumes more power (about 240 Watts) when in benign standby than a 55 inch Sony LED/LCD TV set does when in actual use. Consider: 240 Watts of standby (just idle, no signal output) power consumption is equivalent to continuously burning four 60 Watt incandescent light bulbs without providing any light; just heat. That’s conspicuous waste.

Special Footnote: Unlike solid-state power amplifiers, vacuum tube power amplifiers do not exhibit stable fixed voltage gains. Their intrinsic gain will typically vary by several dB, dependent on nominal load impedance. To minimize this undesired variance, the output transformer often exhibits multiple output taps, e.g. 4Ω, 8Ω, 16Ω. When the nominal rated load impedance permits (i.e., rises), the use of these higher Zout taps will assist in stabilizing the voltage gain. (Refer AudioXpress, issue dated Feb. 2020, p. 64, fig. 5)

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Meet the Staff

Meet the Staff
John J. Puccio, Editor, Publisher, Reviewer

Understand, I'm just an everyday guy reacting to something I love. And I've been doing it for a very long time, my appreciation for classical music starting with the musical excerpts on the Big Jon and Sparkie radio show in the early Fifties and the purchase of my first recording, The 101 Strings Play the Classics, around 1956. In the late Sixties I began teaching high school English and Film Studies as well as becoming interested in hi-fi, my audio ambitions graduating me from a pair of AR-3 speakers to the Fulton J's recommended by The Stereophile's J. Gordon Holt. In the early Seventies, I began writing for a number of audio magazines, including Audio Excellence, Audio Forum, The Boston Audio Society Speaker, The American Record Guide, and from 1976 until 2008, The $ensible Sound, for which I served as Classical Music Editor.

Today, I'm retired from teaching and use a pair of bi-amped VMPS RM40s loudspeakers for my listening. In addition to writing the Classical Candor blog, I served as the Movie Review Editor for the Web site Movie Metropolis (formerly DVDTown) from 1997-2013. Music and movies. Life couldn't be better.
Karl W. Nehring, Contributing Reviewer

For more than 20 years I was the editor of The $ensible Sound magazine and a regular contributor to its classical review pages. I would not presume to present myself as some sort of expert on music, but I have a deep love for and appreciation of many types of music, "classical" especially, and have listened to thousands of recordings over the years, many of which still line the walls of my listening room (and occasionally spill onto the furniture and floor, much to the chagrin of my long-suffering wife). I have always taken the approach as a reviewer that what I am trying to do is simply to point out to readers that I have come across a recording that I have found of interest, a recording that I think they might appreciate my having pointed out to them. I suppose that sounds a bit simpleminded, but I know I appreciate reading reviews by others that do the same for me -- point out recordings that I think I might enjoy.

For readers who might be wondering about what kind of system I am using to do my listening, I should probably point out that I do a LOT of music listening and employ a variety of means to do so in a variety of environments, as I would imagine many music lovers also do. Starting at the more grandiose end of the scale, the system in which I do my most serious listening comprises an Arcam CDS50 CSD/SACD CD player, Goldpoint SA4 Passive Preamp, Legacy Audio PowerBloc2 amplifier, and a pair of Legacy Audio Focus SE loudspeakers. I also do a lot of listening while driving in my 2016 Acura RDX with its nice-sounding ELS Studio sound system through which I play CDs (the ones I especially like I rip to the Acura's hard drive so that I can listen to them whenever I want) or stream music through the system using my cell phone. For more casual listening at home when I am not in my listening room, I often stream music through the phone into a Vizio soundbar system that has remarkably nice sound for such a diminutive physical presence. And finally, at the least grandiose end of the scale, I have an Ultimate Ears Wonderboom Bluetooth speaker for those occasions where I am somewhere by myself without a sound system but in desperate need of a musical fix. I just can't imagine life without music and I am humbly grateful for the technology that enables us to enjoy it in so many wonderful ways.
Bryan Geyer, Technical Analyst

I initially embraced classical music in 1954 when I mistuned my car radio and heard the Heifetz recording of Mendelssohn's Violin Concerto. That inspired me to board the new "hi-fi" DIY bandwagon. In 1957 I joined one of the pioneer semiconductor makers and spent the next 32 years marketing transistors and microcircuits to military contractors. Home audio DIY projects remained a personal passion until 1989 when we created our own new photography equipment company. I later (2012) revived my interest in two channel audio when we "downsized" our life and determined that mini-monitors + paired subwoofers were a great way to mate fine music with the space constraints of condo living.

Visitors that view my technical papers on this site may wonder why they appear here, rather than on a site that features audio equipment reviews. My reason is that I tried the latter, and prefer to publish for people who actually want to listen to music; not to equipment. My focus is in describing what's technically beneficial to assure that the sound of the system will accurately replicate the source input signal (i. e. exhibit high accuracy) without inordinate cost and complexity. Conversely, most of the audiophiles of today strive to achieve sound that's euphonic, i.e. be personally satisfying. In essence, audiophiles seek sound that's consistent with their desire; the music is simply a test signal.

William (Bill) Heck, Contributing Reviewer

Among my early childhood memories are those of listening to my mother playing records (some even 78 rpm ones!) of both classical music and jazz tunes. I suppose that her love of music was transmitted genetically, and my interest was sustained by years of playing in rock bands – until I realized that this was no way to make a living. The interest in classical music was rekindled in grad school when the university FM station serving as background music for studying happened to play the Brahms First Symphony. As the work came to an end, it struck me forcibly that this was the most beautiful thing I had ever heard, and from that point on, I never looked back. This revelation was to the detriment of my studies, as I subsequently spent way too much time simply listening, but music has remained a significant part of my life. These days, although I still can tell a trumpet from a bassoon and a quarter note from a treble clef, I have to admit that I remain a nonexpert. But I do love music in general and classical music in particular, and I enjoy sharing both information and opinions about it.

The audiophile bug bit about the same time that I returned to that classical music. I’ve gone through plenty of equipment, brands from Audio Research to Yamaha, and the best of it has opened new audio insights. Along the way, I reviewed components, and occasionally recordings, for The $ensible Sound magazine. Recently I’ve rebuilt--I prefer to say reinvigorated--my audio system, with a Sangean FM HD tuner and (for the moment) an ancient Toshiba multi-format disk player serving as a transport, both feeding a NAD C 658 streaming preamp/DAC, which in turn connects to a Legacy Powerbloc2 amplifier driving my trusty Waveform Mach Solo speakers, supplemented by a Hsu Research ULS 15 Mk II subwoofer.

Mission Statement

It is the goal of Classical Candor to promote the enjoyment of classical music. Other forms of music come and go--minuets, waltzes, ragtime, blues, jazz, bebop, country-western, rock-'n'-roll, heavy metal, rap, and the rest--but classical music has been around for hundreds of years and will continue to be around for hundreds more. It's no accident that every major city in the world has one or more symphony orchestras.

When I was young, I heard it said that only intellectuals could appreciate classical music, that it required dedicated concentration to appreciate. Nonsense. I'm no intellectual, and I've always loved classical music. Anyone who's ever seen and enjoyed Disney's Fantasia or a Looney Tunes cartoon playing Rossini's William Tell Overture or Liszt's Hungarian Rhapsody No. 2 can attest to the power and joy of classical music, and that's just about everybody.

So, if Classical Candor can expand one's awareness of classical music and bring more joy to one's life, more power to it. It's done its job. --John J. Puccio

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Readers with polite, courteous, helpful letters may send them to classicalcandor@gmail.com

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"Their Master's Voice" by Michael Sowa

"Their Master's Voice" by Michael Sowa