By Bryan Geyer
As commonly creditedto Mark Twain*…
“It’s easier to
fool people than to convince them that they have been fooled.”
—and—
“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)
