December 2004 Tech's Files
Amplification Primer: The Sound of Vacuum Tubes
Part-2: Preamp Tubes, the 12A?7 Dual Triode series
Click here for link to Power Tube Pix
©2004 and updated (schematic added) December 2008
links fixed March 2012
by Eddie Ciletti'Tis the season to snuggle up to a warm ampli-fire with a good book (or this magazine). Amplification is the topic, specifically, the sound of vacuum tubes. I must have been a good boy this year because thermionic specialists, Groove Tubes (GT) and electro-harmonix (EHX), filled my stocking with goodies. Preamp tubes will be evaluated this month; power tubes will be reported next month. Researching and writing about this subject is like trying to control a volcano, so expect to see continued reports throughout the year as well as expanded on-line versions.
SONIC DISCIPLINE
A triode vacuum tube and a single transistor are the most basic forms of amplification. The initial difference is obvious: size! Tubes are "big" on many levels, but in the most literal sense, real estate dictates population density. This form of sonic discipline, an enforced minimalism, is generally responsible for the "color" we associate with tubes, especially guitar amps. Neither "Color" nor the lack of same is vacuum tube exclusive (think API and Neve modules). It is quite possible to design a tube circuit that is more linear and less like the "expected" signature using a combination of less aggressive sounding tubes, more gain stages and some negative feedback, for example.
Miniaturization — in the form of the Transistor and followed by the Integrated Circuit — ultimately increased the number of gain stages that can be squeezed into a smaller space. The resulting performance "improvements" have reduced distortion in the window between nominal levels all the way up to clipping (guitar levels). When done right, linear amplification — the proverbial wire with gain or "neutral" sound — has its place in every facet of the recording — especially the monitoring process — so long as we can still play with our crayons too…
LABEL, LABEL, LABEL
Over twenty-four tubes were evaluated, a small sampling of which are detailed in Table-1. To avoid potential mix-ups, shrink tubing was applied as a label. Each tube was cycled several times through a series of tests. Both the hardware and the test parameters were fine-tuned along the way to make the results as meaningful as possible.
The first step was tester to measure each triode section on a TV-7/U military-type tube tester; the resulting number is only significant relative to the "minimum reading" as stated in the operator's manual. Some applications require identical performance from each partner in a dual-triode, such as when driving a pair of power tubes. For a screaming hot preamp, the gain becomes a priority, hence the use of the 12AX7, a.k.a. 7025 and ECC83. High gain comes at a price, an increased potential for noise and microphonics, but there are alternatives…
NOTE: The unbalanced LOOP-THRU (Isolated O/P to pedals) feature was disabled for these tests and used as an input.GAIN, GAIN, GAIN
The preamp "emulator" is a versatile test circuit with three modes: High Gain, Low Gain and Variable Treble Boost, the latter not used for these tests. (The circuit is detailed online as Figure-1.) For the 12AX7, the difference between HI and LO gain options was about 6dB, less for the other tube types. Before attempting to measure distortion, it was necessary to determine the point just before asymmetrical hard clipping, which, at about +35dBu, generated 10% distortion using the GT-ECC83 (selected for the V-1 guitar amp position) as reference. The level was then dropped until the high-gain and low gain distortion readings saddled 1%. This turned out to be 18dB and 12dB down from +35dBu, for the respective gain options, for the same GT-ECC83. Consider this window the nominal level region. NOTE: This tube is modeled after the European cousin to the 12AX7, hence the name.
dIsToRtIoN
From low gain to high gain — a mere 6dB change — the distortion increased 2x to 4x with one exception. For all tubes save one, the distortion was primarily 2nd order, an octave up from the 1kHz test frequency. Here’s your big clue, folks. In music, octaves add richness to an arrangement. In an audio circuit, harmonic enrichment makes things sound bigger. When the tube in Table-1, GT-12AX7M-2 was scrutinized for why it behaved differently than all the others, it was found to have about as much 3rd order harmonic distortion as 2nd order.
In the test circuit, adding a cathode bypass capacitor increases gain (and distortion) by eliminating a form of negative feedback. The Notes Section of Table-1 show that this simple circuit variation is the cause of the distortion and not the 6dB level change. Without the bypass capacitor, the distortion at +35dBu is cut in half.
JUDGEMENT DAY
Even without listening, the distortion and circuit variations clearly demonstrate the "big magic" most often attributed to tubes is primarily circuit related, of course the tubes help! And not just that there is 100x to 1000x more distortion than an opamp, but the broad window of "harmonic embellishment" from nominal to just before clipping. An opamp at the threshold of visible symmetrical clipping generates about .5% distortion, consisting of mostly odd-order harmonics.
Lest you become fearful that ALL tube circuits have this much distortion, please note that with the exception of guitar preamps, most hi-fi / preamp designs employ more finesse along with a bit of negative feedback to lower distortion. For example, Total Harmonic Distortion plus Noise (THD+N) for the D.W. Fearn Mic Preamp is 0.2% and this, after passing through two transformers, two gain stages and an impedance converter — that’s two or three tubes worth, none of which are of the12AX7 variety. The 12AY7 has 3dB to 5dB less gain, depending on the circuit, and about one-third the distortion even in the high gain mode.
NOISES
There were two tests for noise (measured and by ear) for spectral (hiss and "rocks") and microphonics. All low-noise metal-film resistors were used in the test preamp as a way to minimize the color and noise variables. (From a guitar-o-phile’s perspective, the carbon variety may be preferred and I am told that modern carbon types perform better over time than their 40-year-old predecessors.) The "spectral" noises, when present, were generally constant. Very few of the tubes had major noise issues partly because several had been pre-selected. Noise measurements and audio samples will be provided online.
"Microphonic" describes a mechanical sensitivity that can be tested by tapping on the tube and listening. Ideally the response to such stimulus should be a muted "thunk." A resonant "ding" is a sure sign that the tube will "sing along" if located in the same space as the speaker. Microphonic tubes have a greater potential to feedback and squeal.
IT’S ALL IN THE GENES
There are so many issues and opinions about the sound of tubes, not the least of which are the design variations of supposedly identical types, like the 12AX7 / 7025 and ECC83 family. While the critical stuff is hidden to the eye, the plate structures are the most obviously visible part of any tube. Of my samples - New Old Stock (NOS) or Used Vintage Tube Selection (UVTS) - Mullard, RCA and Telefunken look similar, but never identical, as are what I believe to be earlier Sylvania versions. Later Sylvania-ECG-Phillips types look like elongated versions of GE’s style. There are black plates and gray plates, smooth plates and corrugated plates, long plates and short plates along with some Euro variations that look like other tube types, such as the 6DJ8 and 6GH8. The point of this detour is that these are all models for today’s versions and it is obvious that everyone is experimenting.
If you’d like to know more about the history of these tubes, and how modern tubes are selected, the large format Tube Amp Book (by Groove Tubes) is a wonderful resource. It might surprise you to learn that GT reveals the sources for all of their tubes. On the opposite coast, look closely at the tubes that have come through the Sovtek / EHX pipeline (via New Sensor Corp.) and you’ll see their heritage as well. All pay homage to their forebears. I believe you can still get good tubes by paying extra for the pre-tested versions. Buying untested, off-the-shelf tubes is a crap shoot.
SUMMARY
Recording, audiophile and guitar enthusiasts may appear to have dissimilar needs and preferences, but it’s not as if tubes were made specifically for guitarists, then or now. The 12AX7 was a consumer-grade tube then, and while the 7025 was its low-noise "select" version, in both cases, it was designed to minimize the tube count by providing more gain than any other tube. As such it tends to be wild and unruly, so if your guitar amp (V-1 or V-2 position), mic preamp or Chinese Tube mic is too noisy or microphonic, try a selected, low-noise, low microphonics version. Conversely, replacing the 12AX7 with a 12AY7 will chill things quite a bit. Have it your way! With the 12A?7 series, you are free to experiment without fear of damage (even if it’s not a perfect circuit match). With known good tubes, performance is then in the hands of the designer. I certainly learned how circuit variations affect distortion.
The power tube tests will follow next month. Click here for sidebars worth detailing tube and transistor basics plus the preamp emulator schematic.
Eddie would like to thank Aspen Pitmann and Steve Good at Groove Tubes, Rick Stevenson at Electro-Harmonix for supplying the tubes and D.W. Fearn for assistance with this article.
REFERENCE LINKS
 |
|
 |