What about the KT120?

We are occasionally asked about the KT120 tube. Most of the inquiries make an observation about some competitor adopting the tube and making claims for its virtues. In many cases these same manufacturers did much the same previously for the Russian made Tung-Sol 6550 and the Russian made Genalex KT88.

Generally speaking, the KT120 is an instance of Eastern European & Russian tube makers assuming that scaling up the internal structure of a tube makes everything better. However, this is not unambiguously true with vacuum tubes, so let’s get out the KT120 datasheet and look at the results, both pro and con (or, if you just want the bottom line, your KT88 based VAC amplifier should only be used with tubes that conform to the KT88 specification).

On the negative side:

1) Stray capacitances are increased by the larger structures, in this case by 50% to 70% for tetrode operation. The KT120 datasheet does not give the numbers for triode connection, so no conclusions can be drawn there, and that typically is more critical. Increased stray capacity reduces high frequency bandwidth. In amplifiers that have marginal stability this conceivably could lead to parasitic oscillations. It’s off the KT88 spec, and would have been rejected if it showed up in a production batch at the old British GEC/Genalex/M-O Valve group.

2) The heater current draw is up by as much as 22%. This puts a significant additional load on the power supply, and could lead to reduced heater voltage with attendant sonic degradation, or in some cases cause the power transformer to overheat and fail. Again, off spec for a KT88.

3) The data sheet shows transconductance about 9% high. Ordinarily not a significant concern, but it does relate to another problem in the characteristic curves that we’ll get to in a minute.

4) The KT120 data sheet is showing a surprisingly high 14% THD figure for typical operation. That’s much too high, and one might speculate about a typographical error, if not for the characteristic curves… we’ll get to that in a minute, too.

5) The maximum allowable grid resistances quoted for the KT120 are much lower than for the KT88, representing loads on the driver tubes that might be 1.12 x to 4.3x heavier than with a KT88. The typical reason for limiting the maximum grid resistance value is due to gas currents in the tube. So, this number might suggest that the manufacturer is concerned about gassy tubes.

6) Examining the plate family characteristic curves for the KT120, the first thing that emerges is the extremely sharp transition between the vertical and horizontal sections of the curves; the ‘knee’ is very sharp. The KT120 shows this behavior at much lower power levels than does a good KT88, and this may lead to more complex distortion with reactive loads (i.e., loudspeakers).

The second thing that stands out is the area of ambiguity that develops in very low output levels extends over a much larger range with the KT120 than with a good KT88. Further, this misbehavior continues to twice the power output level than it does with a good KT88.

Next, the spacing between the stepped grid potential curves is much less even than with a good KT88. Ideally, when you increase the signal on the grid by a unit of voltage, you want to see the same percentage increase in output current regardless of your starting point. In this regard, consider 5 volt grid increments, starting at -5 v (near max power), and then going down. With a good KT88, the intervals are 100%, 100%, 69%, and 56% as you head toward cutoff. With the KT120, it is 100%, 78%, 56%, and 39% … which is much less linear, and tends to confirm the surprisingly high THD figure quoted in its data tables.

Lastly, due to the behavior just discussed, the tube does not cut off cleanly at the expected point; it’s about twice the proper spec point for a KT88.

Now, positive points:

1) In triode mode, the max plate/screen voltage rating is up 50 volts over the KT88 to 650 volts. In the tetrode mode, the plate rating is up 50 volts to 850 volts, but the screen rating is unchanged at 600 volts (thus the effective rating for normal ultralinear techniques is still 600 volts, just as with a normal KT88).

2) The anode dissipation rating is up 43% over a standard KT88, so you could push more idle current through it, and potentially get more output power if you also raise the plate voltage (and thus make the amplifier incompatible with the KT88 or 6550). However, the screen dissipation rating is unchanged at 8 watts, so there is more to consider in practical design calculations than first meets the eye, and it might be difficult to exploit this.

3) The KT120 ratings showing an 8% increase in maximum cathode current, but for the tetrode mode only … this makes no sense at first blush, and so may be a typo. Either way, the 230 mA KT88 spec and triode KT120 spec is more than adequate.

The practical results:

1) Plugging a KT120 in place of a KT88 will not give you significantly more output power any more than switching one KT88 for another might. Keep in mind, though, that even with the same production run of tubes, one pair might yield 95 watts and another pair 118 watts; that’s just part of the normal manufacturing batch variation. Maximum power output generally corresponds to how ‘hot’ the tube wants to idle. The more negative the required grid bias voltage, the greater the possible drive signal swing before the onset of grid current, i.e., more watts out. However, hot tubes are not necessarily the best sounding, most stable, or most linear.

2) More power output on a consistent basis generally will require some redesign of the amplifier, which may in turn make it incompatible with KT88s or 6550s..

3) You should expect more distortion with the KT120, possibly particularly at moderately low listening levels and with highly reactive loudspeakers.

4) The situation is much as it was with the first generation EiRC KT90 (Mk I) back in 1990. You could redesign an amp to give more power, but the tube didn’t really conform to KT88 specs and had higher distortion.

5) If you use the KT120 in amplifiers designed for the KT88 or 6550, on rare occasions it may fry a power transformer.

6) You’re likely to give up a few kHz of high frequency response.

7) It might last longer in very hard service if you leave the settings as they would be for a KT88.

8) An amplifier developed to exploit the KT120 fully would not be compatible with the KT88. Since the KT120 is only made by one factory and has very little history, there might be difficulties with retubing such an amplifier in the future.


At the end of the day, it’s always fun for marketers to advertise a magical new ingredient, so it’s understandable that the KT120 has made quite a splash. Many manufacturers using it probably are hoping that it will be a more reliable power tube, and the comparison is clouded by the fact that many of the currently manufactured versions of the KT88 aren’t done as well as they should be, so this is not a knock against other manufacturers giving them a try. However, do not expect the KT120 to be a panacea.

Earlier I noted that there has been a progression with some other amp manufacturers from the Russian TungSol 6550, then to the Russian Genalex KT88, and now to the Russian KT120. At each change, the new tube is hailed briefly, and then thrown aside. At the introduction of each of these tubes we have tested them and found them to be wanting with respect to reliability and sound quality in comparison to our normal KT88; thus, we have not adopted them and do not recommend them.

VAC does not approve use the KT120 for VAC instruments. Our KT88 based amplifiers should only be used with tubes that conform to the KT88 specification.

Some Thoughts on Amplifier Design

(Including why VAC does not make single-ended power amplifiers.)

By Kevin Hayes

Some audiophiles, drawing on single-ended experience, will assume that a triode amplifier produces vast amounts of second harmonic distortion. Interestingly, the triode vacuum tube in and of itself is the most linear amplifying device yet devised. It produces the least distortion, and that distortion is predominately second harmonic, which is relative musical in sound. By contrast, pentodes produce greater distortion, and the third harmonic tends to dominate. A transistor generally looks like a very bad pentode.

To state the obvious, a single-ended circuit must be Class A1 or A2. A push-pull amplifier may be Class A1, A2, AB1, AB2, B1, or B2. Class A indicates that each output tube handles the full cycle of the audio signal, while AB and B allow some of the devices to cut-off during a portion of the cycle. Subscript “1″ indicates that no grid current is drawn by the output tube, while subscript “2″ indicates that the output stage enters the grid current region of operation. In the grid current region, the impedance presented to the driver stage is abruptly lower, and drive power is required, not just drive voltage. The grid current region tends to be rather non-linear, and most designers will avoid it. Single-ended and push-pull circuits may be built with triodes, beam power tubes, pentodes, or the latter two in ultra-linear (“partial triode”) mode.

In a Class A push-pull circuit, there is a natural cancellation of even-order harmonic distortion products. The cancellation is not complete, of course, but it would be unusual to see large amounts of second harmonic distortion from a push-pull circuit (Radiotron Designer’s Handbook, 4th ed., 1954, page 571).

Applying this to the Renaissance Series, the circuit is strictly Class A1, and is push-pull with the exception of the very first 6SN7 triode, which operates under conservative conditions and is thus relatively free from distortion (Moir, High Quality Sound Reproduction, page 264). This single phase splitter triode is interesting, in that the very same electron current flow creates the antiphase push and pull signals, which, given equal impedances within the amp (they are), match exactly. As such, the signal is not being Cuisinart-ed as with most phase invertors, an objection voiced by many single-end advocates.

Note that a push-pull circuit has no significant ability to cancel odd-order distortion products. If low distortion performance is required, one must avoid the generation of odd-order harmonics in the first place. A good triode tube meets this requirement.

Three difficulties are encountered in the design of a single-ended tube power circuit. Firstly, there is no mechanism to naturally cancel even-order harmonic distortions. Secondly, significant new distortions may arise in the output transformer. Thirdly, available power output is greatly limited in a single-ended design, such that it will be spending more of its time in overload for a given volume level.

For background, recall the old children’s science project in which a length of wire is coiled around a nail and then connected to a battery. The DC current from the battery flows through the coil to create an electro-magnet. The primary winding in a single-ended output transformer is similar to this, and also creates an electro-magnet. The full DC current for the output tube(s) flows through the transformer primary and strongly magnetizes the core of the transformer. Thus, much of the core’s ability to couple the audio signal is used up by the non-audio DC current, and causes the core to saturate asymmetrically with audio signals (Radiotron, page 247). Even below saturation, this DC bias increases distortion, especially at low frequencies (Moir, page 283; Radiotron, page 217). Adding parallel output tubes for more power directly increases the DC magnetization current, thus exacerbates the distortion problem, and requires that more primary inductance be designed into the output transformer.

To deal with this, a less saturable core alloy is often used, but this causes poorer coupling of the audio signal (Radiotron, page 207). Alternately, a large “air gap” may be introduced into the transformer geometry, which again causes aberrations in coupling. In most cases, a greater amount of core material is used, which may in turn increase some low level (B-H) non-linearities. The final result is either a higher degree of distortion (all harmonics with the second dominating, increasing with decreasing frequency), a measurably peaked frequency response, or both.

Radiotron summarizes, “…fairly high distortion has the effect of apparently accentuating the bass…It should be emphasized that this is not the same as true bass, and does not constitute fidelity” (Radiotron, page 616) and notes that this trick was used “In small [radio] receivers, in which the loudspeaker is sometimes incapable of reproducing the bass” (Radiotron, page 676).

Since the distortion in the single-ended transformer is asymmetrical, a system based around this type of amplifier might be more sensitive to absolute polarity.

In a fairly complete summary of single ended output transformers, Duncan Kelly concludes, “Direct current is thoroughly undesirable in audio transformers” (Transformer Distortion, Audio, March 1959, page 44).

These problems do not arise in a push-pull amplifier, in which the primary halves are oriented in opposing DC directions (Moir, pages 282-284; Radiotron page 207). The DC magnetization force is thus canceled and is not an issue unless the push and pull output tubes are adjusted to draw different currents. Any imbalance in DC idle current will lead to greater distortion at low frequencies, just as in a single-ended design (Audio Cyclopedia, 2nd Ed., 1969, pages 1449-1450). The Renaissance Series maintain a high degree of DC balance due to the self-correcting nature of 300Bs under individual cathode bias.

Please note that the distinction between push-pull and single-ended Class A triode designs does not stem from the tube itself, but from the natural distortion cancellation in push-pull and from the transformer problems in single-ended. Since a single-ended transistor amplifier may omit the output transformer, it may display yet another set of characteristics.

How the ear deals with the characteristics of a single-ended tube power amplifier is quite interesting. The human ear is a non-linear encoder of information, and excess second harmonic blends in to form the impression of an additional sub-harmonic. This technique was deliberately employed in small radios in the 1940′s to create a richer sound, then referred to as “synthetic bass” (Radiotron, pages 616, 676). The Radiotron Designer’s Handbook notes, “It should be emphasized that this is not the same as true bass, and does not constitute fidelity.”

The frequency response errors of some single-ended tube amplifiers tends to create a high frequency boost and a low frequency cut, in one case approximately +/- 3 dB (Stereophile, Jan. 1994, page 108). The subjective effect of the low frequency loss might perhaps be partially offset by the second harmonic distortion.

Earlier I noted that the triode could be the most linear of amplifying devices. I left this small hedge because it is possible to build a rather flawed triode as well. The 300B is a highly linear tube. In fact, the high voltage supplies in the Renaissance Seventy/Seventy do not vary by one volt over the range from idle to clipping, indicating an absence of rectification effect (distortion). The type 845 is also a very linear tube, although requiring higher drive voltages, which can result in more overall distortion. The 211 is a bit more problematic; it requires a large drive voltage and drive power to deliver full output. In such operation (Class A2) the tube is said to “draw grid current.” Entering the grid current region may cause a sort of crossover behavior as the driver stage is abruptly called to provide significant power into a suddenly lower impedance load (Moir, page 281; Ravenswood, Fixed Bias, Audio, Feb. 1958, page 48). Amplifiers running subscript 2 operation often may be identified by the use of a power tube (2A3, 300B, etc.) in the driver position. The 211 and 845 also require very high plate voltages (800-1200 VDC), about twice that of the 300B, and desire a higher load impedance, both of which complicate output transformer design.

It has been asserted by some contemporary designers that one can not hear second harmonic distortion of 10% to 20%, such as may be produced by some single-ended tube amps. However, I find no corroboration of this, and in the Renaissance Seventy/Seventy hold the sum of all harmonic distortion, including the second, to approximately 2% at clipping without negative feedback.

It is also worth noting that multi-grid tubes, such as the KT88, connected as triodes often do not exhibit linearity comparable to the 300B, 845, or 211 tube types, although this connection may have some advantages over traditional pentode/beam power operation.

In any event, I do not think that THD as such is actually what we hear. I believe that it shadows something that we do hear in the context of analogue tube equipment. As a case in point, there was a 1987 Journal of the Audio Engineering Society (JAES) publication of a study by Dolby Labs’ Louis Fiedler, in which, if memory serves, .005% THD in a digital system was clearly audible to all listeners. Several times this amount would not be detectable in a similar tube analogue set up. Some other measurement likely will be found significant in the context of the a/d/a cycle, and will probably be meaningless when applied to tube amplifiers. At the end of the day, the human auditory system is a marvelously arcane recognizer of patterns, and we are not able to mimic it with our test instruments.

Feedback is another interesting topic. Traditional theory gives feedback high marks, but this analysis changes when we consider that the “error” signal is fed back into a non-linear amplifier. Due to this, feedback may lower the overall level of distortion, but it also multiplies its order. For example, if an amplifier naturally produces second harmonic, feedback will create a second harmonic of that second harmonic, which is the fourth harmonic. If the basic amplifier has second and third, the fed-back amplifier will contain second, fourth, sixth, and ninth. As is well known, the higher orders of distortion are more objectionable to the ear than lower orders, and odd orders more offensive than even orders. Thus it may be possible to lower the level of distortion products and still have the distortion be more audible.

The application of negative voltage feedback also reduces an amplifier’s measured output resistance, i.e., it raises the “damping factor.” Here again, the measurement fails to capture the essence of things. In the case of a feedback amplifier, better control of speaker motion is said to occur because the speaker’s excess motion creates a voltage (the back e.m.f.) which enters the feedback loop via the amp’s output terminals. The amplifier then acts in a manner opposite the error signal to correct for it. However, like many theories, this is an oversimplification and, in practice, the opposite result may be obtained. There are several reasons:

  1. The motion of a speaker’s voice coil former may not match the acoustical output due to cone break-up modes and room acoustics.
  2. The motion of the coil former is being sensed by the voice coil. The coil is designed to be a good driver, but is a lousy sensor, primarily due to its high inductance, which will create phase anomalies in the back e.m.f.
  3. The back e.m.f. may pass through a cross-over network, which will again alter phase and frequency relations.
  4. A differing back e.m.f. from another driver may be summed in via the crossover, making a composite signal that does not match either individual driver.
  5. The speaker leads may cause additional phase shifts.

By the time the error signal reaches the power amplifier it is arguably an erroneous error signal. As the power amplifier attempts to correct for this signal, it may actually do the exact wrong thing with respect to the speaker’s acoustic output. Subjectively, I have noted that high feedback amplifiers tend to give the bass a one note boom on certain speakers, and tend to create an electronic glaze in the midrange, possibly attributable to this process.

With regard to damping, I suspect the best approach is to design the amplifier to have as low an output resistance as possible in a static sense and use little or no feedback. As it happens, the minimum natural output impedance is obtained from a low mu triode amplifier (Williamson & Walker, Amplifiers and Superlatives, JAES, April 1954, page 79).

None of the foregoing is an endorsement or condemnation of any particular amplifier design. The engineering information seems against single-ended tube amplifiers; to be fair, however, perhaps the added distortion offsets something else in the recording chain, at least under some conditions. Then again, perhaps something we do not yet know how to measure something that is better with single-ended designs. The critical ear will help provide the answer: if, for example, part of the sonic character of a single-ended design is attributable to excess 2nd harmonic distortion, then that amplifier will probably sound somewhat full, mushy, or thick, even on instruments that should be clean and fast. This is the characteristic I perceive in such amplifiers.

Nothing made by the hand of man is perfect. It seems to me that the audio designer’s task is to push the frontier of compromise as far away as possible, and then to balance the imperfections in a fashion that serves musical truth.

As we often say, in a battle between theory and the real world, the real world always wins. Or, as Daniel von Recklinghausen once said, “If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you’ve measured the wrong thing.”