6550 NOS TUNG-SOL & KT88 NOS GENALEX in 2025 – Part II

Introduction:

The 6550 and KT88 are two legendary power beam tetrodes in the audio world. Introduced in the 1950s by American Tung-Sol, the 6550 quickly became a standard for high-power amplifiers; shortly thereafter, British MOV/Genalex responded with the KT88, a beefed-up version designed to handle higher voltages and dissipation. Both, in their original NOS (New Old Stock) versions, enjoy an extraordinary reputation among audiophiles and technicians: the original “black plate” Tung-Sol 6550s and the Genalex Gold Lion KT88s are often considered absolute references in their category.

Below, the sonic characteristics of these two iconic valves will be compared in detail, analyzing: three-dimensionality of sound , musical experience and emotional involvement , tonal balance , mid and low range , tightness and control of sound , as well as indicating in which contexts of use each tends to be preferred.

Sound characteristics comparison

Three-dimensionality of sound

One of the most appreciated qualities of the original Genalex KT88s is their exceptional three-dimensionality and soundstage rendering. Many enthusiasts report that with these tubes the sound literally “detaches” itself from the speakers, filling the room and projecting a wide and holographic soundstage. The spatial presentation is airy and well distributed, giving the impression of being in front of a live event. The NOS Tung-Sol 6550s also offer a wide and deep soundstage, populated by focused and stable images. In more than one audiophile review, for example, they speak of a “wide and deep soundstage, populated by solid images” , highlighting how these 6550s are able to accurately reproduce the position and body of the instruments in space. The differences, therefore, are more nuanced than substantial: the KT88s tend to perhaps give an even more airy sense of space (thanks to their high end extension and the harmonic richness of the band extremes), while the 6550s present the scene in a very natural and coherent way. In both cases, we are at the top as far as three-dimensionality is concerned: several listeners have described the experience with these tubes noting how “the emotional content of the music is conveyed in a splendid way” and how the voices appear realistic and “present” in the sound space. This indicates that both the Genalex KT88s and the NOS Tung-Sol 6550s are capable of an engaging and credible spatial presentation, although with a slight advantage often attributed to the KT88s in terms of airiness and the ability to completely “detach” the sound from the speakers.

Musical experience and emotional involvement

In terms of emotional involvement and perceived “musicality,” experts agree that both tubes are excellent, with some differences in character. Many audiophiles find the KT88 (especially the original Genalex) to offer a smoother, sweeter, and more engaging presentation than the 6550. In other words, the KT88 is considered a “more musical” tube, capable of making listening emotional and enjoyable: its silky, harmonic-rich mid-highs give voices and instruments a refined character that draws the listener into the music. Conversely, some find the 6550 to be slightly “cold” or more analytical – for example, it has been noted that this tube introduces a certain grain or harshness in the mid-highs regardless of the manufacturer, which can reduce the feeling of musical fluidity. A historic tester of Audio Research amplifiers says that he preferred to insert KT88s instead of 6550s precisely because, in his opinion, those who only use 6550s “don't know the true musical potential” of the amplifier.

However, not everyone agrees with this view. There are also those who attribute to the Tung-Sol 6550 NOS an extremely engaging and natural character, perhaps even more “musical” in certain contexts. For example, a very experienced tube technician observes that the Genalex/GEC KT88 can sound “forward and a little sterile at times” , while he believes that the Tung-Sol 6550 “present the music in a more elegant way” . In fact, in more than one detailed review of the Tung-Sol 6550 black plate, it is emphasized how they make you “listen to the music rather than the system” , thanks to their ability to convey the emotions of the recording spontaneously and without effort for the listener. These discrepancies in opinions indicate that emotional involvement can depend a lot on the synergy with the system and on personal taste: there are those who perceive the 6550 as a warm and enveloping sounding tube (even described as “soft, bright, velvety” ) compared to a more energetic and vigorous KT88, and those who feel the opposite, finding in the KT88 greater sweetness and musicality compared to a more “raw” 6550. In short, both tubes in question excel in transmitting the musical message; the KT88 tends to impress with the refinement and expressiveness with which it chisels the sound (often giving an immediate emotional involvement), while the 6550 offers a transparency and balance that in the long term are equally gratifying and can deeply involve the listener in a more subtle way.

Tonal balance

From a tonal balance standpoint, the original 6550 Tung-Sol is generally considered a very neutral and linear frequency response tube. Historically, Audio Research and other manufacturers chose 6550s specifically to achieve a wide and neutral response across the entire frequency range, with solid bass, unemphasized mids, and extended but controlled highs. Genalex KT88s, on the other hand, exhibit some tonal differences: they are often credited with a slight emphasis on the midrange (the famous midrange bloom that many audiophiles appreciate) and a slightly more open and sparkling high end. In practice, KT88s can sound more “open” and bright in the highs than equivalent 6550s, which instead maintain a softer character in the highs. A direct comparison showed, for example, that a quartet of original Genalex KT88s produced an “airy and silky” high register, while modern (reissue) KT88s were closed and even aggressive in the treble – a sign of the refinement of the NOS compared to the current versions. On the other hand, the black plate Tung-Sol 6550s are known for having very extended but never strident highs, keeping the sibilants soft and natural: the high frequency transients (of instruments such as the acoustic guitar or the triangle) are reproduced in a delicate and refined way.

In the midrange, as mentioned, the KT88 often adds a slight fullness : this translates into perhaps rounder vocals and instruments with more body in the mid-bass. The 6550, being neutral, can seem drier in these frequencies – which for some is a plus (more transparency), for others a minus (less warmth). An interesting anecdote from a user sums up the tonal differences in a picturesque way: “KT88 = (loud, powerful sound); 6550 = (soft, clear and smooth sound)” , indicating that the KT88 offers a more evident and bright impact, while the 6550 has a more delicate and mellow timbre. Although this is a personal analogy, it partly reflects the general idea: the original KT88s tend to sound a bit more sparkling in the highs and robust in the mids , while the original 6550s maintain a “flatter” balance , free from emphasis. However, both cover the entire audio spectrum excellently: there are no significant shortcomings either in the high frequencies (where both the KT88 and the 6550 NOS reach well beyond the useful 20kHz) or in the low frequencies, as we will see shortly.

Mid range and low range

Focusing more specifically on the low-midrange and bass , we can identify some subtle but significant differences between the Tung-Sol 6550 and the Genalex KT88. The midrange (vocals, solo instruments, guitars, horns) in the KT88 is often described as rich and “full-bodied” : the term midrange bloom used by audiophiles indicates precisely that slight emphasis and roundness that makes the KT88’s mids so pleasant and “full”. The 6550, on the contrary, delivers the mids in a very transparent and clean way, without particular emphasis; some therefore find them colder in this area, or report a slight graininess in the high-mids (especially with some examples) that makes them less smooth than the KT88. It must be said that this “graininess” is not found in the best NOS Tung-Sols (which, on the contrary, have extremely fluid mids), but was typical of some later production 6550s. Overall, we can say that the mids of the NOS 6550s are neutral and detailed , while the mids of the NOS KT88s are warm and slightly more forward . The 6550s favor articulatory clarity while the KT88s, having a response a bit more similar to the EL34 in this region, probably render vocals with more warmth and fullness in the fundamental register, hence a more euphonic presentation on certain voices or instruments.

Moving on to the low end , both the 6550 and KT88 are tubes that are known to deliver deep, authoritative bass in a good circuit. The main difference noted by some is in the amount and control of the bass. In many cases, the 6550 is credited with having slightly more bass: for example, Audio Research stated that they preferred 6550s in their amplifiers because they felt they “offered better low-end performance.” In fact, one reviewer noted that “the 6550 tube has slightly more bass,” although the KT88s are certainly not lacking in this area. The original Genalex KT88s, on the other hand, can dig just as deep into the bass, but do so with remarkable quality : the NOS KT88’s bass is described as fast, tight, and authoritative . In a comparison, vintage KT88s were noted to have a bass that was “full of authority and speed,” while some modern replicas sounded slower and sluggish in comparison – a sign that the originals excel at bass retention .

The black plate Tung-Sol 6550s, on the other hand, deliver a very accurate and controlled bass. An audiophile reviewer reports an interesting aspect: “the bass character can initially seem a little light, until you realize how accurate and musical it actually is. As you turn the volume up, the bass strings vibrate so cleanly that you can almost visualize them moving through space” . This means that the 6550s do not artificially inflate the mid-bass (hence the initial impression of less weight), but instead deliver the low notes with extreme cleanliness and realism , bringing out the real textures and vibrations of the instruments. So the “power” of the 6550s’ bass is there, but it is delivered without tails or smudges: control and definition are the key words.

In comparison, the original KT88s offer similarly extended bass but with a slightly different character: thanks to their ability to handle more power, in some amplifiers they can produce a bass that is a bit wider and “softer” in terms of impact, or simply the same. In instrumental settings, some players describe the KT88s as “cleaner and punchier” on the lows, while the 6550s (spec. 6550A version) add a bit of “snarl” (grunt/aggression) when pushed into saturation. This difference is attributed to the different peak current capacity and the way they saturate: the 6550s remain clean up to very high levels, but beyond a certain threshold they introduce a very solid and aggressive bass distortion (defined in the manuals as “heavy metal-like” for its powerful and compact impact). The KT88s, on the other hand, tend not to dirty the lows too much even when pushed, maintaining a cleaner and rounder sound, with less harshness. In terms of pure quantity of bass , however, there is no “lack” in either: both, if well polarized, give very deep bass. The choice is played on the character of these basses: more slam and “physical” impact slightly in favor of the 6550 (as many testimonials indicate), against a slightly more “restrained” but very fast and articulated intonation of the KT88 (which for their part boast more air on the higher octaves, balancing the tonal presentation).

Sound control and maintenance

By endurance and control we mean the behavior of the tube at high power, the ability to drive difficult loads and to maintain sound coherence when dynamics vary. Both the 6550 Tung-Sol and the KT88 Genalex, being high power tubes, are known to handle very high volumes with great stability. However, there are differences in the way they do it. The 6550 are historically considered robust and highly reliable tubes: they handle high currents well and tend to remain linear up to the limit of their power. In fact, technical manuals report that the 6550 "remain cleaner up to the maximum" and, only when really pushed beyond, they produce a very compact and powerful distorted sound. This characteristic translates into an extremely controlled sound and free of unwanted compression as long as you operate in the linear range; in practice this means always tight bass, no "overflow" or indecision in driving the woofers and a dynamic that, as long as the tube is in linear conditions, faithfully follows the signal. When the signal exceeds the threshold, the 6550s return a very solid and defined type of clipping/distortion – as mentioned, almost “heavy metal” , that is, hard, dry, indicating excellent resistance without sudden collapses. This type of behavior is ideal in applications where maximum control is required (for example, amplifying electric basses or subwoofers, or maintaining headroom in orchestral peaks).

The Genalex KT88s, on the other hand, can handle even higher voltages and powers (up to ~800V plate, versus ~600V max for the 6550s according to the classic specifications), so on paper they offer even more headroom. In practice, their handling is comparable if not superior, but the sonic control manifests itself in a slightly different way: as already noted, these tubes introduce a little more harmonic compression in the mid-high range (of a pleasant kind) when pushed, due to their similar nature to the EL34 in that area. So, instead of remaining totally “still” and then suddenly saturating, the KT88s can slightly soften the transition to saturation in the mids, which can give the perception of a less rigid sound. It should be emphasized that we are talking about extreme behavior: within the operating limits, the NOS KT88s offer exceptional control and linearity at least as much as the 6550s. For example, in the listening notes mentioned above on the original KT88s, their ability to return micro-details (such as drumstick hits on wood) is highlighted with agility and speed superior to any other tube, a sign of excellent reactivity and no laziness in following transients. Furthermore, the fact that the KT88 has an enormous capacity to deliver peak current means that, in low impedances or difficult loads, it manages to control the speaker very well, avoiding “graining” the sound. Ultimately, the difference in tightness and control between these tubes is subtle: the 6550 can appear ultra-solid and immovable (to the point of becoming almost “hard” when saturated), the KT88 ultra-robust but with a slightly more yielding/gentle nuance in saturation (which some might interpret as less control, others as musical compression). Both excel at maintaining authority over the sound : for example, on a loud orchestral kick, a pair of vintage KT88 GECs will provide volume and impact without breaking down, as will a pair of black plate Tung-Sol 6550s – any differences will be in the color of the sound tail and the feeling of tightness or fluidity with which the wave is reproduced.

Contexts of use and preferences

Having outlined the general sonic characteristics, it is useful to consider in which contexts each valve tends to be preferred or excel, distinguishing between:

1. High-end home hi-fi systems
2. Amplification for musical instruments (guitar/bass)

1. High-end home hi-fi systems

In high-end hi-fi audio systems, where the goal is the highest quality of music reproduction, power tubes make a significant contribution to the final sonic character. In this context, both the NOS Tung-Sol 6550 and the NOS Genalex KT88 are considered top-notch choices – but the original Genalex KT88 is often cited as the optimal solution when looking for the ultimate in performance . In comparison tests of various types of tubes, the vintage KT88 Gold Lion emerged as the best overall valve , just ahead of the 6550 Tung-Sol: for example, in a famous hi-fi shoot-out , the Genalex KT88 (1969) obtained the highest overall score (4.87/5), closely followed by the 1963 Tung-Sol 6550 (4.77/5), which however was the best of the 6550s. In that same comparison, it was reported that “the Tung-Sol was voted the best 6550, while the Genalex was voted the best KT88 and the best overall tube of the test” . This reflects the opinion of many audiophiles: in top-end hi-fi chains , the Genalex KT88 perhaps expresses the maximum sonic potential, offering that combination of breadth of soundstage, tonal refinement and detail that make it unique. Characteristics such as silky and airy highs, rich micro-details and midrange musicality give a hi-fi amplifier equipped with original KT88s an open and “holistic” presentation, where each instrument finds its place in space and the music flows with engaging naturalness.

That said, the Tung-Sol 6550 NOS remains a popular choice in hi-fi systems, especially among those who prefer a balanced and realistic sound. In some systems, in fact, it may be the preferred one: for example, if the speakers or the chain already tend towards a very open and analytical sound, the Tung-Sol 6550 may provide a slight mitigation of those rough edges, thanks to its slightly warmer/rounder midrange tone and extremely fine grain. Some describe its contribution as adding a sort of “romanticism” to the music, without losing definition. One reviewer wrote that, overall, “nothing beats the Tung-Sol 6550 for consistent and excellent sound in every area; […] the black plate version is the ne plus ultra for a natural and musical presentation.” At the same time, he acknowledged that other tubes can surpass it in specific areas (mentioning the GEC KT88 as the absolute reference in the rendering of certain percussive transients). This confirms that in the high-end hi-fi field, the original KT88 perhaps offers those last degrees of air in the highs and microdynamic rapidity that define perfection, while the 6550 Tung-Sol ensures an overall balance that is difficult to match, with a sound that many define as “more natural and relaxed” . In terms of usage preferences , therefore, in high-end domestic systems the Genalex KT88 tends to be used when aiming for maximum resolution, extension and three-dimensional impact , while the Tung-Sol 6550 may be preferred when looking for greater tonal linearity and sweetness (for example in systems with a very analytical imprint, or for those who listen for long sessions and want a less fatiguing performance). Both, however, represent uncompromising choices: they are rare and expensive valves, often difficult to find in paired quartets today (the original GEC KT88s have practically disappeared from the market and the few remaining examples reach dizzying prices), but capable of raising the sound output of high-level valve amplifiers.

2. Musical instruments (guitar and bass amplifiers)

In the field of musical instrument amplifiers (electric guitar heads, bass amplifiers, etc.), both the 6550 and KT88 tubes have been used in various designs, especially for high-power applications. The choice between the two types depends largely on the type of sound and feel you want under your fingers, as well as the design of the amplifier itself.

• Guitar (High Power) : KT88s are used in some guitar amplifiers that require crystal clear clarity and lots of headroom . For example, the historic 200W Marshall Major featured KT88s to allow for very high volumes with very little distortion – a solution appreciated by guitarists like Ritchie Blackmore who wanted to obtain a very powerful but clean sound. KT88s, having a response partly similar to EL34s in the midrange (a typical Marshall “plexi” tube), are able to give the guitar a gritty sound in the midrange but with more solid bass and more open highs than EL34s. Many guitarists describe KT88s as tubes with a “big, bold and clear” sound on the six strings: in a comparison, it was said that “the KT88s seem wider, more decisive and clear in tone, while the 6550s are a little more muffled and perhaps with less midrange” . This clarity and breadth make the KT88 a favorite among those who play genres that require clean cleans or very open distortion (such as classic rock, hard rock, or ambient, where a KT88 amp can provide a clean platform upon which to build the sound with pedals and effects). On the other hand, some guitarists who play heavier/metal genres or are looking for a certain type of aggressive attack prefer the 6550. In fact, the 6550 is known to provide a more aggressive and “mean” sound when pushed into saturation. In a high gain amp, the 6550 keeps the sound extremely tight on palm mutes and fast riffs, while adding a sort of “bark” in the distortion that many of us associate with extreme metal. One player described the 6550 as having “more low-mids and an angry kind of bark,” compared to the KT88’s being “soft, big, and smooth.” This indicates that the 6550, in a guitar context, can give more push on the mid-low frequencies (useful for powerful rhythms) and a harder distortion, while the KT88 sound more linear and with a softer drive on the highs (useful for clear and clean solos). In summary: for guitar the KT88 is often chosen for cleanliness, headroom and open sound , the 6550 for punch, aggression and gain holding . It must be said that many manufacturers (e.g. Bogner, Fryette/VHT, Hi-Tone/Hiwatt custom ) offer the possibility of mounting both 6550 and KT88 depending on the user's preferences, confirming that there is no absolute "best" but it is a question of taste and style.
• Electric Bass : In tube bass amplifiers, both families are common, with the 6550 historically being the most present. The Ampeg SVT, probably the most famous tube bass amplifier (300W), uses six 6550s in a push-pull parallel configuration: Ampeg chose the 6550s (GE in the 70s) for their robustness and ability to provide powerful bass and reliability on the road . Many rock bassists appreciate the sound of the 6550 in these amps because, when pushed to the volume, it provides that slight aggressive drive to the low frequencies that “dirties” the bass sound in a musical way, adding character (the classic SVT growl ). KT88s can be used as an alternative in some of these amps (bias-dependent), and several bassists have experimented with the substitution: the reported results indicate that with KT88s the bass sound becomes “a little cleaner and punchier” , maintaining great depth but with less audible distortion at high power. This confirms what the Ampeg technical manual also suggests: “the KT88 produces a great low-end but with a sound more similar to an EL34 in the mids and highs; the 6550s are more robust and remain clean all the way up, and when they distort the sound is more solid and with a tighter low-end (ideal for a 'heavy' type of distortion)” . In practice, on a bass amp the KT88 will give big and rounded bass with a slightly more “British” tone in the mids, while the 6550 will offer equally deep bass but a little harder/more defined and will allow you to push the amp into overdrive while maintaining a very dry attack. For the bass player this translates into: KT88 for clean, defined, hi-fi sounds, 6550 for rock, aggressive and “gritty” sounds. Here too, preferences are personal: for example, some people like to put KT88 in their Hiwatt or Marshall-style bass amplifiers, generally obtaining more headroom; others remain faithful to the 6550 for their “classic” punch. In conclusion, in the instrumental field, the 6550 is often preferred by those who want attack, raw power and reliability (many metal and hard rock amplifiers have them or have them as standard), while the KT88 is preferred by those looking for cleanliness, dynamics and breadth (for example for cleaner genres, classic rock, fusion or to have a very neutral basic sound to sculpt with effects).

Conclusion: Comparing the Tung-Sol 6550 NOS and Genalex (GEC) KT88 NOS tubes highlights how both belong to the elite of audio tubes, each with its own sonic nuances. In summary: the KT88 often offers a more three-dimensional, detailed and “musical” sound (silky mids, airy highs, deep and fast bass), while the 6550 exhibits a balanced, neutral and powerful character (linear midrange, very controlled bass and slightly more incisive in quantity). In high-end hi-fi, many consider it a clash of the titans where the KT88 wins by a hair in terms of absolute finesse, but the 6550 retains a unique charm for its natural musicality that some even prefer. In the world of guitar and bass, the choice becomes part of the sound design: the 6550 for grit and stability , the KT88 for cleanliness and openness . As is often the case in audio, the ideal choice will depend on application and taste: fortunately, both the original Tung-Sol 6550 black plate and the Genalex Gold Lion KT88 offer a top-notch listening experience that continues to captivate generations of enthusiasts and professionals. Ultimately, the NOS 6550 vs NOS KT88 comparison does not see a clear “winner,” but rather two very high-level options that should be calibrated to taste and intended use.

“Analysis written by Vacuum Tubes Treasures with the support of technical sources and testimonies collected in specialized forums, historical publications and audiophile comparisons. The opinions reported reflect direct experience and the literature available at the time of writing.”

Sources: Technical and audiophile forums (e.g. AudioAsylum, Audiogon, AudioKarma, TalkBass), historical articles and shoot-outs (Vacuum Tube Valley, enthusiast reviews), and contributions from industry experts were consulted. Citations included in the text are from these sources and support the comparative claims.