KT88Here you will find a small collection of my all time favourite tube, theKT88. I must stress that there are a lot of good versions of this exceptional tube out there and that these are just a few that I feel I have had enough experience with to share my views on.
What type of valve is a KT88?
The KT88 is a Beam power valve with an anode/plate dissipation [originally] of around 35 watts, it is sometimes referred to as a pentode by some people, but in fact the clue is in the title “KT” which stands for “kink-less tetrode” The main feature differences in the Kink-less Tetrode compared to the Pentode [and early Tetrode’s] was the use of two shaped metal plates that ran the length of the grids and was placed outside the grid support rods but within the anode cylinder. The effect of these plates was to confine the electron flow to the path through the grid wires by forming an electrostatic field when connected to a low voltage. This had the effect of repelling secondary electrons back to the anode’ and by not having the secondary electrons reach the screen grid; the kink in the Tetrode curve was eliminated. Thus the Kink-less Tetrode was born.
In short the” KT” denotes a beam power valve with lower distortion: and a more linear characteristic than traditional power Tetrode’s and Pentodes…
The KT88 is one of only a few power valves that was specifically designed and developed for use in high quality audio equipment!
warning! beware of fake KT88,s especialy GEC, Genalex and svetlana…
Svetlana KT88 [Sadly no longer available, Archive information only]
Here we have the original ultra reliable [winged C] Svetlana KT88. I have used this tube in all kinds of amplification and to date, no matter how hard its been run (within its limits) I can’t remember one ever failing! The impeccable reliability of this tube is not its only virtue – its sonic qualities are very good too. It has a clean, crisp top end with plenty of detail, sound-staging is good, with excelent dynamic detail across the audio spectrum. If you’re looking for a modern day KT88 that’s a very, very good all-rounder, then look no further.
Warning! the svetlana KT88’s you will see on this site are the genuine winged C versions…
Unavailable at the moment
JJ Tesla KT88
This is another great tube. Like the Svetlana, it has stood the test of time in reliability standards, but also it has certain sonic qualities that most tube and solid state audiophiles alike crave, e.g. dynamic detail, good mids, etc. Also, it has a very unique bass characteristic; the clue to this is the mention of ‘solid state’. Anyone who has auditioned these tubes will know exactly what I mean. If you get a chance, try them out.
With a massive plate dissipation of 70 watts, the Tung-Sol KT150 is the most powerful octal beam tetrode ever produced. A pair of KT150s can allow an amplifier with a power output approaching 300 watts to be built.
The Tung-Sol KT150 glass envelope is a special egg shaped balloon that was developed to improve thermal dissipation and maintain a superior vacuum for the best sound. The fact that the glass envelope has no flat sides means the Tung-Sol KT150 has no problems with microphonics.
This valve is now well known amongst audiophiles but Tube Distinctions was one of the first companies in the UK to recognise its sonic qualities hence it is fitted as an optional extra in some of our equipment.The KT120 has a plate/anode disapation of 60 watts compared to the 35watts of the original GEC, and 42 watts for the svetlana winged C. I have completed extensive electrical and listening tests on this valve and can safely say with ease that it has exceeded my initial expectations on both counts, Therefore in my humble opinion it is at least equaly as good as the cryo-treated Svetlana. The KT120 can in most cases be used in place of the KT88, However, care must be taken with bias adjustment options in some equipment. If unsure then please inquire.
Thinking of changing from KT88 to KT120 ?
The new KT120 is a very good replacement for a lot of power amps using either KT88 or 6550, however, I must stress that it is not identical in electrical parameters and therefore there are some important differences to consider i.e. they will take a little more current from conventional AC heater supplies [average is around 100ma once valve has settled] which for high quality well made equipment, is usually not a problem, yes, you may find that the mains transformer will run a little warmer etc.
Also, the bias supply usually will need careful adjustment to exactly match the same steady state current as a KT88, however, with the KT120 having a considerably higher anode/plate dissipation of 60 watts compared to even the highest available KT88 [Original Svetlana winged C of 42 watts] it can be clearly seen that the KT120 when run as a direct replacement for KT88 and 6550 will have seemingly a much easier life.
Bearing the potential higher power dissipation in mind, some amplifiers [mainly grid biased] can indeed be tweaked to produce even more power without getting anywhere near the maximum plate dissipation of the KT120.
If you have a [conventional] cathode biased amplifier, care must be taken to make sure that the bias resistor values and wattage fitted for KT88 do not allow the KT120’s to draw excessive current which could indeed lead to premature failure of the valve and equipment. On a positive note then, excellent results can be achieved using the new KT120 valve, but certain tests and precautions should be adhered to before doing the swap over, if your not sure then either consult your equipments manufacturer or a qualified technician for advice.
Well, you’ve probably guessed by now, we have come to the king of the audio beam tetrodes – the original ‘GEC KT88’. this tube was introduced in the late fifties and its appearance bears very little resemblance to those early versions, whose looks can only be described as a KT66 on steroids! But, as the years went on throughout the 1960s, 1970s, 1980s and 1990s, to the present day, this tube was to become the kingpin in many a serious audiophiles listening room. In my opinion, it has unbeatable sonic and reliability qualities and has been used in an abundance of equipment (not all audio) with exceptional results. If you already own some treasure them, on the other hand if you are financially able to pay the ever-increasing prices for new and used versions, grab them while you can !!!
Please note, old stock valves carry a one month from purchase warranty only.
Hints and tips on buying GEC KT88
There is a lot of mis-information being written on various sites thru-out the internet about used GEC KT88 at the moment, so much so that I have decided to share with everyone interested some of the knowledge I have gained over the many years I have been associated with this exceptional audio beam tetrode, especially about what to look for when thinking of parting with your hard earned cash! First of all appearances can be deceptive in judging whether a valve is any good or not, this is especially true of the GEC KT88.
The main reason most people are put off buying used GEC KT88 is because of what I refer to as “brown getter syndrome” this is where the getters [originally silver] have discoloured, and in severe cases disappeared all together! Although this is an indication that the valve has had many hours of use, it is by no means the “be all and end all” sure way of knowing if the valve is any good. On the contrary, I have had many GEC KT88’s over the years that fit this description, especially ones that have come from industrial and communication facilities, that said I have been surprised time after time with how good a valve in this sorry state has tested. Many although having little or no getter left, upon testing still had full emission!
Shorts and heater to cathode insulation
Testing for shorts and heater to cathode insulation will reveal much more about a valves condition; this is especially true when considering the KT88.
One of the main faults a GEC KT88 can suffer from is low heater to cathode insulation; for e.g., when a KT88 is in good condition it will generally have a relatively high heater to cathode insulation, typically in excess of 10 Meg ohm, [10 million ohms] after high usage this insulation can start to break down, and in severe cases cause total cathode to heater shorts, resulting in catastrophic failure of the valve, therefore testing the heater to cathode insulation will result in a much better understanding of how good the valve is, rather than just relying on physical appearances. Note, in many cases I have had GEC KT88 of new appearance that upon testing have revealed very low heater to cathode insulation.
Warning! be wary of fake GEC KT88,s they do exist! if your not sure of what you may be buying, make sure you look carefully at the internal structures, and compare them with known genuine items, if your still not sure then seek profesional advice before you part with your hard earned cash!
Early and late versions
The very first GEC KT88 was very similar in appearance to its earlier and less powerful little brother the KT66, all be it fatter, and could indeed have been considered in both size and power as a KT66 on steroids, also it had only one getter [top] as did the first of the later classic shape.
But the version we all recognise is the one pictured below [right] with its silver base ring, three getters, and green/blue GEC logo. The lesser known earlier [and much rarer] version is the single getter type which although less desirable amongst audiophiles[due partly to fakes] is no dud, and with the ever decreasing availability of the three getter type the single getter version should now be seriously considered.
The later versions of the GEC KT88 [below left] are often [but not necessarily] slightly shorter and have the pale yellow and green/blue logo, which is also a different shape to the original early logo, the silver base rings also differ in size.
Last but not least, the very last of the original versions of the GEC KT88 were produced around the late 1970’s, [up to 1979] however’ sometime in the 1980’s another version of GEC KT88’s appeared which’ on close inspection differed slightly in appearance having “shiny black” anode plates’ unlike all earlier versions which had matt grey/black plates, also it was not un-usual for these later versions to have double or even triple stampings denoting GEC KT88 CV5220 etc. these late versions are sought to have been of Chinese manufacture [though not confirmed] and therefore are not as desirable as all the earlier versions’ and are not in the same league sonically, therefore they can sometimes be sourced at a more reasonable price, while stocks last!
Rule number one,
When considering purchasing a set of GEC KT88s make sure they have been thoroughly tested, this is equally as important for new and used versions,remember “appearance is no guarantee that the valve is any good” If the seller has no means of testing the valve then I hope that what I have written here will help you to be aware of what you may be getting. Test results for cathode insulation on GEC KT88s is very important, and can tell you a lot about the condition of the valve and most importantly its expected longevity and reliability aspects.
Last but not least, brown getters, badly worn logos, etc are no indication of a KT88s true electrical condition, thorough test results are the only sure way to determine whether the valve is any good, and “remember” nice clean appearance is not an automatic guarantee that the valve is any better [electrically] than one that has “battle scars” from many hours of use!
There is [in my experience] no modern direct equivalent KT88 that combines the sonic qualities and the impeccable reliability of the original GEC!!!