The KT88 / KT66 /6550 / KT90 / KT120 Updated Directory
Updated: 01/01/2013
The purpose of this KT88 / KT66 /6550 / KT90 / KT120 page is to give an overview about what KT66, KT88. KT90 and types and brands exist, and what you can do with the this tube. Also the most common Data sheets are given for these tubes, and links to KT88 and 6550.

Historical KT88 and 6550 and KT90 Data sheets here (choose Data sheets / Manufacturer scans, then "K" and the rest is clear)


Chapter 1
Chapter 2
    Not really off topic: The TT21 and GU50 tube
    KT88 Art Impression

KT88 Tube Curves measured

    Testing of KT88 with the L3-3 tube tester
    KT88 Data sheets
    KT88 Links
Chapter 3
    Not really off topic: The 8417 tube
Chapter 4
Chapter 5
  The infamous KT94 and KT100
Chapter 6
    KT88 Links
Chapter 3
    Not really off topic: The 8417 tube
Chapter 4
Chapter 5
  The infamous KT94 and KT100
Chapter 6


The age of tube developments has ended around 1935. Indeed after this date, there was no jump forward in tube technology. It was only refinement and miniaturization, which was new, but not an invention.

Historically, there has always been competition between Europe and the USA, breaking things down to a level, such as the refusal to use a metric system in the USA, or the refusal in Europe to set industrial standards. Of course each side invented their own tube sockets, as if there were not enough of those yet. Even today it can be very clearly observed that a popular tube socket will make a tube more accepted, and vice versa. The most popular socket is certainly the octal socket, though technically speaking it is not the best.

Add to this, the Europeans have patented the pentode. So for 25 years, the Americans had to pay a licence or find another solution. Well, and as things are with competition, they tried another solution, called the beam tetrode. A beam tetrode is a work around the pentode patent, and has no other purpose. A beam tetrode works like a pentode, but has the third grid replaced by deflection plates. That works also, but still not as nice a real third grid. However the beam tetrode did it's job and the best know family member of this kind is probably the 6L6. Today 6L6 is used mainly in guitar amplifiers, because of it's "distorted" sound.

After the patents ran out, the USA gave up on the beam tetrodes, and real pentodes were made there as well. I want to put up front this situation with the pentode patent, because it is often forgotten, but still many of the tubes and developments resulted from this situation.

In Europe the EL34 was "the" big pentode which was the result of a long evaluation. It starting with the AL1, the first large output pentode, for radios. After that AL4 was developed. Then with 6.3Volts EL3 came and with the coming of the octal sockets, EL34 was born. In the USA it was the more sloppy 6L6 that won the race. The EL34 is a real pentode, and the 6L6 was the beam tetrode, and also the "El Cheapo". In the UK there was the KT66 invented which was a better quality 6L6.

After this in the USA from the 6L6 the larger 6550 was derived and in Europe from the KT66 the larger KT88 was derived . Saying that KT66 and 6L6 are almost the same that leaves us with four very popular power pentodes. Any other differences are just playing with numbers and pin-connections. The end of the evaluation is the KT88 and the crown is the EL156.

However EL156 was a part of the socket type variation war, every country trying no make OODST sockets. (Our Own Damn Socket Type). Many really great tubes got forgotten before they had a chance to get remembered. In the list of forgotten belong beauties like the EL3n, but also EL156 and EL509, and GU50.

The EL156 never had a break through because of the socket. This is a pity, because the EL156 socket is much nicer than Octal. It has a guide pin like Octal, but then 10 pins with clear positioning too. So unlike with Octal, no idiot can press in the tube in the wrong position, by using two hands. So any broken off guide pin can not occur with EL156. After El156 there was a side step called EL509 which was unique because it had a frame grid and Zirconium gettering. However it uses a magnoval socket, another socket type people didn't want any more. .

Mother of all Audio Power PENTODES.

Smaller pre-type of EL34. Beautiful coke bottle glass, and perfect pentode inside. No NOS spotted, but good used tubes are cheap. Collector's item.

Low cost, reliable tube. Cheap and good beam tetrode, with some distortion though. Break through of the beam tetrode.
Better construction 6L6, made in UK . This is a beam tetrode. KT stands for "Kinkless Tetrode".
Better construction 6L6, made in USA
Better construction 6L6, made in USA
Real pentode Can do higher voltage. Works very nice in Class A also.
larger 6L6
Upgrade of 6L6, designed by TUNG-SOL.This is a beam tetrode. KT stands for "Kinkless Tetrode"
Original idea by EI - Yugoslavia. This is modified EL519 which works like a KT88.
KT94, KT100
So called higher power KT88. Part numbers are abused by the Chinese. Can be anything from a KT88 to a 6550, with a larger glass bulb.
This tube is no KT88. It draws more heater current, and it has another operating point. The operating point being "not KT88" shows to me this was knocked together, by using existing tools. With dedicated tools, a real high power KT88 could be made, compatible to old versions.
Switching tube for TV deflection. Can do very high plate current peaks, can withstand extreme heat, also due to magnoval socket. Looks like a larger EL34, but linearity a a compromise as with all switching tubes.
Original audio tube. Something like very large EL34, but 10 pin socket
German army tube. Larger than EL34, but with army socket
Russian army tube. Same as EL152, but yet another socket. Best and finest tube of this list. Strong a s tank. SECRET TIP! Pictures here

PART 1) KT66
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The 7581/A tube is an industrial strength version of the 6L6GC, which would make it the American equivalent to the British KT-66. The ratings 7581 are roughly the same as the 6L6GC while the 7581A has higher ratings. The 7581 has a plate voltage rating of 500 V--35 watts (5 watts more than the 6L6GC) and a screen rating of 450V--5 watts.

This the THE KT66, made by MO Valve, England. This pair is from 1970, and was used into the legendary QUAD amplifiers.

This is the same tube, but in a black glass version. Black glass is electrically better. When you really like the physics of tubes, this is the preferred version.

Here is a close up from the above tubes

The 5881 is the industrial grade 6L6. It has Zirconium coated plates, making the plates act as a getter. The screen grid was carbon coated, giving it some overload of capacity (The total is 3 Watt!) . The control grid is gold plated, giving it lower leakage. Cathode is stand-by proof. Low leakage socket. How do we know all of this? From the above paper :)

PART 2) KT88
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KT88 Connections:
Metal Shield Ring
Screen grid
Control Grid
not connected

KT88 Connections

KT88 by The General Electric Co. Ltd., Magnet House, Kingsway, London WC2
Source: KT 88 GEC 1957 Data sheet
Heater Voltage 6,3V
Heater Current 1,8Amp
Max Plate voltage 600V
Max Plate voltage 800V peak
Max Screen grid voltage 600V
Transconductance 11,8mA/V
Plate impedance 12kOhms
Screen Grid dissipation 7W max
Plate dissipation 40W max
Plate current 230mA max

specification and max ratings(*)

(*) These KT88 specifications are provided 'as-is', with no warranty of any kind. Use of these parameters shall be entirely at the user's own risk.



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Don't we all want to read from the getter how good the tubes are ;) Well sometimes you can, sometimes not.

It's amazing how bad a KT88 getter can be and still the tube works. On the left picture you see a tube that was for sale an auction website. It is clearly visible that this getter has gray edges. Also some part of the getter is gone. This tube probably tests fine, but what the buyer doesn't know is, most of the lifetime is gone. On the right picture you see two good used tubes that were offered an Internet tube dealer homepage, for very high prices. The left tube getter looks fine. The right tube has some of the getter edges gone as well, and the GEC stamp is badly burned. These are the brown type of getters, at the end of lifetime these start to develop concentric rings, of some blue-gray color and after more use pieces of the getter start to disappear. After that continues, the tube gets gassy. At this moment they probably test still fine.


I was send this picture by someone who found those on Ebay, and who was afraid they were fakes. Well, yes they are fakes. Inside is the Chinese KT88 system clearly visible.

What is even worse about these, they were offered as NOS, and they are in the box with a bad vacuum getter. You can see the gray ring around the getter. With all getters, the edges are thinner than the middle part. So once you see the edges are no metallic looking any more, this means some part of the getter has been used up. This kind of getter wear out comes from use. This is not air leaking to the inside, but this is plate out gassing which causes this coloring. The print however is clean, not burned. So this is a real bad case, old worn out Chinese tubes were cleaned, re branded and re boxed as Genelex.


  1. A great part of the lifetime is gone.
  2. Marking is faked. Inside is no Genelex KT88.

Fake Info

Here is a typical worn out getter. Here is the same kind of getter, but still good.

*** Matching of KT88 ***
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Bernardo Santos, from Brazil send those picture about selection of GEC. Thanks Bernardo!

This is how the GEC KT88 was selected. The text is: The Genelex Power Output tubes in these cartons have been matched to each other on the finest test equipment available. When tested these tubes were found to be within 1% of each other. The company is B.I.C. or BRITISH INDUSTRIES CORP. Port Washington, New York, USA.

This is how the B.I.C tubes look like. The sticker is a bit different from most others, but it is genuine, and inside is a genuine GEC tube. Note: Be careful, this sticker is being perfectly faked by the Chinese at the moment. So by itself this sticker means nothing any more. These days are over. You can only trust the tube inside piece parts.

About Test data for the KT88

Here you see the test limits for the KT88. Not by just any company who was selling those, but by the one-and-only-real M-O Valve Co in London. This is historical rare information.

Note however, these certificates do NOT represent actual test data of the tubes in any way whatsoever. Though they do a great job in giving you that impression.

What is hand-written in there, is nothing but general information, and NOT related to the particular tubes inside the boxes. If you have 100 of those certificates, they are all the same. So on the one hand it is good they provided that information, at least they say the tubes comply with something. So you know how to check them, and you can say if they are good or not. On the other hand, and this is really a dirty way to fool you, they give you the impression these tubes are hand-tested, and you would think what you read on the certificates are the results. Well, this is NOT SO. If you are skilled with data sheets, and read these certificates carefully, you will see this is just hand written information which they took from the standard data sheet. It is fine when the tubes comply with that, but if not they would have been rejects anyway. So this test certificate is plain bull... made for amateurs that think many numbers are many good. On the other hand, don't take it too negative, the MO tubes are great quality anyway. It is just the so called test data is for the birds.

You need to know a few things here.

  1. About GOLDEN LION. The MO Valve company is not existing any more. NEW SENSOR in New York today owns the brand rights of the "GOLD LION" brand. That is all they own. They do not use the original tools, neither are their new tools copies of the original tools. The GOLDEN LION tubes, sold by NEW SENSOR are made in Russia, and inside I see the plain old Electro Harmonix construction, as they used always, which resembles their 6L6 optically. The MO Valve company rather looks like the TESLA NOS. So really something different. You can think of this what you want, and you should. The Golden Lion tubes made by new Sensor have nothing to do with the originals, made by the MO Valve company. Also, the quality of New Sensor doesn't even get near. The boxes and printing, and optics are perfect quality. Better than the tubes itself. I have sold the New Sensor for some years, and I was extremely disappointed by those. The ones I had showed all the typical errors, KT88 can have. That is: Heater-to-grid shorts, after postage. Is the post to blame or the factory? I don't know. All I know is, they damage quickly. Another problem, the most occurring I had, was vacuum problems. The getters get white edges quickly. Though many times the tubes still work, I do not trust such tubes. Another problem I saw often, was screen grid current is too high, this over-heats the screen gird, the carbon layer evaporates , this damages the whole tube. They short in the end, blowing out the amplifier fuses. So I do not recommend the expensive GOLDEN LION tubes, sold by NEW SENSOR. For the half the price you have Electro Harmonix by NEW SENSOR, and to me they seem identical. Quality is the same (medium) level. Problem with NEW SENSOR tubes, was for me, they develop gas on stock. They were fine when I received them, but sometimes tubes can be on stock for a few years, and I had several that developed gas.
  2. New made GOLDEN LION Test Specifications. What specifications? Today, many manufacturers blow a cloud of smoke around the question, how tubes should be checked for quality. For a serious tube dealers, trying to do a good job, this becomes a problem, as he has no way to do incoming inspection. For instance NEW SENSOR the #1 maker of KT88 has always refused to give the data for incoming inspection. They just say the tube ARE already tested, so they are good anyway. So problems can get passed on to the buyers before you know it. My opinion is, a tube always needs to have so called acceptance limits SPECIFIED by the manufacturer. Interesting, on those historical certificates, the acceptance limits are there indeed. For instance Plate Current is between 121 and 174 mA. With that information, your tube dealer can test what is a good tube, and what is a reject. I tried to get incoming inspection data from NEW SWENSOR. After many rounds of discussions, I got the following stupid answer from Mr. Morrisson by phone, who was at that time the chief technical engineer of New Sensor New York. "We only present you the data as on the boxes. We tell you the tubes are 100% tested. We will not supply acceptance level data. The tubes are good. We sell most tubes into the guitar amplifier market. The HiFi market is only a few percent of our sales anyway. We can easily do without it if we have to. I am sorry but this is how things are. So take it or leave it". Well.... I decided to leave it, after they refused to replace broken tubes to me, that lost vacuum just from resting in my stock room, unused. My personal opinion about GOLD LION NEW SENSOR: Too expensive. Not any better than Electro Harmonix which costs only half. Risk on vacuum problems is too high for what they cost.
  3. GOLDEN LION NOS FAKES. Oh sure these exist! .Inside are plain chinese tubes.
  4. The blue Genelex stickers, always used to be some proof of originality. Well unfortunately, these stickers are now faked also.

So you see, IF the new copies Gold Lion copies are as good as the original, as they say, here I see a sharp difference. Since with the originals from the real M-O Valve Co in London, the test limits were simply ADDED with each tube shipped, and you don't need to ask. Another difference is, when you ask they don't even tell you how to check the tubes. They are already checked, and "good", and that's it . That's what they said to me.

What is a good new made KT88 to use for amplifers? What follows is only my personal opinion, from selling tubes since the last 20 years. I would prefer them in this order:

Number 1 NOS with test data, which is all you need. As you can blindly trust most other data, such as leakage etc. The holy grail of course, no realistic solution of everybody.
Number 2 Svetlana C-Logo KT88. Very hard to get.
Number 3 Most realistic good choice: Chinese KT88. Seem to have the highest generic quality, but the Chinese mix a few bad tubes under the good, always. They get away with that, and they know it. So what leaves the factory is a strange mix of mainly good tubes, with some few bad ones amongst them, no good matching, and not enough burn in. This makes them very suited for professional sellers that have really good equipment. (Meaning not just a tube tester). In industry this is called "uptesting". So a good dealer separates the bad from the good, and offer the good ones to his loyal customers. For the less good ones there is a nice market, on auction web sites, where fast delivery counts mainly for good product feedback.
Number 4 Electro Harmonix - Sovtek - Svetlana S-Logo. All the same tubes, just another jacket. You can take the lowest priced, as the more expensive ones are not any better.
Number 5

New Sensor made GOLDEN LION. They seem identical construction as ELECTRO HARMONIX, though I had often vacuum problems with GOLDEN LION.
Number 99 not recommended

JJ, Fullmusic.




Not really off topic: The TT21 and GU50 tube
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This is a picture of the TT21. These are like a KT88, with a top cap, allowing higher plate voltage. Look at the typical shape of the plate, with the six markings, three on each side. This is one way to recognize the original KT88. (TT21 and KT88 have the same plate. TT21 has it's own data sheet, it differs mainly because of maximum 800V plate voltage. They used to be cheap on Ebay, but boy... these days have gone.

TT21 has 37,5Watt plate dissipation, same as KT88

Did you know this? TT22 has 45 Watt plate dissipation.

You like "another" tube than KT88, well it's a bit off topic, but try GU50. They're low cost and much better than KT88. GU50 is amazing quality Russian military old stock. (and yes we have those in the price list, under NOS;)

TT21, TT22 can be found here

You like "another" tube than KT88.... try GU50 (similar to EL152). They're lower cost and compared to GU50, KT88 is only a consumer tube. GU50 is extremely strong and very low distortion. Amazing quality Russian military old stock. (and yes we have those in the price list, under NOS;). Peak plate dissipation is 55Watt. Note: These can not directly replace KT88, because the bias setting is another, and also the sockets are another type. We can not help you with modifications, also not help you how to understand to do it, when you have no idea. We can only give you the data sheet, and it is no black magic when you're a specialist. To me it looks the bulb is hard glass. Much different from the fragile KT88 glass. The metal piece can be removed from the top. When using a hot air gun, the glue will get soft and brittle away.

GU50 data sheet can be found here

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Don't let anyone mislead you. If there is one tube that a whole crowd of fakers have jumped on, a long time ago, it is the KT88. Every self-respecting faker has "finest" KT88 for sale, and his own brand. Of course, some respectable companies also make finest KT88, but how to keep the those apart? Apart from the Tesla re-brands and re-makes, these are probably all China tubes and.... ALL come from the same factory in China.

When I first ordered tubes from CEIEC China in 1997, there was the sales engineer, who some of yo perhaps still remember, his name was Mr. Li. He told me: Sir, what brand do you want on them?... I said: Well, your factory brand of course! They said: yes sir, we have that too. I was so shocked by this. This shows how easy it is to get "your own" great brand of Chinese tubes. However when you know the Chinese better you will learn that the way to get quality from them is hard, very very hard, and impossible for most. For them, something that "works" good is a fine product. They sure can make quality but they prefer to sense very delicately what your minimum acceptance level is, and simply send you that. So you won't complain, and they can send you the lower quality, that they know some other company will reject. (or has rejected, and send back). A lot depends on how having a long history with your purchasing channel, and how they have learned about you, and what you find out and not find out.

This problem comes from one particular and actually quite normal Chinese trade method. That is, you are supposed to pay the price they ask, when it is a reasonable price for you personally, like they know it, and like you know it. If you THEN bargain on the price, they regard you no future business partner any more. You lost your face. Speak in our words, they regards you dishonest. Next thing is, they send you lower quality, that they feel better suits your dishonest price. This they do with an oriental smile on their face, and you get screwed of course. For them this is not screwing, it is giving you what you deserve. The other way around doesn't work, like giving them a real good price and expect real good stuff. They think you must be pretty stupid. So you see, how hard this is, and why so many bad Chinese tube stuff is around. There just is no good way to talk straight with them.

No information about the SOUND of each KT88 tube here. Only technical experiences.
Audio Classic
Sorry folks just REBRANDED Chinese tube
Golden Dragon, England
Sorry folks just REBRANDED Chinese tube

SEEN IN 2003

Sorry folks this is REBRANDED TESLA tube

Fake Tesla KT88

Sorry folks just a REBRANDED Chinese tube. Offered to a tube dealer in the USA, in March 2007.

Original Tesla NOS

Original Shuguang


This is a small little picture here, but look at this better.

Wow! My respect for the nice faking of the special material GEC sticker, which was (UNTIL NOW) seen as proof for genuine MO Valves. Well forget about this sticker as from now on. So this is NEW. Get ready for the finest GEC faked from China!!!!!!!

With KT66 (not pictured here) this makes it very difficult to distinct Chinese KT66 from GEC. Reason is they are made with the same tools. At EBAY these KT66 are seen already. You are looking for troubles? Then go buy from Hong Kong sellers original GEC KT66 or KT88. Good luck with it!

This KT88 tube I received from my friend Frank Geeraerts from Belgium, as a gift, because he knows I collect "fake". pictures. Another very raw fake. As you know, in China product faking Western products is a national hobby. Copying is not seen a crime there, by almost nobody. The only problem for the Chinese with respect to copying is possible export problems. For the rest there can be nothing bad with a business that us a good business.

Only the inside of the tube is not so nice faked, inside is a plain stupid Chinese fire cracker.

Do you see the white speckles in the back of the tube? That is chipped off cathode coating. When little particles of this come in between the grids, it sparkles inside the tube and it gives the famous fire cracker sound from your speakers. Also the rings around the getter flash show that there is some gas in the tube.

On the back is stamped: KT88 / 6550. Must be the latest invention. It is either KT66 or 6550. Probably 6550, because they're 10 cents cheaper to make, and 10 cents at 1000 tubes is 100$.

Just marketing Name / brand name. Tubes are from China or Russia.
Sorry folks just a REBRANDED Chinese tube - gets a bit boring

Sorry folks this Mullard tube is a GEC tube. Not a bad one!
PHILIPS Miniwatt
Probably TUNG-SOL.
Just a brand name. KT88 could come from China or Russia.


On an auction site, I find some wild stories about "so called" tubes for the Czech army, made by TESLA. If you believe it, you come in tube heaven.

Some research on the TESLOVAK brand lead me to a USA company PENTALABS, though on their own website I saw nothing about TESLOVAK. So I can not verify this.

For me the only question is, what factory made those. Specially with four of the five letters of the "TESLA" brand seem borrowed, as well as the logo letter type.

Inside, I see DEFINITELY NOT systems from TESLA.

Just marketing Name / brand name.
Just marketing Name / brand name.

This is from the people who bought the Western Electric 300B production, they announced a quad of Golden Lion for 1700$. I found it here

To me it seems, more than one company claim to own the brand name. Check below:


This tube is a Russian made tube, in the REFLEKTOR company. They make this tube since 2007 and it is said to be a true copy of the Genelex GOLD LION. Which it is definitely not. The plate construction is the same as normal Electro Harmonix tubes. It fails the characteristic of the original Golden Lion.

Moreover, this is my personal experience with the RUSSIAN tube, I had many with bad vacuum, first buyers send them back with problems, usually a spark is reported. Then the tube tests not very strong, but "just ok". Store such a tube for a year, and I find it on stock with a white getter. This problem I had so many times with ELECTRO HARMONIX tubes, it became boring. My impression is, the GENELEX branded REFLEKTOR tubes are not any better, just more expensive.

So not they are not a copy of the original Marconi Osram Valve ENGLAND company, they are totally different tubes, in terms of internal construction and reliability. The only thing that is the same is the brand name and the logo.

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This is really BIG business is China. The search for "Better" KT88 tubes is the most craziest I have ever seen. These do not exist, but the holy grail most be somewhere. So people look on Ebay for something "nice" and optics come first. The latest cry is to use Blue Diamonds or Crossed Sward, or whatever new names the Chinese have invented. And hey.... business is business... the stuff sells. For them it is a kind of fun making, and the greatest fun is, we buy it kind of massively. So here are just some pictures. If you want to know how these sound, you missed the point. There is no other sound from another glass color. A small warning I would like to give for carbon plates. Here the marketing bullsh.. went too far. Such tubes can never be reliable. (read the rest below)

PSVANE KT88 TII, with a gray shielding used. The shield looks nice, but I have no idea what the practical purpose of this is.

I wonder if the put the side getter on the inside of this shield? Probably not. But... where has the side getter gone then?

I hope they did not leave it away? Since with KT88 a side getter is a must have. This place of the tube gets very hot, and that's what a getter needs.

PSVANE KT88 Black treasure. Shugang tube with another jacket.
Shuguang KT88m, this time with black coating, purpose of the coating unknown.

KT88 fullmusic with carbon plates.

Definitely the use of carbon is not recommended. This will lead to low lifetimes, and is only advised for thoriated tungsten tubes, ever since the history of tubes. And no... this engineering law has not changed recently. Not that I heard of.

Against all knowledge, it has been tried with 300B some years ago, and it these were the lowest lifetime 300B ever, called 4300B.

Also the Carbon tubes from Alesa Vaic were a disaster. But.. who cares. We all want to learn from our own mistakes.

It has not been tried with KT88, so here is the next experiment, but I have seen enough of it. Do you believe in this? Well good luck.


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SOUND? No information about the SOUND of each KT88 tube here. Only technical experiences.

  • KT88 Genalex Gold Lion (one top getter)
  • KT88 Genalex Gold Lion (one top getter, 2 side getters)
  • KT88 GEC (one top getter, 2 side getters)

Genelex / Gold Lion.

National Electronics

National Electronics REBRANDED anything. Many Russian and Chinese tubes, as well as USA tubes. It was (and still is) the tube brand of Richardson Electronics.

As far as I have seen, they write correctly write the country of origin on their boxes.

I was told by email that the tube pictured here was made in the USA, on CETRON equipment, and it was available only for a short time, and it was very expensive. You can click on the next line picture, it gives a high resolution picture, and you can nicely see the details of the inside piece parts. I think those are genuine.

Message was send by Steve Heitert in Nov 2009. Thank you Steve!

Jac, you requested any further information on the National KT88 that was made in
the USA.

Attached is a photo of a matched pair I purchased from Richardson, Geneva, IL around 1990. I think the matched pair cost around $250 vs the typical $75.00 for Chinese tubes back then. A couple of years later, I purchased a second pair from Richardson for around $200 and they were physically the same tube but came with the "Jadis" logo on the glass! I put this pair in one of my Quicksilver KT88 amps and on first turn on one of the tubes had a "melt down". Richardson replaced this pair with a pair like the ones in the photo and they performed very well. I am not sure whether there was any definable sound improvement with the National tubes (over the Chinese tubes), but I can honestly say the Nationals lasted at least twice as long as the Chinese tubes.

GEC England

Later Production Tesla KT88-S
Nice looking!

Original TESLA KT88 Data Sheet


Why TESLA KT88 is NOT JJ KT88!

The Czech Republic State TESLA electron tubes factory was split in several companies, during privatization. Here is as far as I can reconstruct what happened. Some parts manufactured those yellow color street lamps, and mercury light bulbs, etc. I don't know what happened to this part.

The part that made the Audio tubes, like EL34, ECC83 etc, was sold to Jan Jorgo, who owns now the JJ company.

The part that made the transmitter tubes, and some magnetron tubes was sold to Mr. Malinivic, a very friendly cigar smoking gentleman. His office looked like a cafe because of the smoke. He called the factory TESLA VRUSOVIC. He bough this plant including the buildings. They made some pretty interesting transmitter tubes, some had laser cut grids from a solid block of graphite. I was there and I saw it myself, it was an amazing small machine, but worked beautiful. So they just blast away all graphite, until a solid part is left that is actually is as fine as a tube grid, with very fine "wires" of graphite. These are miracles of manufacturing. Just touch this grid, and it breaks. Once in the tube they are indestructible.

There was at that time great dissatisfaction about the profit on the transmitter tubes, and some klystrons and magnetrons they made. These tubes have to be evacuated in ovens for four or more days, using lots of energy, and using up the lifetime of the manufacturing equipment. Comes to that a declining market for transmitter tubes, that could not be compensated with higher prices. TESLA VRUSOVIC thought they would like to build a KT88 also, because profits were better than with transmitter tubes. However, all tools were in the hands of JJ, so he had to remake them all, which he actually did.

HOWEVER.... it is not only tools that make a tube. They found that out the hard way. They lost most of the knowledge, lost un replaceable machines, and happily fired what they thought were useless old workers. Modern times were coming. Good-bye Lenin. They later learned these old men, were senior tube experts that can't be found and they couldn't make (good) KT88 without them. When they realized what they had done, they couldn't get them back. I have visited that production line myself in 2003, but that was after the loss of expertise, and it was in a dying condition. They were building the KT88 line up from scratch, re-furbishing broken tubes, etc. All if that was very hard to do, and actually what I saw was a big mess.

Getting back 25 years of experience, but from whom, and how? Questions.... questions... Many bad tubes were the result of these attempts. I was looking and the production line, it looked a bit sad.

There was from 2001--2003 some retro production of the TESLA KT88, with the blue logo. I had three charges of them, and the overall failure rate was 10% within the first six months use. From this the statistic calculations say you have 35% chances on ONE bad tube in a QUAD. As you can understand, this made this business not very pleasant to do.

APRIL 2006. TESLA FACTORY PERMANENTLY CLOSED. This moves TESLA KT88 permanently to the Hall of NOS Tubes - END OF PRODUCTION. Perhaps somebody will smell money and re-start it again, it may happen.

OLD Production
The good stuff!

Try to get the old style, white printed TESLA KT88. These were made in the days when the whole TESLA conglomerate was one company. THESE ARE THE ONES YOU NEED, BUT WILL NOT FIND.

When I read the babble on audioasylum, the experts confuse me. Imagine, Tesla was trying to sell those, and the smart guys on Audio Asylum were asking the usual questions, like who has heard them, and did anybody have problems with them. So after a while they were all talking about problems, and nobody said anything good. (and nobody ever heard the tube playing). Particularly that is so characteristic of this forum.

And then the usual surprise: "OUT OF PRODUCTION". Now look what happens... Help! where can I buy 10 quads...who knows a dealer who has some? All I can say is: Folks, you should have supported this production as the factory was still alive. They were only 35$ one, new off the factory. Today you have to outbid your colleagues on Audiogon or Ebay for minimum 250$ for a pair, that is "said" to be new. I have two of those left, the ones you see on the left. They're not for sale.

After that, the tools were sold to JJ. That's another factory, they use other materials, and there is a different company philosophy behind it, and it's just not the same tube any more.

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No information about the SOUND of each KT88 tube here. Only technical experiences.

Electro Harmonix

Most excellent KT88 factory selection "PL" I found to be better than 0,5mA per Quad in most cases. You can calibrate your tube tester with those! Mechanical quality is superb. Failure rate is close to 0%. No problems reported. The BEST KT88 from new production.
Old Svetlana
They now talk of a "new" Svetlana KT88, and "old" Svetlana KT88. It's best to speak about USA marketed S-Logo Svetlana and C-logo Svetlana. The two have been fighting over the brand name. The Svetlana we sell in this website is from the USA company. I tried to contact the Russian Svetlana but very quickly I was in contact with several people, in totally different locations, who all claim to be the sales manager and are in charge for collecting the money. My conclusion: Buying from them is Russian roulette.

S-Logo Svetlana
To my opinion the most reliable. These tubes are made in the Reflector factories, one of the older producers of Russian tubes. Sales and management is under USA supervision. These people are committed to Quality. They supply not really cheap, but for that you get perfect material. My Conclusion: The best Svetlana to buy today.

Valve Art
Reliable company, a bit messy with shipments, but that's life when dealing with Chinese Selection is good, but not as superb as with EHX KT88. The question is if you need that. Matching is a few mA difference per Quad, where below 5mA is considered a "Good" match by most. For the rest the quality is very good, and failure rate is just as low as with EHX and Svetlana. Well made, using fine materials, and best tube bases (sockets) I have seen. Their polished tube bases are unique, and very good looking. Nice glass work. Failure rate is close to 0%. No problems reported.
JJ Czech Republic has bought the complete equipment for making Audio tubes, from Tesla, in the 1990's, when TESLA thought the market for audio tubes was not interesting any more. Tesla wanted to focus on Transmitter tubes only. What a BIG mistake that was from Tesla. In the years after the sell off, the audio market soared, and transmitter tubes are sold less and less. The people at JJ are clever, and focuses very hard on making a very good KT88, and they succeeded. Today the JJ KT88 is a plug & play tube, almost as good as the Russian tubes. Conclusion: An interesting KT88, made on original old TESLA Machines. Not using TESLA Filament materials though.
Shuguang China


Most excellent factory selection. "PL" is better than 0,5mA per Quad. Interesting glass shape, like 1960's Tung Sol. Quality is very good. No problems reported. Occasionally sales in the Internet is seen from obscure sources. There IS NO OFFICIAL KT88 factory sales in Russia, although every time again some Russian unknown dealers pop up, and say they have a "connection". Don't believe them. All sales is organized via USA, and they have their official worldwide dealers. So buy Sovtek only from official dealers, and you're safe.
Brand is acquired in 2004 by American company. Tubes are Russian made. Theoretically TUNG-SOL but these are Made in Russia. First experiences from 2005 production showed loose solder contacts at the tube base. Very nice packed, and very good matching.

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2Mb File. Download make take longer

This is where all Chinese KT88 come from. The difference is just another jacket.



KT88 Art Impression
by Kees Soeters. Thank you Kees!

The KT88 Tube
Attention: This picture is just an art work.
***At Emission Labs there are no plans to produce these tubes****

*** KT88 Tube Curves measured ***
with SOFIA
(the BEST tube tester ever build)
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Many of the tube curves you find in the internet, are made on a drawing table, with a so called "bend ruler". I have one too, this is a ruler that you can bend. It's relict from my early work experiences where I had to draw circuit diagrams and technical drawings with pen and ink. Computers were non existent. We used real blue prints, hand made. I used to do this for two years, it was my first job when I was 18 years old. (I started my studies in electronics at the age of 26). Such a bend ruler can do great art jobs, it's a miraculous tool, hardly known even at the days of hand made drawings. You can bend it in any curved shape, and it has a soft metal bar inside which fixes the shape. Let me give some attention to this. It's ideal to fake draw curves, as you can copy the shape of a curve with it, and draw with a nice pen this curve on a blank piece of paper. Of course you can change the shape by hand. When you have an eye for this, you can EASILY unveil these, hmm let's call it FAKE tube curves. It was done so ever since tube curves were .presented. You can have your own opinion about it, it such curves are FAKED. Myself I think yes, but manufacturers may think and say they're real. Well and they are, if they represent reality. Penciling in a few missing lines was done often.

Bend ruler for drawing tube curves.
Scotch Tape for PCB layout


These are cut out of the JJ KT88 tubes curves. There should be common understanding, that the distance between the tube curves must follow a logical pattern, otherwise there is something wrong with the tubes.

Another way to make curves by hand is, use that old kind of tape to make printed circuit board drawings by hand. Note, PCBs were around since 1960. With that tape you can make beautiful curves, you can buy it in every line width, and you can pull if off,and stick it back on. I have seen many tube curves made with that. To me, the JJ KT88 have the optical impression being made that way.

Now from the curves here, it can be seen, distance1 is longer than distance2. How can distance2 then be shorter than distance3? It looks to me, the -35V line has it's right corner slipped up a bit. Also you will see the end of the line is a bit random, which is another sign for artist work. Another way to make curves by hand is, use that old kind of tape to make printed circuit board drawings by hand. Note, PCBs were around since 1960. With that tape you can make beautiful curves, you can buy it in every line width, and you can pull if off,and stick it back on. I have seen many tube curves made with that. To me, the JJ KT88 have the optical impression being made that way.

When looking at the original (copy here)l, you will find more of those spots. It seems to me, these curves are hand drawn. Please don't misunderstand me, this is no attempt to say something negative about JJ tubes. Actually this method is as old as tube curves are and there is nothing wrong wit it, as long there are no attempts to "improve" the curves that way.



Yet, today I prefer computer made curves, such at he one below here, as presented by Audiomatica for the first time in 1993.

KT88 Curves
KT88 triode mode connection

KT88 Curves
KT88 ultra linear mode connection


KT88 Curves
KT88 pentode mode connection.

KT88 Data sheets
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Maximum ratings
Typical operation -- Push-Pull, cathode bias
Typical operation -- P-P, fixed bias tetrode, cathode biased ultra-linear
Typical operation -- P-P, fixed bias ultra-linear, cathode biased triode
Installation and outline
Tetrode plate characteristics
More tetrode plate characteristics
Triode and ultra-linear(!) plate characteristic

KT88 Links
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What follows now, are external links
you leave

Duncan amps
Push-pull KT88 class
Klaus Boening
Virtual Valve Museum
Black Forest Audio
Best Sounding KT88 on Audio Asylum Forum

KT88 announced by Westrex corporation
Link to KT88 specification sheet

KT88 T-shirt
Want to see the rest?





PART 3) 6550
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The 6550 was designed by Tung-Sol, New Jersey U.S.A. and first introduced in 1965. It was the "better" 6L6, with higher output better stability. It could work at 600VDC and a screen of 400VDC, and Plate dissipation of 35 watts. We need to say here, that the MO Valve company had already build an improved 6550, it was the KT66. In 1957 the M.O.V. of England striked back with the another improved tube, their famous K.T.88, later sold as G.E.C. KT88, The KT88 had higher ratings than the 6550. It had 800VDC plate, a screen of 600 VDC, and dissipation of 42 watts, outdoing all of the previous products.

Silently and unnoticed, TELEFUNKEN GERMANY build their EL156, which is heavier and stronger than a KT88, but at that time it was not noticed by the USA "crowd" and hey... even today it isn't. So don't tell it to anyone, and secretly pick up those NOS EL156 from Ebay :)

Not really off topic: The 8417 tube
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The 8417 is a different tube than 6550. It can handle 60V more on the Plate and 100 Volts more on the Screen, but transconductance, bias and Impedance are another. This tube has some electrical resemblance with the German EL503.


Here you see some pictures of the RCA 6550, which is probably made by Tung-Sol

PART 4) KT90
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Basically the KT90 is a modified version of the EL509, a TV deflection tube still used in OTL amplifiers. (Check articles from the menu to find out who is the father of OTL. ) Anyway.. The EL509 is very unlinear which doesn't matter for a cathode follower like OTL, with an amplification factor of 1x, and 100% feedback. The "chassis" of the EL509 was modified for more linearity and then you get the KT90 or KT100. So it's no BIG power KT88 it's another tube made to work like a KT88. A real alternative if you want to hear another sound.

Made today by EI Yugoslavia Shuguang China JJ Cz republic and EHX Russia.

Historical KT88 and 6550 and KT90 Data sheets here (scroll down to KT88 or 6550 or KT90)

we're working on this part. We'll open an old KT90 and make some pictures from the inside. Now now.... please visit us later.... Thanks!

Click on image for full size

By Michaelson & Austin TVA-1. Power 70W
Click on image for full size

By GEC Ltd in December 1957. Power 30W.
Click on image for full size.

By GEC Ltd in December 1957. Power 50W
Click on image for full size

By GEC Ltd in December 1957. Power 100W.
Click on image for full size.

By by GEC Ltd in December 1957. Power 400W.
Click on image for full size.

PART 5) The infamous KT94 and KT100
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So called higher power KT88 made in China. Part numbers have been abused by the Chinese. These have a larger size bottle, but inside can be anything from a KT88 to a 6550. Most of all what is missing is a clear, FACTORY data sheet. Any self made data sheets by the Chinese re-branding workshops is not what I mean.


PART 6) The infamous KT94 and KT100
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Now this is something very interesting,