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Historical KT88 and 6550 and KT90 Datasheets here (scroll down to KT88 or 6550 or KT90) |
History:
Between USA and Europe, the competition was not in both trying to make the best tube of one type. The competition was, each tries to make a different tube that the other side of the ocean didn't have. Like this each side thought to be more clever. In Europe, the EL34 was "the" big pentode, which was the result of a long evaluation, starting with the AL1, a radio speaker-output tube for class A. 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 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. The EL156 never had a break through because of the socket being not American.
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6L6
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Low cost, reliable tube. Break through of the beam tetrode. |
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KT66
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Better construction 6L6, made in UK |
7581A |
Better construction 6L6, made in USA |
5881 |
Better construction 6L6, made in USA |
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EL34
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Real pentode Can do higher voltage. Works very nice in Class A also. |
GU-50 |
Russian military tube. Fits somewhere in the 6L6 family. Superb quality, and rugged a tank. |
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6550
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larger 6L6 |
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KT88
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Upgrade of 6L6, designed by TUNG-SOL. Real Pentode Can be seen as a big EL34. |
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KT90
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Original idea by EI - Yugoslavia. Modification of EL519 to work like KT88. |
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EL156
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Something like KT88, but no octal socket. |
PART 1) KT66
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
KT88 Connections |
KT88 by The General Electric Co. Ltd., Magnet House, Kingsway, London WC2 |
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. |
*** GETTER QUALITY ***
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| 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. |
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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. This kind of getter wear out comes from use. It is no "air" inside, but tube outgassing which causes this coloring. Conclusion: These are not NOS Genelex Golden Lion. These are heavily used tubes, only 10% of lifetime left. Faked tubes from China. |
 Here is a typical worn out getter. |
 Here is the same kind of getter, but still good. |
OK, two off topic tubes, but you need to know them
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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 recognise the original KT88. (TT21 and KT88 have the same plate. TT21 has it's own datasheet, 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 disipation. 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 pricelist, under NOS;)
TT21, TT22 can be found here |
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You like "another" tube than KT88.... try GU50 (similar to EL152). They're low cost and extremely strong and low distortion. Amazing quality Russian military old stock. (and yes we have those in the pricelist, 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 modifictations, also not help you how to understand to do it, when you have no idea. We can only give you the datasheet, and the idea. GU50 datasheet can be found here
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**** THE NEVER ENDING KT88 RE-BRANDING
LIST ****
(THIS IS WHAT YOU CAN GET, BUT MAY NOT NEED IT)
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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, these are probably all China tubes, and.... ALL come from the same factory. When I first ordered tubes from CEIEC China in 1997, Mr. Li said: 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 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 sure have good ways to sell the junk to someone too, and before you know it that someone is you. So a lot depends on how having a long history with your purchasing channel. The 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, and when they know it, and you know it. If you THEN bargain on the price, they regard you no future business partner any more. 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 is no good way to talk straight with them. |
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No information about the SOUND of each KT88 tube here. Only technical experiences. |
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Sorry folks, just REBRANDED Chinese tube |
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Sorry folks, just REBRANDED Chinese tube |
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KR AUDIO PICTURE |
Sorry folks, this is REBRANDED TESLA tube |
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Sorry folks, just a REBRANDED Chinese tube. Offered to a tube dealer in the USA, in March 2007.
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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. 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. 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. Alos 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$.
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Just marketing Name / brand name. Tubes are from China or Russia. |
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Sorry folks, just a REBRANDED Chinese tube - gets a bit boring |
National Electronics |
National Electronic REBRANDED anything. This one looks Russian |
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MULLARD |
Sorry folks, this is a REBRANDED GEC tube. Not a bad one! |
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Probably TUNG-SOL. |
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Just marketing Name / brand name. Tubes are from China or Russia. |
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Just marketing Name / brand name. Tubes are from China or Russia. |
| Just marketing Name / brand name. Tubes are from China or Russia. Sometimes from USA, some people believe in it. | |
WESTERN ELECTRIC |
This is from the people who bought the Western Electric 300B production, they anounced a quad of Golden Lion for 1700$. I found it here To me it seems, more than one company claim to own the brand name. Check below: |
 GOLDEN LION from REFLEKTOR Factory |
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. The plate construction however looks like the normal Electro Harmonix tubes to me, and fails the characteristic of the original Golden Lion. |
**** THE KT88 OLD PRODUCTION LIST ****
(THIS IS WHAT YOU NEED, BUT YOU CAN'T GET IT)
| SOUND? | No information about the SOUND of each KT88 tube here. Only technical experiences. |
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Genelex / Gold Lion. England |
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GEC England |
 
  
  
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Later Production Tesla KT88-S Nice looking |
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, who called this 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. 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. TESLA VRUSOVIC had found out, they would have liked to build a KT88 also, because profits were better than on transmitter tubes, but 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 unreplaceable machines, and happily fired what they thought were useless old man. Modern times were coming. Good-bye Lenin. They later learned these that KT88 senior tube experts 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. 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. LATEST NEWS: APRIL 2006 FACTORY PERMANENTLY CLOSED. This moves TESLA KT88 permanently to the Hall of NOS Tubes - END OF PRODUCTION - FINISHED - OVER AND OUT. |
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I don't know what this is. Who can comment....?? Please let us know! |
OLD Production Tesla KT88-S 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. I read the bla bla on www. audioasylum, like: are they good.... are they this or that... and nobody says anything good. (and nobody ever heard them, but nobody admits that) Well you know who stupid this forum is. And now while it's too late, the forum gives hints to each other where to buy them. What a weird situation. Folks, you should have been buying this tube to keep the factory alive. They were only 35$ one, new off the factory. Now you have to outbid people on Audiogon for 250$ for a pair, that is "said" to be new. |
**** THE KT88 CURRENT PRODUCTION LIST ****
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SOUND?
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No information about the SOUND of each KT88 tube here. Only technical experiences. |
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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
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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 SV-KT88-PL |
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 VA-KT88 |
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. |
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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. |
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Sovtek KT88-SOV |
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. |
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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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**** THE BIG TUBE FACTORY LIST ****
2Mb File. Download make take longer.
Link here
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KT88 ultralinear mode connection. 
KT88 penthode mode connection. 
KT88 Links Audiomatica
KT88 announced by Westrex corporation
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PART 3) 6550
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 :)
8417 (like 6550)
The 8417 Is very similar to a 6550 but it can handle 60V more on the Plate and 100 Volts more on the Screen. Black Base & Clear Top. Two O-Shaped Side Getters.
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Here you see some pictures of the RCA 6550, which is probably made by Tung-Sol |
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PART 4) KT90
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 Datasheets 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!
By JEAN HIRAGA 1980. Click on image for full size |
By Michaelson & Austin TVA-1. Power 70W Click on image for full size |
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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 |
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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. |
