Useful info here: http://www.audiotubes.com/6dj8.htm
And because I know that there are just too many lazy people who don't care to read this link, here I go again with the quotes (read the whole thing from the link if interested, because I left some interesting stuff out to save space):
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The 6DJ8 is a twin triode in a small 9-pin miniature glass package, with a very high transconductance for each triode unit.
6DJ8:
This is the standard incarnation of this tube. GE and Sylvania both made USA versions, and are fairly good quality. RCA tubes were often made by either Mullard or Amperex in Britain or Holland. The most popular NOS vintage tubes are those made by Telefunken, Amperex, Siemens or Mullard, and are usually also labelled ECC88. See the next paragraph for this description.
ECC88:
This is the European designation for the 6DJ8 type tube. It is the same tube as a 6DJ8 and often the tube has both type numbers on it. The popular vintage tubes here are those by Telefunken, Amperex, or Mullard. The Telefunken are said to sound the best, and are the most difficult to find of all of these brands. All Telefunken tubes have a tiny diamond shape molded into the glass on the bottom, and have a very chalky ink on the surface which often is partially wiped off. Beware! These tubes are being reproduced in China, and look like the real thing, complete with diamond mark. The tipoff is the label: the fake labels will not wipe off.
The Amperex can be found with a white label and the treble clef logo (rare), the Bugle Boy cartoon tube logo, Amperex in white ink, and the orange world logo. The orange world logo is the newest, and can be found in both standard "L" bracket top getter, and the rarer (and said to sound better) "A" frame top getter. The A-frame looks like a little metal "A" holding up the getter element on the top of the tube internal structure. The Bugle Boy is the most popular, almost to being a "designer" label, which has driven the price up and supplies down. I believe any of the Amperex / Philips Holland 1960s and 1970s 6DJ8 tubes are excellent, no matter which one you use. Mullard 6DJ8 tubes are excellent as well, but are more difficult to find. They often made 6DJ8 tubes for other labels, like RCA, Zenith, and other USA brands.
6922:
This is one of two premium versions of the 6DJ8 tube. First of all, it is the same tube as the 6DJ8, and will work wherever a 6DJ8 is needed. It has premium features such as low noise, low microphonics, and usually a longer lifespan. Sylvania made a version that is JAN military spec without gold pins. Amperex made all of theirs with gold plated pins and most have the PQ shield logo, standing for Premium Quality. Some of the later gold pin Amperex have the orange world logo.
Philips (the parent company of Amperex) owned a number of tube brands, and many were never seen outside of Europe. Most were actually made in the same Heerlen, Holland factory that turned out the Bugle boy 6DJ8 and PQ 6922 Amperex. Watch for tubes labeled E88CC with brands like Valvo, R/T, RTC, Miniwatt, Dario, Philips, and Adzam. These tubes are identical to the Amperex PQ and Philips SQ (Special Quality) types more often found in America, and are perfect if the Amperex is not available.
RCA 6922 tubes were made by Siemens in Germany, and also have gold pins. These are great tubes, but are not as plentiful. Since the military and some large industries (Tektronix) bought the Amperex made tubes in huge batches, that is what is on the surplus market today.
E88CC and E88CC/01:
This is the European equivalent to the 6922, and is a quality step up from the standard 6DJ8 or 7DJ8. Telefunken made a fantastic version of this tube, complete with gold pins.
CV2493:
A rare military version of the E88CC/01. Some have both types on the label, some only the CV2493. This can be considered the British or Dutch version of the German Cca. Very low noise, carefully matched triodes, and these are batch tested to meet military specs. Usually only available in the Mullard brand.
CV2492:
This is a European military (usually British) version of the 6922 / E88CC type vacuum tube. Unlike the US military version, these tubes were never made in the USA. Most of the older examples were made in either the Mullard Mitcham, UK factory, or the Philips/Amperex Heerlen, Holland factory. These are fine tubes that have passed various demanding military specification tests regarding ruggedness, heat and shock resistence, and heater life.
E188CC:
A bit rarer than the E88CC, the E188CC is usually found labelled as such by Siemens or Telefunken. Amperex and other makers (Sylvania for example) used the industrial 7308 marking, Amperex Holland and France used the E188CC label. It is basically the European equivalent to the 7308. The E188CC has gold pins, and is near the top of the 6DJ8 food chain.
The Siemens version from the 1960s is quite rare, and is rated by many audiophiles as a jewel on par with the best Cca tubes, or the cryo-treated 7308s. It has low noise to the vanishing point, and a wonderful, airy top end and soundstage.
7308:
This is probably the ultra 6DJ8 type tube commonly available. It has all of the features of the 6922, but the triode sections are also matched to each other, and the tubes all fit within very tight specifications. Therefore, any 7308 should match any other 7308 within the same brand. The Amperex versions were again made in New York, complete with gold pins, and often have a number etched into the side of the glass. Amperex made these for industry as well as the military. The Amperex versions have the PQ label, the military type usually is labeled "USN-CEP", but I have seen versions made for Stromberg-Carlson and Hewlett-Packard, with white ink labels. These all had the numbers etched into the glass, and all are the same tube. The USA made military white label types have been rated "Best of All", over other 7308, 6922 and even Cca tubes, in several well-documented 6922/7308/Cca listening tests. Therefore this particular type is becoming scarce and prices are rising. There were a few of these made in Holland, but most were made in the USA, therefore the Holland tubes are RARE and usually command a higher price. The Holland PQ versions have the PQ with stars on either side of the letters, and the words "Premium Quality", where the USA types have the PQ inside of a shield logo.
Again, there are some nice vintage USA made standard pin military and industrial types available from Raytheon and Sylvania, just to name a few, and these are quite good at a price currently far below the European vintage labels.
E288CC:
This unusual tube is basically a 7308 / E188CC in a slightly taller bottle. The filament current draw is also slightly higher but for most applications it is plug and play compatible with the 6922, 7308, Cca, or even the 6DJ8. Physical space is a consideration, since the tube is about one-half inch taller than the rest of the tubes on this page. It features gold pins, factory screened triodes for low noise, 10,000 heater life, and all the other great features that make the 7308 and the Cca such high-demand tubes. Since it is a relative unknown, the prices are about the same as good 6922 tubes, and far below that of most 7308 or Cca types! I have only seen this tube in Valvo, Telefunken, or Siemens brands. Worth giving a try if you want the best but cannot afford the soaring costs of NOS 7308 or Cca tubes.
Cca:
Whew, these babies are so scarce there isn't even much info out there about them! Mentioned by audiophiles, usually in a reverent whisper, these gold pinned gems are about as good is it gets in 6DJ8-land. Matched triode sections, low noise screening, 10,000 hr. heater life expectancy, carefully controlled frame grid winding, low microphonics......the list goes on and on. This is regarded as one of the most detailed and three-dimensional sounding tube ever made in this family of tubes. Most of what is available is Siemens or Telefunken made, although there are a few very rare examples of Philips or Amperex Holland made out there.
6H23:
This is a Russian made equivalent to the 6922 or E88CC, but will work in place of any 7308 or 6DJ8 type as well. Ordinarily the Russian tubes are just not of a high enough quality to go head-to-head with NOS German, Dutch, or UK tubes. These are a possible exception. The so-called "rocket logo" type from the 1970s and early 1980s seems to hold it's own against NOS tubes costing 3-4 times as much. These NOS Russian rockets have been critically acclaimed in several online discussion boards for their detail and smoothness. At current prices, these are about one-third to one-fourth the cost of NOS Western Europe types, and so their value is enhanced even more.
7DJ8 / PCC88:
This is an unusual tube that must have had a limited range of specific uses. It is virtually identical to the 6DJ8 except for the heater rating, which is 7 volts. You may want to try a 7DJ8 for the longer heater life in a 6.3 volt circuit, and the lower price!
TELEFUNKEN, SIEMENS, VALVO, LORENZ, and other German made NOS: These tubes are usually characterized by an impressive open "air" at the top end. The soundstage is large, even in mono applications these tubes have a great 3-D image. The midrange is ruler flat, and the bass is tight and accurate. These tubes have a fine sense of dynamics, and most are impressively quiet. These are not "warm" tubes, and to some ears their lack of midrange warmth may be heard as bright. I tend to think of them as accurate, and their clean, focused sonic image is astonishing.
AMPEREX, PHILIPS, MAZDA and other Holland/France/Belgium made NOS: These tubes are a great balance of a clean, airy top end, nice midrange warmth, and accurate bass. They are very pleasant, clean, and musical to listen to in hi-fi applications. The white label Amperex PQ type or USN-CEP (same tube, the USN was made for the military) made in the USA are considered one of the best 7308 or 6922 types of all ever made. The D-getter and pinch waist rare types are also highly regarded, and are very quiet as well. The orange label types run a close second.
MULLARD, GENALEX, BRIMAR, and other British made NOS: Like a warm British jacket of the finest tweed, these glorious tubes have an attractive sweet warmth in their midrange and lower regions. The top end is silky and pleasant, without being rolled-off. The best of these tubes retain a fine sense of "air" at the top, and the upper midrange is smooth and liquid. These tubes reproduce the human voice, especially female voices, with haunting realism. The 1970s Mullard made have an attractive sparkle at the top with the rich bass, and these tubes are usually priced less than the older types.
RCA, RAYTHEON, GE, SYLVANIA, and other USA made NOS: This group is very diverse. The older RCA, GE, and some other brands of 6922, 7308, and 6DJ8 were often made by Siemens, Mullard, or Amperex, and usually are a bargain. The USA made 1960s vintage 6922 and 7308 are nearly always made for the military, although there were some early industrial versions made by Westinghouse and other brands. They are fairly good and usually priced very low. Their sonics are reasonably well balanced. The 6DJ8 tubes made in the USA, and the 6922 or 7308 USA tubes made AFTER 1975 are generally best used in test equipment that calls for them, as they are not particularly attractive audio tubes. The earlier 1960s versions are somewhat better, including the greyglass GE. Their prices are usually very low, putting them on par with the Russian or Chinese types. You may want to try them if on a strict budget, for they will surely outlast several sets of current production tubes, and probably still sound better than the Russian-Chinese trash. Overall, the USA made tubes are a nice surprise with their low prices as compared to the European types.
