This mythical tube deserves its own topic, so I guess I'll start it here. Since the 60s there are five main types available. The Xf1, that stopped around 62, I will not discuss it further because surely Marshall has never used them. Then you have the Xf2, which can be distinguished by its welded plates, and double ring getter. Around the end of 68, they transitioned this Xf2 model to a single ring getter. The color of the base still changes in the next years from brown to black, and mid 73 the Xf3 model is being introduced, which has stapled plates instead of welded plates. The top mica spacer still has the 10 dents. Then around mid 75, the Xf4 model comes into play, which can be distinguished by only have 4 dents in the mica spacer. So by these differences, even without seeing any codes, you can easily identify the Xf model of the Blackburn EL34. The Xf codes were also used for older Philips tubes produced in the Netherlands, but they can be easily spotted by the light brown base, the double D getters or the presence of X or L codes. Then dating them becomes easy, after the "B" which stands for the Blackburn factory, the first digit represents the year. So you cannot make any mistakes, since there are no overlaps. B0E4 stands for 1970 on an Xf2, while it is 1980 on an Xf4, and also the last code of the batch of EL34 tubes ever produced in the Blackburn factory. The 2nd letter represents the month of production, A being January, L being December. The last number is the week in which it was produced, so ranging between 1 and 5. Having collected around 100 of those, 85% of them are used, I have already obtained a decent data set using the Maxi Matcher 2 tester. First of all, how can you see that you have a true NOS/NIB tube? It can be easily recognized by the getter flash, that is shiny and the barium metal deposit has a specific pattern. If you don't see this pattern, I wouldn't trust it's really a new one. Check the pictures in the next post. I use a specific system to grade my (used) tubes. First of all, how good does the getter flash look? It's really the best indication of how many hours the tube has burned. If you see it's almost gone, you can be sure it has seen thousands of hours. Also discoloration (black/brown), or rainbow-like colors are never a positive sign. If you see tubes advertised that don't clearly show you the condition of the flash, you better ask for that or assume the seller is hiding something. I've seen many advertised that are shown in a way that hides the true condition of the flash. But enough on that. This is the first sign you need to look for on a good tube. Secondly, how good is the tube still looking. Does it still have a good label? If you see brown or black discoloration around the plates, the tube has overheated. Sometimes you can also see how the heat left a mark on the glass at the position of the holes in the plate. Not so good either. How dirty is the base? If it's rough, but the glass looks nice and shiny without any label, you can be sure it has been sitting in an amp for a long time, and it was just cleaned until it shined again. A tube that sat in the original box still has a very smooth base. When the pin guide is missing, you can be sure some idiot pulled it out of an amp with no intention of recovering it, but then somebody found the tube and still thinks its worth to sell it. I have a handful of those, and some do measure ok, but surely this is a bit more risky to install, and value wise you surely shouldn't pay too much for it. Also corrosion on the guide pins is always a very bad sign. A tube that sat all his life in an amp or box typically still has nice and shiny pins, and even the NIB tube has markings below, from testing it back in the Blackburn factory, but also the tube tester of course. Third, my tube tester. It measures plate current at 400V -36V bias. The spec according to my tester is 20-50mA under those conditions. From all my NOS/NIB tubes, the lowest measures 20.9 mA (Xf2 from '62), the highest 44.5 mA (Xf4 from '79). I also have 22.2 30.9 25.9 27.6 28.4 42.7 40.3 33.7 37.2 31.2 39.7. This large spread was a surprise for me. From my used tubes that so aren't burned up, the lowest measured 8.3, an Xf2 from '63 which still had a decent getter flash, but clearly visible rainbow markings. Can it still be used by just biasing it correctly in the amp? If I find a tube that matches it in the amp, then I guess so. Next, it measures transconductance in mMhos. The limit for a good tube is 4 according to the tester, but on the first Maxi Matcher it was 5. This is also where I put the limit for a good tube. To obtain this value, it typically requires that the tube pulls at least 26 mA under the 400/-36 condition. The transconductance is depending on the current it pulls, so what I do is sort my best tubes on the amount of mA they pull, and then see what kind of transconductance value they have. If you have tube A and B that have a test result of 30 mA, and one tube measures 5.5, and the other 5.1 mMhos, you can be sure tube A is better than B. In almost all cases, the getter flash will tell you the same story. If the tubes measures below 25 mA, and the flash isn't looking too good, it's probably not a good tube anymore. Also, you can have a visibly worn tube that draws 40 mA with a 5.2 mMhos reading, while a NIB tube measures 27 mA and 5.1 mMhos, the second tube is the one you would rather have. So the tester is not always giving the full story. And this probably applies for any other tester. I buy my tubes when they are reasonably priced, without test results. A person who has a tester, typically asks for a lot more (I can't blame them), and is not necessarily honest on its performance. Looking at value, surely if you see welded plates (Xf2), they are worth more than stapled plates (Xf3-Xf4). If you can't see the plate construction, but you see a lot of dents in the top mica, then it's either a Xf2 or a Xf3 and they are worth more than an Xf4. If you see the typical 4 dents, then don't overpay for them. If I see the typical getter flash of a NOS/NIB tube, I'm prepared to pay more for it. Sometimes I get lucky with old amplifiers that still have the original tubes, but then your main focus should be on the condition of the flash. If you see its almost gone, then the tube is also almost gone. But if you see for example a small Geloso tube amp from the mid 60s where the flash on the tubes still look more then decent, it's probably not used much and the tubes (which will be Xf2 double ring) can still be in perfect shape. And finally, you have to go and make matched pairs from them. Having them pull the same mA in 400V -36V bias is not enough to match them, if your amp works on 450V plate voltage, the conditions are different from the tester and they can be drawn apart, even when they are reasonably close transconductance matched. Since there is one more important thing to check, how the tube sounds like, it's a good practice to look for the matching tube in the amp starting from the data of the available tubes. This way you can also hear that the tube still sounds the way it should. Like mentioned below, ultimately doing this on a cranked amp is best. For a bass amp this is of course easier to do with 50W than with a guitar amp. An attenuator is probably a good idea. So, I have around 50 tubes that meet all the criteria of a good tube (including my NIB tubes); and 50 that don't. The plan is to collect the good ones (e.g. Angus Young or Joe Bonamassa can always contact me), and use & abuse the less good ones (for example when some of the flash is already consumed). When the flash is really almost gone, why not bias it to (or over) it's limits, and use it for a recording amp until it goes out with a bang and a broken fuse.