Important note: Note, this page is also linked from the Emission Labs factory page, and in that case we only refer to such amplifiers that can use the Emission Labs tubes, like 2A3 and 300B, etc. For other cases, this page also gives information about other tubes than Emission Labs, like KT88, 6550, etc.
The (not complete) list of amplifiers that need special attention
When you are going to replace tubes, for most amplifiers this will work fine. However when you do have a problem, the cause of the broken tubes can be the tubes, or something else, and we want to see here what this "something else" means.
Luckily we can say problems occur not very often. However if it does happen, in the first place we see tubes with signs of severe overload. On the second place come unwanted noises. A dead tube can happen also, but this is relatively rare.Note that the root cause of the problem can be related to what you see and hear, or it can not be related to what you see and hear. Or even more difficult, you see clearly "A" and after a while the root cause appears "B". For instance replacement tubes have crack noises, and the old tunes were fine. Then after a while it appears you have a 220V amplifier connected to 240V mains. People will say, 220V or 240V, is that so important..? The answer is: Yes, when you don't want to damage the tubes, it is important. Unfortunately you find that out after one KT88 of a pair has short circuited, the other tube of the pair died from that also, and the two remaining tubes can not be retro matched to another pair. So it costs you a new quad. Things like that we want to show here, and prevent them from happening of course.
So by just looking at all possible factors, we make sure we do not forget one. When the case stays unclear, perhaps the mostly like explanation does not apply to the case.
Please read these three lines carefully. It is to understand under which view this article is written.
Wrong conclusions: In case you see a particular problem with one tube of a pair or quad, the normal reaction is, you will try this tube in other sockets. If you do so, it may happen the problems shows still with this one tube only. Now you may be tempted to say: "problem follows the tube", and from there you conclude "it must be the tube". This can be a mistake! It can happen, that three tubes of a quad will forgive an amplifier problem, and one tube will not. There is no "must be" that a tube has to forgive amplifier problems. In some cases an amplifier cam have extremely high in-rush current during switch on, like four times average current can happen. If so, this is an amplifier problem, since in-rush current with a cold tube must be avoided under all circumstances. Specially with KT88 it can happen in such a case that one tube out of a quad develops a spark inside. And of course, the experiment is repeated many times. It will be always that one particular tube, and it is plugged in all the sockets, and of course the problem "follows the tube", but... the problem is the amplifier. After 20...30 times, this tube will short circuit, self-destroy, and by this destroy the other tube of the pair as well. So the owner by default gets very upset, puts back in the old tubes (that worked fine always) and will refuse to understand the mistake with the amplifier. Then, to make it worse, the manufacturers almost never admit a mistake. I remember only two cases.
Another situation can be, a quad or an octet works fine for some weeks, then the first tube develops some problem, then another comes with the same problem, and so on. Also here, the wrong conclusion "it must be the tubes" is a easily made. So all in all, this diagnosis requires some experience.
My honest judgment is, that with amplifier tubes 50% of the tube problems we see are above nature, and with rectifier tubes that number is above 90%. Reason is that rectifier tubes are more difficult t use than the average"designer" thinks. Even when a mistake is obvious, like far to big capacitors, they do not admit it, and simply continue making the product that way. Another classical generator of problems is mixing tube electronics with semiconductor electronics. This is far more complex then expected, and problems of arise because the switching on (and off) effects of the circuit are completely unverified. I can write a book about all the mistakes I have already seen. Perhaps I will do so.
Did you know when you point your finger at somebody, three fingers of your hand point to yourself? These are the things that can happen:
The following table, is some personal experience, how to best replace the tubes in this model amplifier, and we write it here, simply to prevent problems.
Amplifier list in alphabetical order
BOTTLEHEAD the electronic circuit of the "Paramount" 2A3 amplifier is not approved by us for the use with our tubes. (Guarantee voids) |
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CARY good and much recommended amplifiers. They push the 300B or 2A3 tubes close to the limits, which is of course possible, but you need to be aware of that. We recommend EML 300B-XLS for the amplifiers using 300B, and EML 2A3-S for for the e amplifiers using 2A3, and there is else to take care of, other than the manufacturers instructions. If you want to use EML 2A3-mesh, you have to use those that are selected by us for a grid voltage not more negative than -47V.(So a tube of -48V should not be used). This is the voltage that is written on the outside of the boxes. |
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Kondo (Japan): Model KSL KEGON This amplifier hard-wires two 300B tubes in parallel. (see schematic) So, without any precautions whatsoever, the terminals of the 300B tubes are connected in parallel, and that's it. This is dangerous with random tubes. Since this method was used here, we need to see how we deal with this. It means, this amplifier will only work safe on a pair 300B tubes with special selection methods, and definitely can NOT work on unselected 300B tubes, and also not on just any matched pair. What you need here is not just matched tubes, you need SELECTED tubes. So matched tubes just fit together, and selected tubes have specified values. IN THE KEGON, you need EML 300B tubes with factory tested grid voltage from -56V.... -60V, by the Emission Labs test method. We give only guarantee when you use EML like this. Otherwise, guarantee will void. No other brand tubes when bought from JACMUSIC are guaranteed for the use in the KEGON. So please take care of the above, and you will love the results and the wonderful sound of this amplifier! WARNING I have seen a 1991 circuit diagram without bleeder resistors on the High Voltage Caps. This is extremely dangerous when you open the amp, even when "off" the power supply caps can be charged still. Today the "CE" qualification does not allow this any more. |
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Kondo (Japan): Model NERO This amplifier hard-wires two tubes in parallel. So, without any precautions whatsoever, the four terminals of the 2A3 tubes are connected in parallel, and that's it. This can create severe problems. Since this method was used here, we need to see how we deal with this. It means, this amplifier will only work safe on a pair 2A3 tubes with special selection methods, and definitely can NOT work on unselected 2A3 tubes, and also not on just any selected pair, which someone's says are " selected very tight", but it is not targeted for what we need here. Note, that there are two methods to match tubes. Method1) take a fixed grid voltage, and measure the plate current. This is most commonly used, because it works fast. With the 2A3, you would use 250V Plate, apply -45V grid, and then measure a plate current, of 40....80mA depending on the tubes, and write that value on the boxes. After that match the tubes in pairs. So the test set up is static. You plug in the tube, wait for the warm-up, and then measure the plate current. Method2) Take 250V PLate voltage, apply a variable grid voltage and hand-adjust it until you get the required plate current for this tube, which is 60mA for a 2A3. So you get a grid voltage of say -40....-50Volt and we write that on the tube box. After that match the tubes in pairs. Method1 is the common method, because it works fast. It is done by most manufacturers, like EHX, Sovtek and by the Chinese. At EML we prefer method2, and by coincidence this is just what you need here to get a safe matching for your NERO. IN THE NERO, you need EML 2A3 tubes with factory tested grid voltage from -43 to -47V. Then use following table:
We give only guarantee when you use tubes under the above conditions. Otherwise, guarantee will void. So please take care of the above, and you will love the results and the wonderful sound of this amplifier! |
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VAC 30-30 Good amplifier, but puts heavy stress on the 300B tubes. Not recommended for standard 300B tubes. We recommend 300B-XLS tubes for this amplifier. |
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VTL 185 Adjustment range is not very high, and (good) new KT88 tubes may draw not enough current. When replacing with Svetalana S-logo, Sovtek or EHX, it is recommended to use KT88 that show test values much above 60mA, by the factory test method. For other brand tubes we don't know the limits, but you need the high current ones here as well. (Note, there us no such thing as standard test methods for tubes. So at factory test conditions, like a 60mA factory tested tube, this will give another setting in almost any amplifier) |
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VTL. Model: ST 150 Adjustment range is not very high, and (good) new 6550 tubes may draw not enough current. When ordering Tung-Sol 6550, it appears you need tubes that are tested above 60mA, by the factory methods. The same values apply for EHX 6550. (Note, there us no such thing as standard test methods for tubes. So at factory test conditions, like a 60mA factory tested tube, this will give another setting in almost any amplifier) |
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