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Frequently Asked Questions
Last Updated19-Sep-2016e -->e -->

1. What to do in guarantee case?

Best is to read the guarantee conditions first, you find that at the EML website, here. If you feel, the guarantee does not cover your case in a appropriate way, let us know.

2. Do the tubes need a burn in?

Yes they do! Good Burn-in will assure maximum lifetime and develop the final sound. These tubes are burned-in initially, to do the factory testing, after tube data becomes stabile. However emission of the cathode is not homogenous at this moment. Emission is build by many small islands, overlapping each other. This will not give the final sound yet. The burn-in process will take place in the first 50-100 hours under normal use conditions. This means it is best to switch off the amplifier after each use, and in the beginning not use the tubes longer than 4 hours at one time. Many short use periods have a better result than few long periods. The tubes need the „cold“ periods in between for best formatting of the filament. Some blue glow effect on the glass will disappear during burn-in, or may take longer to disappear eventually. What you observe, is a fluorescent effect, and it is normal with new tubes, even so proving cleanliness of the glass. For good burn in, use different loudness levels from the beginning, and increase the maximum loudness gradually. If tubes with very little use were switched off longer than 12...24 months, it may be necessary to repeat the burn in. So tubes that were not used for some years, may sound unpleasant, and simply need a new burn in.

3. Can I plug in 300B-MESH in any 300B amplifier?

The answer is: Often yes, but not always. 300B-mesh must be used at typical 22 Watt dissipation, and maximum 28 Watt. For the rest EML300B-Mesh is compliant with historical data sheets from Western Electric. So you need to know for sure, your amplifiers runs at a dissipation of 22....28 Watt, and you can just plug it in. In all other cases, you need to do something to get the dissipation (plate current) down. Read the 300B-Mesh data sheet for more information about this.

4. How does the sound of 300B compare to 300B-XLS, or 300B-mesh?

New is, in all data sheets is a paragraph called "Sound Character of this tube". To compare two tubes, just compare the sections of the corresponding data sheets

5. Can I use the tubes in horizontal position?

Yes you can, but even if the tube is horizontal, two orientations are possible. The best orientation is when you have the two thick pins in vertical position, and there are no restrictions. If the two thick pins in a horizontal position, there is a small risk on a short circuit, due to filament sagging. In this case you do this at your own risk. We know of quite some people doing it this way, and we never had a complaint, so we believe the risk to be very small, or perhaps theoretically only. In this case don't knock on the working tubes. (See also the next FAQ)

6. Are EML tubes microphonic?

There is no good answer to this, because no standardized measurement exists. From our side we have done what we could, such reducing mass of the filament suspension systems, and using triple mica in some of the larger tubes. Unexpected for many, the inner system may not be attached to the mica too tight, or a ringing effect will occur. So if you hear some small mechanical noise while shaking a tube, this is not creating microphonics, rather preventing it. Besides, the electronic circuit plays a large role in the overall microphonics. Good and bad circuits exists. So you can have amplifiers where microphonics is never an issue, whereas with some other amplifiers microphonics is an issue from the beginning. With the last category, strong focus comes on the tubes of course. People try to judge the microphonics by tapping on the glass, but the relation between tap-sensitive tubes and microphonics is not very direct. When you tap on the tube, some inner system noise gets audible, such as resonance of the filaments. However, such noise will not appear under normal use conditions. Microphonics means the acoustical sound signal gets in, through the glass, which is another path, and follows other rules. Better is not to tap on a working tube, because you will stress the filaments. While glowing, these are fragile, so they should not be tapped on when hot. To some extent, mesh tubes are less microphonic, since the woven mesh wire by nature is one of the best damping materials, also used in industry for this purpose. This does not apply for the fake mesh tubes that are made in China, these are solid plate with holes in it. This has only the optical effect, and nothing else.

7. Can I tap on a working tube?

The answer is no. The filaments are red hot glowing, and the coating powder is easily loosened due to filament vibrations. Also, the filament's lifetime can be shortened, due to crystallization effects. This can be triggered by vibrating the read glowing heater wires like a guitar string. (the sound you hear, when you tapping a working tube). Once crystallization starts, it will slowly continue. On the crystal's edges, the wire can break, at a much later point in time. Cold filaments are fully resistant against shocks.

8. Can I serialize tube filaments?

No you can not. All tubes ever since 100 years, are made either for voltage powered filaments, a for a serial circuit. There are no other tubes on this planet. A real current specified tube is made such, that if you inject exactly this current into the filament, it was made for, you will get the specified voltage with a tolerance of maximum 5%. When a filament is voltage specified, and you use another current as this tube would draw by itself, you will have problems. When you force a fixed, precisely regulated current into the filament, you can not say in advance what the voltage is going to be. Deviations of 10...25% can occur. The explanation is, a tube filament is a very nice constant current user, quite independent of the voltage! Another problem comes when you serialize two tubes of 5V, the one is 3 Ampere, the other is 3.2 Ampere. These are normal, good tubes at 5V. However if you serialize such tubes, the voltage will not distribute evenly. The unbalance can be extremely large, since a constant current user will not follow Ohm's law.

9. Can I use an electronic current source for the filament?

No you can not. People try to do interesting things with electronics. Sometimes for the sake of doing it in an interesting way. If you want to go this way, keep in mind thermionic tubes are a 100 years old technology. If something "better" was waiting to be invented, it would exist already. There is no "better sound" if you use a current source, and no longer lifetime. Even so, for best lifetime, you have to use the tubes as intended. The following is widely unknown, yet it is very basic information. That is: Tubes have VOLTAGE driven filaments, unless specified otherwise. A tube filament is not a resistor. So, it follows NOT Ohm's law at all. You can try this with an old tube, and make the voltage/current chart. To your great surprise you will find the filament is a constant current user, when you are in the range of the normal voltage +/- 20%. Even worse, you will find it possible to overheat this old tube at 50% more heater voltage. However you will find it is impossible to overheat this old tube at 50% more heater current. If you do, heater voltage will become too high, and the heater will break.

So you see, you want a constant current through the heater? Well all you need to do is connect the heater to a constant voltage and you're done! However if you connect the filament to an externally forced current source, you are doing a very curious thing. You are serializing a constant current user, with a constant current supply. That can only work as long as these two have the same value. In all other cases, something unknown will happen. Even with very small differences, the resulting filament voltage jump 25% higher or lower. Which is sure not what you intended.

Here is a numerical example. Suppose you have a 5Volt , 1Ampere tube by the data sheet. The 5V has little tolerance, the 1Ampere has high tolerance. In fact, this characterizes a voltage specified tube. This 1 Ampere is for instance +/- 10% tolerance. So your individual tube draws for instance 950mA at 5 Volt. However you ignore the 950mA, and you pump in 1000mA because you think the "data sheet says so". Then this 50mA additional can NOT flow, since the tube is a constant current user, here we have one with 950mA. The conflict comes now with electronic circuits, which have no other possibility, as increase the voltage until 1000mA flows, no matter what. This results in a filament voltage will be much higher as you would expect by Ohms law, since this law does not apply for a constant current user. The resulting voltage can be 6Volt instead of 5Volt. Which is not what you want. Another problem which occur is, the tube may not start to glow at all. Reason for this is, initial filament current of a fully cold tube is appr. 4...10x higher than normal, and some abnormalities can occur with the electronic circuit. Like it thinks there is a short, and it shuts down.

Link to article at the jacmusic website.

10. Can I have 10% tolerance on the filament voltage?

There is general misconception, if a parameter has no tolerance specified, you can take 10%. If you have too high filament voltage, this will shorten the tube lifetime. Reason is a very delicate balance, where the filament active layer is evaporating on the one hand, and regenerated from the inside at the same time. This balance is indeed very delicate, and it works best at the correct voltage. As a rule of thumb, lifetime is reduced 3%, for each percent the filament voltage is too high. If we take the 2A3 tube as example, it is made for 2.5 Volts. At 2.8 Volts, you are at +12%. Then 12x 3% = 36%. So lifetime is reduced by appr 1/3. This is so for all tubes, also NOS or Chinese.

If you are above +15% another failure mechanism occurs, which is re-crystallization of the filament, much faster than normal. The wire will get brittle, and can no longer withstand the spring tension. Eventually this will lead to filament breakage.

Damage by too HIGH filament voltage is permanent.

Damage by too LOW filament voltage can often be undone, by simply use the tube at the normal voltage. If the problem was only moderate, it will self repair, provided the tubes are new.

11. I use an AVO Mk4 Tube tester, but the test results of the tubes are not good. Are the tubes defective?

The AVO Mk2, Mk3, Mk4 are very nice for small tubes like 12AT7 etc, but not suited for triodes with low plate impedance and high plate current. AVO admits this when you look in their data tables for the 6080 tube. Also you will not find the 300B in most of their data books. That should tell enough. You can work around this, when you have a pair of new EML 300B (or 300B.mesh). The factory test values of our tubes are very accurate and guaranteed to reproduce within 5% on any good tester. With the AVO you need to set the plate voltage as on the EML tube box, and initially use -60V grid. Then change the grid voltage until you have plate current as indicated on the EML tube box. With this grid voltage you should measure the transconductance. The value should be close, to what you see on the EML box now. Leave the tester as is, and plug in the other EML tube. If both tubes show the correct transconductance this way, they are ok and your AVO also. Any differences are due to the AVO measuring with AC voltages instead of DC, and also many AVO need a new calibration, and by default are nor accurate on the 100mA setting, while using tubes of impedance below 1000 Ohms. This is a shortcoming of the AVO, which the factory has always tried to hide. Even the 2A3 measurement results, they publish faulty in the data book, so to hide this shortcoming. At under tube testers, you will find a correction formula to use. Like this you can measure with the AVO the normal way, enter the result in this formula, and the result is the REAL plate current as it should have been. If you use the formula, results will be almost correct, but do mind the EML method id DC heated, so you need also test at a corrected grid voltage, of half the heater voltage. When also apply this correction, AVO results must be 100% correct.

12. I use the Amplitrex AT1000, but Emission is not 100%. Are the tubes defective?

No, probably the tubes are fine! The problem is rather the Amplitrex software is not 100%. This is a long explanation, but interesting for users of AT1000. The hardware of the Amplitrex AT1000 is capable to to test Emission, and a lots more things. It seems made to my by a hardware designer, and not by a software specialist. Moreover, the they did not look into history, what already existed , and what state of the art historical tube testers can do. What the AT1000 is doing, it measures plate current and then relates this to the average plate current of the data sheet. Even so making an error of half the heater voltage with Directly Heated tubes. So if the average plate current is 60mA by the data sheet, and a tube under test gives 50mA, this is 80% of Average plate current. Amplitrex says then EMISSION is 80%. Please excuse me, but this is real nonsense. Emission is not the same as plate current. High plate current is the result of strong emission. Not: High Emission is the result of strong plate current. From this alone, you can see it is not the same.

Emission tested completely different, but it is beyond this explanation here. The best we can do, is give a method to say something about the Emission with the AT1000 anyway. Fact is, what is measured by Amplitrex is deviation of the average current, and since they express that in "percent", you can compare it quickly to the tolerance. Which is a good idea. However what is the tolerance, and what is good or bad? Whit that, Amplitrex produces only silence. Since nobody like 80%. Specially when the do not say 80% of average, but 80% of Emission. This is so dead-wrong, you can see this test program was not even reviewed by somebody who knows better.

So how to say if a tubes is a good or a bad? Let's see what an expert company said about this, in 1959. We refer to Funke Germany tube tester company. So what is the tolerance of Plate current for new tubes? In the blue book, Max Funke writes in 1959, +/- 30% for European tubes and +/- 40% for American tubes. Meaning, a tube may have this tolerance, and it is regarded "good". So you get plain wrong information by the Amplitrex, telling you so called "emission" is only 90%. Though what they observed is 10% deviation of average plate current, which frankly speaking is a fine tube.

Emission in reality can be at 130%, whereas average plate current in 90%. Since emission and plate current is simply not the same, such a situation can very well occur.

So how to test Emission? This can only be done with an impulse test. AT1000 could not handle this for larger tubes. First, we must talk about another issue with the AT1000: Gm is tested the wrong way, if done so in the FIXED bias mode. Of course you will measure something, but what were you looking for? Probably you are trying to compare the measured Gm, with the data sheet value. Well, that is ok, but then you must test under the same conditions, and AT1000 is not doing so in the default mode, which uses a fixed grid voltage.

In historical datasheets, have for me more meaning than anything else. Here, Gm is always specified at 100% plate current. Not at a random plate current which results by coincidence from the fixed bias mode. For this reason, EML tubes are ifactory tested by setting the plate current exactly to the data sheet average value. If you are confused now, it is the same confusion Amplitrex had. To get the idea what is the right thing to do, we stay with the numeric example we already used above. So suppose a tube has 60mA in the data sheet. Then you CAN test it in fixed grid voltage, but that will give any random plate current between 70% and 130% of average, for a 100% good tube.

However in historical datasheets, Gm is always measured at the same plate current only. That is for good reason, as Gm changes along with the plate current. So it is only logical, at 80% plate current ( 50mA instead of 60mA) you likely find Gm probably at 80% too. So you are disappointed!

Now, test the tube the right way, which is: Test a 60mA tube at 60mA. Suddenly, you will find the SAME tube producing Gm close to 100%. I know this is confusing, but take some time, to read this a few more times, and repeat some of the tests as described here.

A work around, saying something about Emission with the Amplitrex, is as follows: Take the datasheet and check at what plate current the tube was tested. Then take the required grid voltage for this, from the datasheet. Multiply this value with the transconductance. This gives a number, that you will find in no datasheet. However since Gm is in mA/V and Ug is in Volt, the unit of this number is mA. (because mA/V * Volt = mA. This indicates it is a current flow, and it's a has more to do with Emission than Ia, Gm or Ug each by itself.

Here is a numerical example for the 2A3 tube. Datasheet value: 60mA at 250V plate and -45V grid. Gm = 5.2 at those conditions. Multiply 45 with 5.2 gives you 234. Remember this value: 234mA. It is a characteristic for a good emission 2A3

Now, do the same thing with the tube under test. (WIth Directly heated tubes, you need to substract half the heater voltage to the grid voltage. This correction is needed for all testers that test with DC, and have forgotten to correct this in software. And yes, with the Amplitrex it was forgotten) So test at the 60mA and no other value. This is done at the auto bias setting with the AT1000. This will result in a result for Ug and Gm.

Suppose that needed -44 Volt Grid at the AT1000. Substract half the heater voltage of this. Heater is 2.5V. So substract 1.25 from -44. There is a minus sign, so you get -45.25V. If Gm tested was 5.5 (at 60mA!) the result is now 45.25 x 5.5 = 248. Divide 248/234. This gives 1.06. Emission is 106% by this method.

Whereas this tube would give a "so called" emission of 98% if you don't look what you are doing, and just press "TEST" on the AT1000. If a tube has plate distance tolerance (by drop error, or production tolerance) meaning Ug will go up an Gm will go down, or vice versa. So the multiplication method will make no mistakes, whereas the original AT1000 method will make errors then, like come up with emission of 110% for tubes that are 90% in reality, or vice versa.

Conclusion: For Emission tests, use the above multiplication method, and compare it with the RCA datasheet values. If you are close, the tube is new. If you are clearly below, the tube has reduced emission.

13. There is a white spark inside the tube, at switch on.

Sparks in vacuum, if they exist at all, stay only very short, and since there is no path to damage (only vacuum...) there is usually no damage. However you do need to find what causes this, because if repeated very many times, the cathode wire will chip off particles at the spark point, and loose quality that way. The question already gives a clue to the answer. As observed, this is a switch-on effect. So it has to do with the way, the tube is switched on. Basically any NEW tube can be forced to spark when it is set to draw too much anode current, while the heater is not fully warm yet. Too much anode current, means exceed the data sheet peak value. This condition cam sometimes take place at switch on. Aged tubes have less peak emission, and have not the capability any more to draw so much current, to initiate a spark. So sparks occur mainly with new tubes. The question is to blame the tubes for this, or the amplifier. The wrong approach, is saying it helps to replace the tubes, though it often helps indeed. You may be replacing the tubes with the old ones, or some others that have less emission. Replace the amplifier for another brand may also help indeed. Neither of those methods is a good one. What you need to know, is the maximum anode current exceeded, short before the spark occurred. This is the right approach. Then if you verified this (with a scope), you know where you are. When the answer is yes, the peak current was exceeded, then you have a problem with the amplifier. In the opposite case, it was the tube. There are a few ways to solve an amplifier related spark problem, but for this it needs some expertise. With rectifier circuits, we find mostly the first capacitor value is higher than maximum. We see sometimes 5x higher as allowed. A weak solution, is select tubes that can take the abuse. If the spark occurs only occasionally, it will disappear when the tube gets older, and looses emission due it's to age.

The main reason for sparks with Triodes, is too high coupling capacitors. As these are empty at switch on, these have to be fully charged at the working voltage, which is the supply voltage normally. In a common circuit, this means a charge peak current flows through the capacitor. This current flows through the grid resistor, as there is no other path. Specially if this resistor is too high, and the capacitor is too large, you may have much voltage resulting from this, that off-biases the tube at that (short) moment. If amplifiers have an analog plate current meter, you will see indeed the plate current over-shoot 30...50% sometimes. This is the risk zone, and may work out harmless. However when the off-bias becomes so large, a POSITIVE grid voltage occurs, this may spark sometimes the tube. Specially when they are new. The solution is to change the RC network. Practically speaking, make the grid resistor or the capacitor smaller, so the whole effect reduces, and the spark will be gone.

14. There is something glowing down inside the tube bulb. Is that a defect?

What you see here, is a current dependant resistor, in vacuum technology. You will see it as glowing element, but the resistance of these wire pieces is variable. This is an old technology, but it's an Emission Labs invention, to use it this way. Please refer to the 5U4G data sheet. for a general explanation of this.


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