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.

Yes they do! Good Burn-in will assure maximum lifetime and develop the final sound. These tubes are burned-in initially. This burn-in process will be extended in the first 50-100 hours under normal (home) use conditions. This means you must 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 occasional blue glow effects will disappear during burn-in, or may stay. This is not gas, and it is normal. 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 months, is necessary to repeat the 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 excists. 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 excists. 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 (fool's) mesh tubes that the Chinese make from 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. When doing so, please keep in mind thermionic tubes are a 100 years old technology. It means, if something "better" was waiting to be invented, thousands of people before you would have done so already. There is no "better sound" if you use a current source, so the only logical reason would be to get longer filament lifetime. However for best filament lifetime, you must use the tubes as intended. All tubes on this planet have VOLTAGE driven filaments, unless specified otherwise. What you may not know, a tube filament is not a resistor. 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%. So you see, you want a constant current? Well all you need to do is connect the tube to a constant voltage and you're done! However if you connect the filament to an externally forced current source, which is not the same as the individual value of the particular tube you are using, the resulting filament voltage can be easily 25% higher or lower. To say it in another way: Suppose you have a 5Volt , 1Ampere tube by the data sheet. This 1 Ampere is for instance +/- 10% tolerance. So your individual tube draws for instance 950mA at 5 Volt, whereas you pump in 1000mA because the data sheet says so. Then this 50mA additional can NOT flow, since the tube is a constant current user, here with 950mA. The conflict comes now with electronic circuits which will simple 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. 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. 10x higher than normal, and some abnormalities can occur with any electronic circuit.

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

There is general misconception, if a parameter has no tolerance specified, one 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 tube show the correct transconductance this way, they are ok and your AVO also. Any differences are due to the AVO measuring with AC voltage on the plate and the grid, instead of DC.

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

No, the tubes are probably fine. The hardware of the Amplitrex AT1000 is able to test Emission, but the software is not testing emission at all. The actual plate current is related to the average plate current, and the percentage of that is called "emission". Sorry, but this is definitely NOT how an emission test is done. For the plate current result that you will see under this condition, keep in mind that a new (+100% emission) tube has a plate current somewhere of minimum 60...70% and maximum 140...200%, in FIXED BIAS mode. Yet all of these are fine tubes with 100% emission. These numbers are from the Funke Manual 1959, which is the only handbook I know to be clear on how to test a tube in general. Also they put this into practice with it's undisputable tube testers. By the AT1000, also Gm is tested wrong in the FIXED bias mode. Of course you will measure something, but we are not looking for "something". In order to compare the Gm with the data sheet value, you must do so under the same conditions, at that is always 100% plate current. Not at a random plate current which results from the fixed bias mode. Furthermore there is a serious issue with the grid voltage interpreted wrong by the software, but this gets beyond the scope of the FAQ here. Conclusion: This is very good tester, but it has it's issues too. We recommend to use this tester in the AUTOBIAS MODE only, and results should be identical to the test results as we write on the tube box.