This is a comparative test, for which we used a new Fullmusic tube, a new EML tube, and an NOS Sylvania tube in it's original box. In this test we measured how the tube bias depends on the filament voltage. There are several reasons why we prefer this change to be as small al possible. Most of the larger Hickok tube testers, even use this method for the "Lifetime Test" circuit. Consequently, the Fullmusic tube 2A3 as seen below in the table, did not pass the lifetime test the on the Hickok 750 I have.
1) In many amplifiers, the filament voltage is not electronically regulated, and is depending on the mains voltage variations. This means if the mains voltage changes 5%, the filament voltage will also change 5%, and this will lead to another bias. How much that change is, you can see in the table. We marked any deviation in red, where the bias changed more than 10%, and also it must be said that it is very unlikely the filament voltage comes near 2.1 or 2.8 Volt. However, deviations of 5% up and 5% down of the mains voltage during the day is nothing unusual, and then we talk about a total change of 10%. For this reason, it is extremely important that the filament transformer does not add it's own deviation to this, due to production tolerance, or uncareful design. In practical situations it may be that a small percentage is added to this by the transformer.
2) Another reason why we prefer this variation to be small is, that we at Emission Labs pay much attention to highest Emission reserve. Such a tube will tend to be more independent of filament voltage variations. For good performance and long lifetime, it comes down to the filament being strong and healthy. If new, such a filament will be able to provide a lot more current than the tube actually needs. So, changing the filament temperature, will have a small effect only on the plate current. On the other hand, if the filament has no high emission reserve, in that case lowering it's temperature will naturally result in lower plate current. This mechanism was already used in the laboratory testers of Hickok (such as 532 and higher - it's the "Lifetime" switch). So the filament voltage was lowered, and the meter may stay in the green still. Also modern tube testers like the Amplitrex AT1000 use this method still today. It must be said that at the Emission Labs production, we use another (direct) method to find out the maximum emission, but observing how the tube reacts on filament voltage changes is an indication for this, and most of all this can be practiced by DIY.
Note, we do not say what is a "good or bad" result for this. We do not even know our self where the "bad" limit is. So what we can do, is make the EML tubes such, that they perform possibly constant over filament voltage.
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