When we use a normal resistor as a heater element, you will see that a 10% increase in voltage will result in appr. a 10% increase in current, so simply by Ohm's law. A tube heater however will behave much different, since it is in vacuum, and has no good way to get rid of the heat. Even so, it is made like that on purpose. The heat should not go away. This will result in an increase of the resistance. On voltage rise, the resistance will go up, and current will rise only slow. It can be seen as a constant current user effect. This effect is remarkably strong. We can only encourage you to try this out with a defective tube, you may have one laying around. When you are at the normal operating voltage, It can quickly be seen, that a 5% increase in voltage will result in only 3% increase in current. Of course actual numbers depend on the tube you take.
When selecting a tube from the catalog, one can distinct three kind of heaters:
The difference is in the intended use.
The constant current heaters, for instance the 300mA type, can all be put in series, and have a similar heat-up behavior. Like this you can save the heater transformer, and heat the tubes directly from the mains supply. In some nice and interesting designs, also the heaters can be used as cathode resistor at the same time. In all cases, current specified tubes of the same range (like 300mA as we have here as example) have a similar heat up behavior. So not one of them will acts as a fuse. It is important to know this behavior is build into the 300mA series by default. Nevertheless, this behavior is not perfect. The old timers amongst us will remember, all TV's or Radios working with serialized tubes often had a tubes with a broken heater, whereas this is quite rare with voltage driven tubes.
The constant voltage heaters, have no particularly specified heat up behavior So some may start up with 4x the rated current, others with 8x. Some will be at 1.3 times the rated current already after 20 seconds, others need one minute. Even with two of the same tube types from two manufacturers, this can have quite some differences. Also within one manufacturer, depending on when or where the tubes are made. Sometimes the heat up behavior is specified, and can be found in added numbers or letters like 6SN7, 6SN7GT, GTA or GTB. GTB for instance is a fast warm up tube, which is very pleasant for TV's as they should work as soon as possible after you turn the TV on.
It is evident, you can not serialize random constant voltage tubes, even if the current is the same, as one may act as a fuse, and blow out. So here we see already one difference between constant current heaters and constant voltage heaters: You can not always serialize constant voltage tubes, even if the heater current is the same! Suppose you take two 5U4G rectifiers, made for a constant 5V each. You just serialize them, and connect them to 10V. This will go wrong if the tubes come from two manufacturers, or even from the same manufacturer, but from different production sites, or other date codes. One will heat up faster than the other, and the 10V will divide unevenly. So there will be a moment where one has 6V and the other 4V. After a while they will balance at 5V or close, but the heat up was not good for either tube, since one was over heated, and the other under heated. Both is not bad for the lifetime. However with series heater tubes, this is not going wrong, as they are MADE for this. I am always surprised to see over and over again that most people, including professionals do not know this. When they are told, they do not believe it. When we present proof, they ignore it. It's just as if things may not be different as they were thought to be. Here is an experiment, that will change your mind. Serialize two incandescent lamps of the same current, but another voltage. Connect them to the rated voltage with a switch. You may burn the heater of one of them. After you have seen this, and understood, perhaps that changes you mind about a few things.
So far, this was the introduction. Now comes why this technical note was written.
The constant voltage heaters may not be connected to a current source. Electrically speaking this is a contradiction by itself, and we will try to explain that here.
It can be understood better when you accept the constant current user effect of a tube heater. Let's give a realistic, numerical example. We take the well known 300B by Western Electric. The heater voltage is 5V. The heater current is 1.2 Ampere, but Western Electric never specified a tolerance for either of those values. This was the beginning of a long road, full of miszakes and errorz. Now what is sure, this is a voltage specified tube. Meaning best lifetime you will get at 5V, not more and not less. There is a strong tendency by users, to take 10% tolerance for anything without tolerance specification. However this is not so. Each 0.1V you deviate, in either direction, will reduce the lifetime significantly (exponential). So if you are 2% off, the tube will not reduce lifetime by 2% but, by 20%. The further you are off, the worse it gets. Most likely +/- 5% is the (deadly) limit. So be nice to the tube, and give it exactly 5V, it is the best you can do. Make very good note, that a few percent is very much. Now, what current will it draw at 5V? Well, most of the time 1.2 Ampere, and most of the time means not always. Add to this the natural tolerance. How much is that? Well here you SHOULD take the famous 10% that people like for unspecified tolerance. Strangely this is commonly unaccepted! If you ask the so called "experts", a 300B is supposed to draw 1.2 Ampere, period! So zero tolerance on that. If you ask the same experts about the voltage, they tell you 10%. This "expert" expectation is a general error. It is based on no data sheet, and no literature, and also it is not in line with what you will observe with real, physical tubes. In real life, a 300B at 5V will draw from 1.1 to 1.3 Ampere indeed, and even 1.35 Amps is no exception.
Now let's see what happens, if you force 1.2 Ampere into a tube that would use 1.3 Ampere at 5V. (so that is 8% above average, for this particular tube). It is evident, the voltage will be lower than 5V. How far below will it be? Well the only thing that counts is a measurement. I did so, on Western Electric tube. The result was: 4.45Volt. This is 12% lower. If the heater would be an Ohm's resistor it would be 8% lower, but due to it's constant current user effect, the effect is larger.
Conclusion: We strongly recommend against current source driving of the heaters.
In case you do want to use a constance current source as heating device, the question raises: WHY are you doing this? This costs extra money, and you expect something beneficial from the investment. There may be a small advantage for heater lifetime, but only if you can make sure, the plate current is switched on, after the tube has reached 5Volts, and that can take two minutes or more with a current source. (MEASURE IT, and you will see!). During all of that time, you are not supposed to have high voltage on the tube. Also the source current needs to be adjustable to each individual tube, such that you get indeed this 5Volt exactly. From the above measurement with the Western Electric tube, you must count with a range of 12%. So not just a little bit. Make sure you do the adjustment with a fully hot, thermally stabile tube, and do not swap the left and the right tube by mistake. Overall the question is really WHY are you doing this. Why are you taking a risk with the tube, if there is no benefit.
Here is an issue with hum. When connecting the DC module with one end to ground, and the other end to the filament, this greatly increases the sensitivity for the tube for AC ripple. So you might be disappointed with the result. In this case you are amplifying the AC ripple with the tube gain now, instead of rejecting it with the classical (differential working) connection scheme. Either you understand this, and it rings a bell. If you not understand exactly what common mode rejection means, we can not explain it here, but just look at these diagrams and use the right scheme. (The diagram is a part of an kits from www.jacmusic.com)
LAST BUT NOT LEAST, THIS ISSUE. Never use a filament with one end to ground and the other to the the DC module, UNLESS you do not run the tube very hot . This causes the one side of the tube to wear out faster that the other. Here is a drawing that hopefully is self explaining.
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