There is a lot of myth about tubes in stand by mode. This article tries to break the situation down to some elementary considerations.

What is originally meant by stand by mode?

We really have to ask this question, because not everybody uses the same idea behind this. STAND-BY means indeed the tubes have already warmed heaters, and are ready to use immediately. This, and nothing else was the original intention of it. Some very good examples are: A military Radio transmitter for field communication. So the field station is always listening, but will only transmit as long as they speak, so to hide their position better. For that, the tubes were continuously heated, and the operator could transmit without any delay for tube warm up. A more recent application is computer tubes, also called switching tubes. So there were only on or off, and not used analog. In the "off" condition, the plate current was cut off by means of the very high (negative) grid voltage. For the cathode quality that makes no difference, how the plate current is switched off. Zero current means zero current, and only special cathodes can be used like that. This requires a special cathode construction. All other tubes, so the ones that are not made for stand by mode, will fail by low emission if used in stand by mode too long and too often.

What is TODAY meant by stand by mode?

That depends if we talk about guitar amplifiers (some 90% of tube production goes into this) or if we talk about HiFi.

Most commonly we find stand by switches on guitar amplifiers.

Introduced first by Leo Fender, who discovered that simply switch on a "half warm only" tube is the worst thing you can do to the poor tubes. This is very true! So he advised to pre-heat the tubes with the stand by switch, before turning on the amplifier. Moreover, it saves the tube life a lot, when the musicians want to switch off their gear for just a moment, and can put it in stand by instead. This was a very clever move, it cost Fender not much more than just a switch, and the amplifiers have been famous ever since for long tube life.

Here is a link to the Fender website, with a good Article about it.

What does it mean for HiFi?

It is a shame, but most HiFi equipment designers do not see the need for a stand by switch. This becomes more curious, as so many of them try to build things better as the others, and I know from my customers they are really interested in the subject. So forgive my clear wordings sometimes here, but all I care about, is good circuitry, which makes the tubes last long. Yet the majority if the amplifiers have saved 10$ on this simple part, which in the end results in longer tube life.

So again, what means stand-by for HiFi?

Well, something else as originally intended by Leo Fender. It was well known that with specific tube equipment, which was never switched off, the maximum run hours of the tubes increased a lot. Like broadcast transmitters running 24 hours per day, would run 1...2 years on one set of tubes. So 20.000 or more hours could occur with tubes only made for the usual 5000 hours. Also a tube oscilloscope would run 10 years failure free, even when switched on every day from 8pm to 6am, and may need not even all tubes replaced. I own a Hewlett Packard HP130 tube oscilloscope, and the first set of OEM tubes (Telefunken <> E88CC) was still in there when I got it.

For HiFi equipment, people wonder if it makes sense, to have the amplifier in stand by mode, I hear this question often.

Then I ask: Well, does it have a stand by switch? If the answer is yes, all I can say: Use it like intended!

Most of the time though, such a switch is missing. Then people use the word wrong. They mean to leave the whole amplifier switched on, even over night, and they call that stand by mode. Indeed this is a very good idea, when you want to hear no music for a while, but still it has to be said, the most damaging thing is the cold switch on. So still on a long listening session, at least one cold cycle will take place. (Unless your amplifier has a pre-heat option)

Indeed, the start up phase of the tube is giving most of the wear out. Here are a few reasons why this is so:

1. The cathode metal itself. Though you can not see that with the bare eye, metals are always crystallized. These crystals are not passive. Over time, they split in smaller crystals, and the material becomes more brittle from that. This can cause the cathode wire of a directly heated tube to break, but also the heater wire of an indirectly tube can break from this. This process of recrystallization takes place in the cold tube as well. Though it is slow at low temperature, the cold (storage) time is of course the longest, even decades, and it can not be neglected.
2. Switching on and off. This is probably the most devastating part, as recrystallization is triggered by temperature CHANGE much more as by temperature itself. So, a slow warm up is always better. And yes, fast cool down is just as bad. So if you design special circuitry for slow warm up, don't forget the slow cool down also. During this phase however, plate current should NOT flow. SPECIALLY for tube rectifiers this applies.
3. High temperature (the use period) makes the recrystallization go much faster, but compared to (decades?) of storage time, the use time is small. Extreme care however should be taken with the heater voltage, because recrystallization accelerates if the normal voltage is exceeded by more than 5%. Above this limit, every single percent significantly reduces lifetime. So 5% is really a very tight limit, and many sins with bad amplifier design are made with this, because it is so comfortable to ignore. However all tube data sheets are very clear about this.
4. Cathode Chemical Structure. Changes with this are the cause of tube aging, but this is the most difficult process of all. So we stick to the final conclusion only. This is: Do not draw current from a tube which is not fully warmed up. Just do NOT do it. Also not with rectifier tubes. If you respect this rule, you can get rewarded by a lot more lifetime. How much more, depends on the design of the amplifier, but 98% use the hardest condition anyway! The hardest condition is: Just switch the whole amplifier on at once, and after a while it begins to play gently. Though inside the tubes, it went not so gently at all. All tubes, most specially the rectifier and other power tubes, were forced to draw a lot of current, like 100mA or more, while the cathode was barely (or better say NOT) ready for it, and just beginning to work. We can say, in this phase, which was only 5 seconds, the wear out was the same or higher, as during the whole rest of the listening session.

So what to do?

Amplifier designers, and people who want to become such: Use EXACTLY the heater voltage as in the data sheet. So when it says there 2.5 Volt +/-5%, this is the tolerance for guaranteed tube life. Not the value for best life, or in other words, not the ideal value. The ideal value is 2.5V +/- 0%, measured at the metal of the tube pins, not ar the solder contacts.

Note, with a tube like 2A3,this 5% means only 0.13Volt deviation. Below 5% or at 0.13V less, tube damage will occur and above 5% (so +0.13V) lifetime will begin to be less.

Actually each single percent deviation is not ideal l for the tube. We see often sometimes so called "bad" 2A3 tubes, which were in fact under heated. 2.5 Ampere is very much, and there are two socket contacts. So the is only 63mV per contact, at 2.5 Ampere reserve, which is really very little.

Moreover it is of course the wrong approach to have 2.5V exactly at the tube pins, and use the tube tolerance to compensate for contact voltage loss. First, this is not what the tolerance was intended for, but also in case of AC heating, there is zero percent left for the mains voltage variation. For 2A3, as a rule of thumb is, use 0.1V more, so 2.6V at the solder contacts, and you are safe. Any really professional transformer for AC heating is set up this way. Like all Lundahl transformers for 6.3V heating are 6.6V at the transformer side. This is indeed 4.5% more and allows conveniently for voltage drop across the wiring, and tube socket contacts.

The user: Prevent the cold start up period, by leaving the amplifier switched on. For instance you want to have a break of a few hours. In that case, it is better for tube life, to have the amplifier simply switched on during this time. The question is always HOW LONG, but that is really hard to answer. I would say, something like 4 hours. Even so, repeated switching on and off, searching for the source of a noise, or whatever reasons occur, is really a very bad thing to do. In that case, you allow the tube no regeneration phase.

What will be the result of good use?

Hard to say, but a doubling of the lifetime, particularly for power tubes seems possible under common use conditions. In case of exceptional few switch on /off cycles, so which probably not reflects what most people are doing, lifetime can even be 5...10x higher.

So in any case, good and careful use pays off. For this, check the article on the top right of this page, or directly from here. This pair of 45 tubes is switched off only 8 hours per day, when the owner goes to sleep.