This subject comes up by email sometimes, so we put the explanations here.
We all try to do things the best way. The intention of Stand-by circuits can be to ensure a fast and noise-free start up of an amplifier. That is why tube amplifiers for electric guitar, often have a Stand-by mode. The Rolling Stones would not want a guitar amplifier on the stage, starting up with "not the ideal sound yet" while thousands of people are listening to it. But also they would not want it switched on, all of the time, with no guitar attached to it, adding perhaps unexpected noises, like a loud bang or a crack, or whatever things can go wrong when they plug in the electric guitar. Here, a Stand-by switch is totally ideal. The fact, this is very bad for the tubes, plays no role, because tubes for guitar amplifiers are abused anyway, and must only give the desired distortion.
With HiFi, things are different.
There is always time to wait for the amplifier to warm up. Tubes here, are no cheap replacement products, and we don't want distortion. The idea of Stand-by circuits for HiFi, is to prolong tube life. However we must understand what causes long tube life. That means two things. One is, doing things right, such as keeping plate dissipation in the recommended range, and using heater voltage as exact as you can. The other is not doing things wrong, such as tapping on a working tube, or use funny heater circuits, which tube gurus sell in the internet. Or, make mistakes while operating the Stand-by mode, if the amplifier has this option.
Stand-by Mode. What is good and bad about it.
The most devastating moment for a tube, is when the amplifier is switched on, and the tube after some seconds becomes capable to supply a small plate current already, but not all of it. It is simply just the warm up phase. Amplifiers without Stand-by switch can not avoid this condition of course. It is during THIS moment, which takes only 5 seconds with a directly heated tube, the tube is mistreated, and this causes a wear out, equivalent to some 2...4 hours of normal use. This causes the infamous cathode stripping, and it's the white powder debris which can be found sometimes in very old tubes. On the other hand, this is nothing to worry about, tubes and amplifiers are made like that since 100 years, and we know what is the resulting lifetime of tubes, doing it this way. EML tubes have what we call slow start filament, meaning we stretch the heat up period a few seconds longer, and this prolongs life time significantly. So far this is without Stand-by mode.
What Stand-by Mode will do
The filament is warmed up fully, before the anode current is switched on. This can be done simply by a hand operated switch, or electronically with a timer. Like this, the additional wear out is avoided, and tube life is maximized the best possible way.
However we must keep in mind, there is also a problem related to Stand-by Mode. If the filament is heated, without anode voltage, this must be limited, and should not exceed 2 hours. Otherwise, some damage mechanism begins to take place. Moreover, the use period following after this, should at least equal the Stand-by time. If the above conditions are respected, Stand-by mode is the best you can do.
So to say it clearly, amplifiers which are in strand-by mode for 23 hours and use the tube for 1 hour per day, will damage the tubes for sure.
The best, and ideal use would be to heat up the filament for one minute or so. Then, activate the anode current, and use the amplifier for whatever use period you had in mind.
At Switch off, use the reverse procedure. Otherwise the starving tube filament gets the same hit, as the waking up tube. So first deactivate the anode current, wait a few seconds and then switch off the whole amplifier. With Electronic circuits this is forgotten most of the time.