I just watched a youtube video which is telling that standby switch only mutes an amp and has nothing to do witch amp tube life. It is not the thing I used to hear from tube amp users. What is the truth?
The video doesn't address the type of situation where the standby switch is useful to mute the amp while keeping the tubes hot.
Here's a realistic situation: you're at the studio using an amp, you want to change rigs, you use the standby switch to mute the amp while keeping the tubes hot so that the filaments do not experience thermal stress from heating and cooling. If you're plugging/unplugging in pedals then just "turning your guitar down" doesn't help. Similarly, if you took a (lunch?) break, it would be better to flip the standby switch, to protect it from someone accidentally yanking out the cables (a contingency the presenter doesn't consider), than to turn the amp completely off. A similar situation arises between sound check and the start of the show. In these kinds of situations using the standby switch saves the tubes from thermal cycling, which contributes to one of tube failure modes.
With respect to powering on/off, I think the presenter is technically correct, but maybe not applicable to everyone. My pattern is to, first thing, power on the amp with standby off. Then go about doing the other setup/prep/tuning whatnot that needs to be done. For the first few seconds to minutes while the tubes are heating you can't play anyway, and leaving the standby off protects the amp from plugging/unplugging pops (I'm absentminded enough that this would be a problem for me if I just turned everything on). Then, once everything is situated, turn the standby on. This makes sense to me and is more about protecting the amp (speakers really) from pops while the amp warms up than about extending the tubes' lifetimes.
Technically, nothing in the video is incorrect, however it strikes me as incomplete in that it doesn't consider the situations where the standby switch can be sensibly used to avoid thermal cycling of the tubes.
I only partially agree with the answers above. The standby switch is turned on when you turn the amp on to pre-heat the tubes for about 3 minutes prior to performance. This prevents stress on cold tubes and keeps the tube system healthier. Think revving a cold car engine - not very good for the engine. I think we agree on this.
When you walk away for lunch, between studio sessions, or between stage sets, I do not recommend turning on the standby switch on most Vox amps (especially the older ones), because that standby switch just builds up current in all the capacitors around the tube system. This I believe results in excessive heating of the tubes (that are already hot) and if a capacitor decides to blow from age, can potentially start a fire. At the very least a very nasty smelling smoke cloud and weak amp. When you want to leave the amp hot and walk away, turn all your volumes down and let the circuit flow the way it was designed to flow. If you need a quick change out of guitars, then flip on the standby momentarily, but don't leave a hot amp in standby for extended periods.
I could be wrong, but my knowledge of electronics is extensive and have seen the smelly cloud come from amps that are left on standby for extended periods. In addition, if you are going to work on an amp, stay away from the standby switch. The standby switch prevents draining of current from the capacitor system. And of course, bleed and check those puppies before poking around them.
It isn't a mute switch. It allows power to go only to the valve heaters. You switch on with Standby active. Wait a bit, flip Standby off. Whether it's worth doing is debatable.
The stanbdy switch is a fraud... it does not extend the life span of the output tubes, an in fact leaving the amp in standby will reduce the life span of the valves due to cathode poisoning.
The standby switch was introduced by Fender in the Bassman line to protect not the valves, but the cheap crumby capacitors they used back then. That "protect the tubes" shitchat was a smoke screen that Fender uses until today, and other manufacturers like Marshall and Carvin just contribute to spread the myth.
So if you have a vintage Fender, do use the damn switch. Or recap your amp with proper rated components and never never touch that switch again.
Old 50's TV and Radio sets never used standby switches, the original Vox AC30 didn't have one. The newer Vox reissues have a FAKE (bypassed by a resistor) standby switch, that they included just to stop the flood of consumer questions about the matter. A switch is cheaper than hiring more people to answer dumb guitarists' questions.
I have a recapped Bassman copy (a Brazilian Giannini Thundersound 3) and rewired it without the standby switch. I repurposed the switch as an 8 <---> 4 ohms selector for the output transformer.