Just to clarify, in the UK tubes are valves, in the USA and possibly elsewhere they are Tubes!
Either way, they are the glass things that glow and get really hot!
Briefly for those of you who don’t know about these things, (and why should you) all valves have heater filaments inside them, needed to heat them to their operating temperature.
After about a minute of heating, the "standby" switch can be switched to ON and high voltage, (in amplifier language the B + ) will be applied to the valves allowing electrons and hopefully sweet music to flow through them. Switch the standby OFF, the sweet music stops but the heaters stay on whist you go for a beer or two. When ready to go again, flick the standby ON and beer enhanced sweet music flows in an instant. WOW 😊
There is a general consensus that a standby switch on guitar amplifiers puts considerable stresses on valve rectifiers and output valves.
Valve amps without a standby switch, all the valves warm up at approximately the same rate and the B+ voltage gradually rises to its peak as the rectifier valve warms up to its operating temperature, sweet music flows. Powering up with the amp in standby, the valves still all warm up at the same rate but the rectifier output, B + is disconnected from the main amp by the standby switch being OFF. Coming out of standby/ standby switch ON subjects the rectifier to full load conditions the moment the switch is thrown. This will/ might/could stress the rectifier valve and possibly shorten its life. However the standby switch still has its uses.
The standby switch is a useful feature for musicians taking a break etc because the amp is always ready to go when needed. But a valve amp with a bridge or silicon diode rectifier, at power on and the standby switch off, can help prolong the life of the output valves if off whilst the amplifying valves warm up.
The reason being that a cold amp when switched out of standby (standby 'on') the full B + is instantly applied to cold valves which over time will strip the special coating material off the cathodes of the output valves. Whereas in standby, the valves can warm up ready to go before applying the B + voltage, thus reducing the risk of damage to the amplifying valves' cathode coatings.
Some say that all this is a just myth, but whether it is or not, I have seen many an output valve, and for that matter preamp valve where loose black flakes or dust could clearly be seen inside the valves if you hold them horizontally and rotate them.
No data sheets or publications seem to state quite what this "special coating" is other than it is there to enhance the electron flow.
My advice: Valve amp with Valve rectifier, from cold both power and standby switches on together at the same time. All valves heat up at the same rate offering some protection to the valve rectifier.
Standby off when taking a break. Unless you have time to wait a minute or so whilst the amp warms up. If so just switch both off together and back on together after your break.
Valve amp with bridge or silicon diode rectifier, check both switches are off. Switch Main/ Power on and wait about a minute while valves warm up, then standby on. Standby off for your beer break, when ready, standby on, play sweet music.
Either type of rectifier, valve or silicon.
To shut down. Main power off wait 30 seconds or so for capacitors to discharge then switch off the standby. Let the amp cool before packing away, maybe a good time for another beer!