This is very true. The tube machines were not friendly to power cycling. Due to the amount of power invloved and loading effects. The preference was to kill B+ (high volts) to the affected section and keep the filaments hot. Further like all thermally affected things temperature cycleing ws not only a filament problem but tube seals (glass to metal) and element movement is problematic. Many high gain tubes have remarkably close spacing of the elments particulary cathode to grid1. Another phenomenum was tubes have materials to grab gasses and ions in them that only work well when hot. Temperature cycling was (and still is) a reliability issue. It was just much more easily noticed when you have more than ten thousand tubes and the differential temps from cold to hot (both filament and cabinet) are so large. The smaller mini and submini tubes were only somewhat better but smaller means closer in smaller space and prescreening weeded out infant mortality at the mechanical level. There were even special tubes developed that would better withstand being at cutoff for extended periods without losing emission. I have a few of these, they are pin compatable with 12AX7 duo triode but have slight differences as they are for extended operation at cutoff. Outside of computers I have years to commercial radio where tubes have had long standing in the transmitters especially at high power. Power cycling was and still is avoided. When the power has been removed such as for maintenance or expecially retubing (done based on time in service) the initial bring up is done slowly to warm up the "getter" (degassing) and to avoid thermal shocking parts. It's also not uncommon to see step starting built into the power supplied to avoid the harsh effect of surge loading the power lines or trying to bring 500uf of capacitor up to voltage in a half cycle (kills rectifiers!). The last bastion of widespread tube use was television and over time even tube sets (particulary those with series string heaters) adopted "instant on" circuits which ran the filaments at 50-70% when powered off and removed the operating DC. This reduced warmup time to nil and also improved the operating life of the tubes and the reciever greatly. Treated right tubes(valves) offer excellent reliability and lifetimes mistreaded and those can be drastically shortend. Allison