I completed my ST-70 from Bob's kit back in 2009, and was so taken by the sound quality that a year later built a set of Mark IIIs as well. All these use the VTA board design (you'll find photos of my ST-70 and Mark III builds in the photos section).
I work from home and my amps are on EVERY day of my life as I listen to music all the time while working.... I fire it up usually around 9:30 AM after my morning meetings and switch off only around 7 PM, sometimes it runs till 11 PM or 12 PM at night. So.... Many years ago in my dim distant past I used to be in the South African Air Force and involved in Early Warning Radar as well as mission control. IIRC I think we were running British Marconi radar systems at the time, and one day one of the radar Techies told me that with the radar systems running 24/7 it means that tubes last a lot longer than if the equipment were power-cycled every day (CRT tubes, transmitter and receiver amplifier tubes - all of of it - high-power Tactical Radar systems in the early 1970s were very much cavity magnetron + vacuum-tube driven).
QUESTION: Would I be extending the life of my amplifiers tubes by leaving it on 24/7 ? Or as an alternative, I once read somewhere of a mod that puts the heaters in "standby mode" - dropping the heater current so that it stops conducting, but still high enough that the warmup time coming out of standby is a few seconds at most. Do anyone know of this mod?
I rotate between my Mark III amps and my ST-70, to try and spread the Wear 'n Tear a bit. I favour the Mark IIIs over the ST0-70 for pure sound quality.... And after thousands of hours of continuous use I have had no problems whatsoever with any one of my amps (other than replacing the original set of KT88 tubes in my Mark IIIs - the Gold Lions I ordered originally must have been a bad batch, the subsequent set now have in excess of 1,500 hours on it).
Your opinion on this is VERY welcome!
-- JunkyJan, BC Canada