i think there is hour ratings in the old RCA handbook. the old tubes run thousands and thousands of hours. groove tubes told folks to install fresh tubes every 6 months. of course they sold a gajillion tubes but... i just brought an early nineteis rivera amp home 2 nights ago and.. walla! groove tubesThat's what the M.O.Valve company quotes for life expectancy on KT66's and they go onto say that 10,000 hours is not uncommon. I think that big Marshall book that Mike Doyle put out nearly 30 years ago has the sheet from M.O.Valve(GEC).
I wonder what the reissue ones that Electro Harmonix has made is..