the key to understanding tube production and sales is to understand that there were unavoidable and significant variances in how the innards of any tube (but especially a small signal tube) were assembled, thus affecting their performance ... (of course back in those days, vacuum tubes were absolutely essential components in equipment much more important than hifi gear making music... so great pains were taken to make good ones with pure materials and extremely careful assembly)
thus it was the screening and testing of tubes that made all the difference...that is why tubes were graded and sorted and branded/marked as such, many many portions of lots were rejected or sold off brand
as time has passed, 40-50-60 years go by, old tube supply doesn’t expand (garage and dusty warehouse finds are possible, but so rare nowadays), yet tubes are in use every day, thus making the need to test, confirm function, quietness etc etc is as important as ever if one is going to spend some real money on ’nos’ or used old stock tubes
thus it was the screening and testing of tubes that made all the difference...that is why tubes were graded and sorted and branded/marked as such, many many portions of lots were rejected or sold off brand
as time has passed, 40-50-60 years go by, old tube supply doesn’t expand (garage and dusty warehouse finds are possible, but so rare nowadays), yet tubes are in use every day, thus making the need to test, confirm function, quietness etc etc is as important as ever if one is going to spend some real money on ’nos’ or used old stock tubes