When did Sylvania stop making tubes?

I have some Sylvania tubes that say Made in USA. Anyone know when they stopped making them in the US?
I believe they were bought out by Philips. Not sure how long they used the Sylvania for afterwards.
In the early eighties when Sylvania merged with GTE. The Sylvania name was then sold to Philips to market electronics and Osram to market light bulbs. By about '83 or '84 there were no US or Western European tube manufacturers of significance.
I just made a truck load delivery to Sylvania on 12-31 in York,Pa. I don't know but this may help you out.