What make is the tube? You should name names... nothing gets a mfr moving to keep their good name like the prospect of a little negative publicity. Tubes can loose their vacuum due to manufacture defects (maybe a bad batch that others bought) so it's to their advantage to act favorably towards you. Nothing's perfect and things break - but 13 months is not a fair exchange for the price.
BTW Nrchy, a wonderful observation!
BTW Nrchy, a wonderful observation!