I found an interesting tube-related article in the Dec-1955-Jan-1956 issue of NATIONAL RADIO-TV NEWS. The article exposed a supply-chain problem during the 1950s where unscrupulous vendors would buy “pull-outs” (duds or near-end-of-life tubes) from repair shops, then manipulate the tubes in a manner that would make them look reasonably new. The tubes were then resold to distributors and repair shops, who assumed that they were buying “factory seconds” or possibly even fresh inventory.
The defect rate was enormous. The first stealth order of 21 tubes showed 14 tubes to be defective; the second stealth order had 17 defects of 20 tubes purchased.
No doubt that some of these gems are still floating around today. While the article seems to suggest that many of the tubes were resold with their original brand name intact, I suspect that most of the “off-brand” or “private label” tubes were using the same unseemly source (pull-outs) for their inventory.
When I test vintage tubes labeled as “Standard Brand”, “Rad-Tel”, “Atomic”, etc…, the defect rate is far too high (in my opinion) for them to have been “factory seconds” or “quality used”. That doesn’t mean that all of them are duds, of course, but that they need to be carefully checked before selling or using.
I would point out that the advice at the end of the article is not completely relevant today, almost 60 years later. It is not uncommon for the tube designation to be hard to read, or completely missing, because some labeling can be EASILY wiped off while cleaning the glass envelope. Furthermore, it is common to find NOS tubes with oxidized tube pins that must be cleaned — sixty years and varying storage climates will “do that.”
[ Here is a PDF ] of the article that I scanned & cleaned up with Photoshop for good readability.