A quick question related to Amperex Bugle Boys

Even if the tester is properly calibrated, it is hard to estimate remaining life of any tube that tests strong--at or near the expected value.  That is because of the way tubes perform with age.  When brand new, a tube will typically test very strong--well in excess of expected value, but performance will quickly fall to near or slightly above the expected value.  The vast majority of the tube's life will be at or near this plateau. and only near the end of its life will its performance begin to decline.  The fall off from there can be quite rapid.  When you get a tube that tests strong, you really don't know where along the broad plateau that the tube is at--is it at the beginning or is it close to where performance will drop dramatically?  It is hard to tell.  If the tube is testing weak or marginal, it probably is well into the end phase--that is about all you can tell from test results about remaining life.  

If a tube is tested on a modern machine like an Amplitrex, the tube is being tested under stress conditions that are better at revealing the age of the tube.  I've seen quite a few tubes that tested very strong on machines like a TV-7 that did not look quite so good on an Amplitrex.  That tester is sort of the gold standard and most tube sellers who claim that they test their tubes will proudly claim use of that tester if they use it (some post screen shots of the readout).  Amplitrex does not use a number scale, like xx/xx, so results given with such number mean that some other tester is being used.

Well the very best by far are the units which allow testing and tracing at numerous operating points. 

Hook the Amplitrex to your laptop and it will trace the tube.  I have never bothered to use mine in that mode.  I like that it gives you real measurements, plus the expected values, and for ultra idiots like myself, it provides the "good," "weak," etc. evaluation.  

The thing with estimating remaining life is that it really depends on how a tube is being operated.  If operated gently, it may last a long time after beginning to test weak.  Also, a weak tube may still be able to do its job extremely well depending on the circuit.  I had a pair of 6sn7 tubes that tested quite weak on the Amplitrex, but the amp sounded good.  I emailed Audio Note (the maker of the amplifier) about whether it was safe to continue using the "weak" tube and I was told to continue using it because, in their circuit, even a weak testing tube could sound just fine.

Some info about the original question. I think the 5AR4s were made in England in the Blackburn Mullard factory but were sometimes labelled Amperex. Phillips was a Dutch conglomerate that owned many tube brands, Mullard, Amperex, Siemens, Valvo, and on and on. Tubes made in one factory could be labelled with the brand of a different factory. The way to tell where and when a tube was made is to check the codes etched into the glass of the tube. here is a list of the codes and how to interpret them:

Phillips Factory Codes

Any tubes you come across today will be "New Code’ so just ignore the ’Old Code’ stuff. Also etched tube codes can be hard or impossible to read, but that doesn’t necessarily mean the tube is a fake.

For example look on page 11 of the linked document for a lower case f and 3, f3 and you’ll see that’s the code for a GZ34 (AKA 5AR4)

There were also many other tube makers who had their own way of identifying tubes, for example, Telefunken, RCA, GE, etc.

Yes you are correct. Almost impossible to fake the acid etched codes. After a great deal of experience, you become aware of the internal structure of certain types from certain manufacturers. Even the thickness and shape of the glass become familiar. One of my greatest irritations currently is the contention that Sylvania Bad Boy 6SN7s extend beyond the true bad boy which is a very distinct tube.