Here we see a classic advertisement from CBS, in which they explain why their tubes were superior to the competition. There are a plethora of examples similar to this copy for the CBS 5U4, and they each struck me personally as cute, clever, and well thought out.
CBS tube advertisement can teach us a lot, if we ‘read between the lines’.
If you cannot quite make out the copy, here is the main highlight. The plates are made from a ‘non-emissive’ material, which takes some explaining. Essentially, the plate heats up enough (from electron bombardment) that electrons are emitted from the plate itself, and attracted back to the filaments! This is not good, so the design of the plate material is such that this phenomenon is all but eliminated. Of course, CBS claims they test their tubes very strenuously, and won’t market a tube that does not meet stringent quality control. This is probably true, as the few CBS tubes I have used always seemed well-made, and performed flawlessly.
We can also see an advertisement below for the GE 5U4GB, which will tell us why their 5U4 lasts a little longer than the competition’s 5U4.
p style=”text-align: left;”>What we are told is that the ‘ribbed’ plates can collect ‘contaminants’ along the ridges, which will build up into sources of back-emission. This causes ‘hot spots’, and the back emission can eventually melt the filament! GE had other advertisements explaining all the developments used in producing their vacuum tubes, each making a GE tube a little better than the competition. I’ll give you a few more examples for a later ‘update’.
Where this is all interesting pieces of esoteric knowledge is as follows. I have seen many Chinese 5U4’s ‘blow up’ in a high-voltage Boogie, or Russian 5U4’s bite the biscuit even in a Matchless. I had originally though no one makes a decent 5U4 anymore; I may be right. I questioned how it could be possible to make a ‘bad’ rectifier, and these advertisements explained it to me. By using cheaper materials, Russian and Chinese tubes may not last as long (this applies to more than just rectifier tubes). Now, that is a pretty broad paintbrush to use; there are good Russian and Chinese tube out there today. But now you have food for thought if you should ‘go through’ 5U4’s a little quicker than you’d like. Lastly, keep in mind 5U4’s have their own evolution. A good tube manual will explain that to you better than I can, so compare the rating for a 5U4 from RC-16 up to anything in the neighborhood of RC-29.