Factory code = "*"
Factory code "B"
Factory code = ">"
Factory code = "D"
"VALVO was an old and renowned german company. their tubes are of excellent quality, and at least as good as for example Telefunken, Philips, Mullard or Siemens tubes (of course - i'm referring to the originals, only).
according to my experience VALVOs are often even better than Teles ! in fact, Telefunken had gotten tubes from VALVO since the early 30s, put the diamond-logo on and sold them as "Telefunkens". at those times nobody really cared inspecting a tube very closely to possibly discover its real origin (knowledge of where to look for what wasn't widely spread, either).
since you're asking,- here's a bit of history: before 1914 the company "C.H.F. Mueller GmbH Hamburg" produced only X-ray tubes (therefore, they were known as "Roentgen-Mueller"). after 1916, probably triggered by the requirements of WW-I, "ordinary" tubes (mainly Xmitting ones) were added to the program.
in april 1924 Mueller created a daughter-firm called "Radio-Roehren- Fabrik GmbH" or just "RRF GmbH" (meaning radio-tube factory); they took over the tube-production from Mueller.
in 1926 RRF was renamed to "VALVO". that name was inspired by the latin word "valvae" (something like door wing, to symbolize the flow-control of electrons), plus the english "valve". since then VALVO used their typical logo, a sort of jagged ring with a stylized "VALVO" written inside, which was kept (almost) unchanged for more than 5 decades.
in 1927 Philips bought C.H.F. Mueller (Mueller had financial problems), and later in 1932 also VALVO (Mullard became a Philips-daughter, too).
BTW, Philips was founded in 1891 as "Philips & Co" for producing and selling light-bulbs. after significant expansion also internationally they changed to "Philips AG" in 1912. AFAIK their headquarter has been in Eindhoven (holland) ever since, till today.
VALVO didn't just produce tubes (incl. camera/pencil/Geiger/PM types, & up to 1 MW klystrons), but a whole lot of other stuff like ceramic caps, piezo-elements, ferrites, smaller motors, transformers, deflection-yokes and (b/w + color) picture-tubes, diodes, transistors, loudspeakers, etc.
it's amusing to see how eagerly people are sometimes pointing out major differences (prices follow accordingly) between tubes e.g. made by VALVO, Mullard, Telefunken, Siemens or Philips. one should be aware of the fact that tubes were frequently exchanged & re-labelled among those and other firms.
as an aside,- Telefunken started off with the prototype of their very 1st EL 34 (metal-base) in october 1954, which was an exact copy of the VALVO EL 34 (Philips had developed the EL 34 years before). at that time, only some slight changes were done by the Telefunken engineers (e.g. the lower pair of cooling-wings was dropped); later other mods followed."
The above quoted from a posting to rec.audio.tubes by Herbert R. (e-mail h_r <at> vcs NO SPAM <dot> de )on 12/08//99