<b> <i>That's a good chronology. Details I think will generally conform to the following:

I've never seen or heard of a Dutch double D Xf1 (Although quite a few Mullard made Xf1s w/"B" codes are labelled "Made In Holland"). Anyone got an "Xf1" with an "X" factory code? I'd be suspicious, but you never know . . .

The first all-Philips EL34 to my knowledge not made in Eindhoven or Hamburg (the Philips metal bases) was designated Xf1, made in Blackburn. These appear regularly with date codes of early 1957, Feb and March, which seems to be the beginning, anyone got an earlier one?Maybe that's why the appearance of Sittard, NL -made brown base/double D version (with "X" factory code) within a few months later was designated "Xf2". These (Dutch Xf2) seem really rare, appeared in second half of 1957. Changed very quickly to "Xf3", I've got examples dated "X7K" where one is "Xf2" and the other is "Xf3". So Xf3 Dutch went from late '57 through mid 1959 (all going by etched manufacturing date codes) . "Xf4" came in by later '59; I've got multiple "X9G" that are "Xf3", and multiple "X9J" made three months later that are "Xf4"s. To this point, these all had "X" factory codes, also brown bases with pronounced concave "spokes" on the bottom. Xf4 continued into late 1961; however, after Jan of 1960 the factory code changed to "L", indicating Brussels. So there are lots of double- D Xf4 with either "X" or "L" codes. The bases did not change on the bottom with the factory code change. "Xf5" (at last we get there, Pup!) appeared by 1962; I think I've seen very late '61 Xf5 but maybe someone can confirm that; most are 1962. The additional character in the code for week-of-the-month first appeared on Xf5, by June 62; Multiple "L2D" specimens still lack it. Late summer of 1962 is about the latest I've seen for any double D EL34; anyone got a later one? So Xf5 was the last of these. Quite a few of the Xf5 version in particular seem to have been selected, labelled and packaged for W. Euro military use, these are exciting to find NIB! Oh, the bases changed during the final year or less, the concave spokes disappeared and thin raised spokes were used instead. This correlates at least approximately with the "Xf5" designation, but maybe not exactly. Anyone got the latter style base (non-indented) on a late '61 Xf4? The Dynaco one shown at the top of the thread is 1/62 Xf5, lots of photos but none show the bottom of the base.

Looking across pieces, these brownfoot double Ds show variation in overall length, flatness of top, orientation of the "D"s, and a few other nuances but none of these seem to correlate exactly chronologically or with the code changes. I've never noticed consistent differences in sound or performance across the 5 year span of these Xf2-Xf5, often use them interchangeably in good sounding pairs. They're all exceptionally good, IMO! Slightly distinct voice compared to Mullard Xf2, I would argue, a little analogous to differences between Blackburn and Heerlen ECC83s maybe, upper midrange . . .

I'm trying to understand better the shift from "X" to "L" factory codes that took place abruptly on these w/o other obvious changes, also for certain cool GZ34s, EL84s, etc. The "L" coded ones still invariably tend to say "Made In Holland". Sittard and Brussels aren't too far apart . . .

To keep the timeline in perspective, the Blackburn made "Mullard" EL34s remained "Xf1" throughout almost all this period (although with different bases, gettters, etc 1957-62). The last of those big base boys I've seen is dated 2/62, and the earliest Mullard Xf2 I've ever seen is one I have with "B2C2". So Mullard Xf2s were coming in as "Amperex" double D Xf5s were having their brief run. At the other end, the fine Philips metal base SY coded EL34s were pretty much ending as the first double Ds started rolling. I don't think I've seen or heard of a Dutch metal base coded later than '57, anyone have one of those?

I hope someone might have additional details/refinements on these interesting tubes . . .

Jeff W."]</b>

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