หลอดสำหรับปรีแอมป์ตู้แอมป์กีต้าร์ Bugera เบอร์ 12AX7A
The 12AX7 was introduced by RCA in late 1946 or early 1947, along with the 12AU7. The original construction was the "long plate" style, identical to the 12AU7 plates. As was common at the time, popular designs were cross-licensed between manufacturers, and since the design was specified to the mechanical level, all American 12AX7s through the 1950s had the identical long-plate construction.
In the early 1950s, various European manufacturers started making their own versions of the 12AX7 (some under license from RCA), and called it the ECC83. The early versions had the RCA-style long plates, but I've seen some some exceptions, such as the Mullard "box-plate" version. Some European tubes started to be imported to America, most notably tubes from Mullard, Telefunken, and Philips (labeled Amperex for the American tube company that Philips bought around 1945). These European tubes were generally made to better standards than the American tubes and typically had lower noise and microphonics. As a result, high-fi vendors started using these tubes. To fight back, RCA and GE came out in 1961 with a re-designed 12AX7 called the 12AX7A that had lower noise and microphonics. They did this by using a shorter plate structure (with the grid pitch and distances adjusted to keep the same electrical specs) which allowed stiffer cathodes and grids. This became the American "short-plate" structure. It seems to be based on the design of the 12AY7 by GE around 1949, which started with a box-plate but by the early 1950s had moved to a short-plate structure. It may also be related to the ruggedized 5751 which came out around the same time and had the short-plate structure.
Interestingly, Sylvania, the other first-tier American Tube manufacturer, made only a long-plate 12AX7A that had more ribbing and reinforcement than the old long-plate. Based on curve-tracer measurements, the Sylvania 12AX7As have somewhat higher bias currents than the standard 12AX7, i.e. at a given negative bias, it draws more current.
As far as one can tell, the 7025 is identical to the RCA 12AX7A, but was made to cater to those who wanted an "industrial" type number. This was kind of a fad with hifi designers in the late 1950s. Maybe it was tested to tighter specs, but most 7025s are marked 7025/12AX7/12AX7A.
Philips (and hence Amperex and the other Philips companies such as Mullard and Valvo) adopted a short-plate design for the ECC83 around the same time as the American 12AX7A came out. With the exception of the box-plate Mullards, all the western European ECC83s were either long-plate or short-plate designs.
On the other side of the iron curtain, the USSR came up with a completely different design for the ECC83/12AX7 that one thinks originated with the 6N2P. There was a shield between the two sections, the plates were even shorter than the American/European short-plates, and the plates were spaced rather far from the grids. Again, the grid and cathode were adjusted to give the standard ECC83 characteristics. This design also showed up in China, and the bulk of newer tubes from Russia or China are of this design.
So, with the exception of the later Sylvania 12AX7As, all these tubes have essentially the same characteristics - the i vs v curves. What, then makes them sound different? One doesn't really know for sure. It can be suspected the cathode material and processing makes the biggest difference. There may be secondary emission effects that depend on the plate material. As far as we know, the getter shape doesn't affect the sound, but if the getter flash got into the wrong places, it could cause leakage and noise. The type of glass used might even be a factor, since different glasses have different resistivities which affects how long electrons "stick" to the inside of the bulb.
One thinks the best we can do is note which tubes sound like what in which system, note it, and then use the knowledge to chose your next tubes. In this sense it is more of a taxonomy than a predictive science
