Regarding the tubes, we often see that 7025's and 12ax7's are just different names for the same tubes, in tube data charts as a matter of fact. Why, especially in Fender amps, we see both tubes bearing different names in a same amplifier ? There must be a difference between both tubes then, even a slight difference, no?
We just have to separate what is true today from what was true when Fender made these amps.
If you look at a
7025 data sheet from the old days, they almost all claim the 7025 is identical to the 12AX7 but has less hum/noise.
The first stages of the preamp have the most amplification following them, so a little hum/noise there has a big impact. Later stages have less amplification following them, so hum/noise
in that stage could be slightly higher while still being largely undetectable in the speaker. The 12AY7 was also originally designed as a low-noise tube for the first stages of an audio amp. Therefore, Fender began specifying the 12AY7 for input stages in the 50's, then switched to the higher-gain 7025 for inputs once that type became available.
The higher gain of the 7025 most likely offset the increased loss of the new blonde/brown and blackface tone circuits.
Eventually, most 12AX7's probably converged to include features to reduce hum and microphonics. Such changes might be how the heater is physically arranged and placed in the cathode sleeve, shortening of plate (to stiffen the structure and reduce microphonics), altering plate geometry, etc. A late-60's RCA 12AX7A might have been just as good as a mid-50's 7025 with regard to these issues, as the manufacturer improved their processes.
And there used to be differences in other similar types in the past as well (just look up all the types that supposedly substitute for a 6L6).
Modern tube vendors will put any label on a tube that sells the tube. They may use/abuse substitution charts to apply various labels to types that they have which are similar to each other, but include some kind of difference. Groove Tubes would do that to denote different country-of-origin, or maybe slap a "7025" label on an otherwise normal 12AX7 on which they had performed some kind of extra testing.
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