7025 vs 12ax7 - which are the differences ?

[kagliostro]

What is to aspect as difference using a 12ax7 in place of a 7025 tube (Fender 6g2 reference amp) ?

Many Thanks

K

[eleventeen]

None. The 7025 is allegedly a low-noise 12AX7. The best 12AX7 is probably quieter than the noisiest 7025 but it's the same tube.

[tubeswell]

I bought a NOS RCA7025 last year expecting great things and was disappointed to find that it has a noticeably higher level of hiss than most of my plain ole' Sovtek 12AX7s

[HotBluePlates]

What is to aspect as difference using a 12ax7 in place of a 7025 tube

For any 7025 and 12AX7 samples you're likely to have, probably nothing.

I bought a NOS RCA7025 last year expecting great things and was disappointed to find that it has a noticeably higher level of hiss than most of my plain ole' Sovtek 12AX7s

The 12AX7 was made for quite a while. After the original 12AX7, additional variants were issued with various industrial numbers (including the 7025) as well the 12AX7A.

If you look closely at all the sheets, all conditions listed for the 12AX7 are identical to the 12AX7A and the 7025. The only claimed changes are lowered hum, lowered noise and microphonics (in some sheets). If you look closely, you'll see the same average noise figures cited in both the 7025 and 12AX7A data sheets, suggesting they are essentially the same tube.

G.E. 7025
Sylvania 7025
Tung Sol 7025
G.E. 12AX7A

If you stop and think for a second, what are the "features" in 50's NOS that dealers keep trying to sell you on?

Aside from black plates, they're described as "long plate" tubes. Notice in the data sheets that the 7025 and 12AX7A sometimes mention lowered microphonics. That's because they shortened and stiffened the plate structure, with some variants adding extra mica spacers. No manufacture ever really went back to a longer plate, higher microphonic tube.

Same for reduced hum. Some changes were made regarding heater coatings, and a number of manufacturers wound the heater in a spiral to relieve tension causing breaks in heater insulation and leading to higher heater to cathode leakage (and therefore hum). The spiral wind might also reduce current-induced hum inside the tube.

Again, once the improved version was being made, the improvements likely made their way into the standard product. I notice that I've seen RCA 7025's that had black plates and were obviously from the late-50's, while I never seem to see RCA 7025's from the mid-60's. However, I have a bunch of mid-60's RCA 12AX7A's...

My point is the improvements that made a 7025 likely made their way into many of the modern 12AX7 offerings to some degree.

As for the extra noise, might either be a bum sample, dirty pins, or ...

[kagliostro]

Many Thanks to ALL

HBP really a very good and interesting explanation, Thanks

K

[HotBluePlates]

You're welcome.

Some of the point I made were derived from Getting the Most Out of Vacuum Tubes, 1960 by Tomer.

[kagliostro]

Thanks again HBP

I've just downloaded the book and want to read it

here the link for those who can be interested

http://www.nutshellhifi.com/library/MostVacuumTubes.pdf

K