PI-Clipping

[stiltamp]

Hi,

how does the 12AX7 long tailpair PI clipping sound, fine or terrible?

regards
Manfred

[Cliff Schecht]

It's typically close to symmetrical clipping so you get a lot of odd-order harmonics without much even. The best mix of harmonics is a nice blend of even and odd, with even-order distortion sounding more musical (occurs at octaves) and odd-order sounding more harsh and thrashy.

I think the goal typically is to avoid distortion in the PI though, at least in most designs. The fact that this stage is typically encompassed in the NFB loop is going to make distortion even harder to achieve.

[jjman]

There's an article out there about how the 12at7 is far superior to the 12ax7 when pushed hard since the max current ratings for each are far apart. But he doesn't get detailed enough IMHO as to exactly what is happening in a 12ax7 under high current that's bad. (Not that I doubt him.) I took his word for it and switched to the 12at7 on my pseudo Express and the PI still clips when I want it to. I have a post PIMV so I can see the PI clipping vs the clamping of the PI from the output tubes clipping. 12at7 amplification factor is a little lower. Cost for NOS is much lower. I didn't notice a significant change in tone but assuming they bias differently a switch could yield different breakup characteristics.

Most Marshalls use 12ax7 so.......

[KT66]

Many of the classic non-master volume amps that we know and love generate quite a bit of their overdriven tone when cranked in their 12AX7 LT PI, including the Tweed Bassman and Marshall derivatives thereof, VOX AC30, Brown Deluxe (6G3), etc.

So, I think the answer is that it sounds pretty darn good.

[stiltamp]

Thanks to all!

cheers
manfred