12au7

[echoes420]

Can anyone here explain to me what exactly is the difference between a 12au7 and a 12ax7?

I haven't found a satisfactory explanation anywhere on the web.

 

[MrMarkIII]

Different pre-amp tubes have correspondingly different "gain factors", meaning they amplify the signal by varying amounts.
For example a 12AX7 has a gain factor of 100, while a 12AT7 has a factor of 60.
So, a 12AX7 has about 40% more gain than a 12AT7.
In the real world, this means that if you get a given level of distortion at "6" with the 12AX7, replacing it with a 12AT7 will require turning the knob up to "10" to achieve the same sound.
This is all approximately, of course, and will vary according to your ears, etc.
It's important to realize that gain factors in pre-amp tubes will have little or no effect on loudness.
Also, some circuits are designed with lower gain, like some reverb circuits, so putting in a higher gain tube may result in more noise, not better sound.
Here's a chart:
http://www.diyguitarist.com/GuitarAmps/SD-Convertible/patgf1.htm
Hope this helps

 

[Timbre Wolf]

I haven't found a satisfactory explanation anywhere on the web.

Must be using the wrong Internet! :mrgreen: I hate when I get sucked into the other one.


Mr.Mark's explanation is very good. I'd only add that gain factors can very significantly affect volume, if you're using your amp for clean settings. If you've got your amp set for saturated overdrive, then the volume may not drop with a lower-gain substitute, but you'd have less overdrive (= more clean sound).

- Thom