12ax7 gain

[J M Fahey]

This is such a worn, well trodded path, that I donīt even *calculate* 12AX7 gin any more, since I have built and obviously tested thousands of gain stages with them so why bbother/waste time?.
To boot, calculations crumble when you have a real world tube before you instead of the datasheet example anyway.
In a nutshell: standard "Fender" stage (which comes straight from the datasheet),around 60X unloaded, which is sort of irrelevant bevause it will always drive *something* so around 50X loaded, +/-10% .
Increasing plate resistor to 220k, a common "high gain" trick, may only change it by 20% if at all, with a few selected tubes, so in practice not worth it.
Of course booteek "designers" copy each other so once one does it, all do

Unbypassd cathode? ... 20/25X , as simple as that.

Recalculating such worn old data sounds to me like paying NASA U$10000 for a satellite picture of your backyard, "just to check whatīs in it".

[uneumann]

Ok right. But the tube is still biased by Rk and I would think bias point effects gain. But maybe it does not. I don't know. Although looking at the grid curves it does change symmetry and headroom of the output.

Bump on this... effect of bias are more noticeable for changes to harmonic content rather than gain.

[Gingertube]

See Chart No 25:
http://www.tubezone.net/pdf/rcachart.pdf
Cheers,
Ian

[trobbins]

The assumption is also that you aren't using the 12AX7 as a point of applying feedback.

[Merlinb]

This is such a worn, well trodded path, that I donīt even *calculate* 12AX7 gin any more.

Quite right. In a guitar amp, there is rarely and point in calculating gain. Unless you're a total noob you know the answer is always going to be 'more than enough'. So you spend your time tweaking the caps and interstage dividers (which cannot be calculated) to get the tone you want. Most of guitar amp design is empirical.