Amp doesn't have a bright cap. It has a tone control with 500pF and 5000pF caps.It defaults to "on," with a value of 4999pF. At that value, turning it on/off doesn't seem to make a difference. If you lower the value around 200-2000pF, it seems to act like a midrange boost, and then towards the bottom of the dial, it lets more highs through the cap. Couldn't tell you whether that's behaving as intended or not, but it seems strange.
I did try to research a bit and normally the 5E3 has a 500pF bright cap, not 5000pF. Changing it to 500pF on the AF3 results in a much stronger midrange.
I dug in deeper into some schematics and you're right. There's no cap on the input. It has a 500pF cap connecting the Bright channel volume to the Tone and a 5000pF cap that the Tone control uses to bleed to ground. My assumption is the "bright cap" value on the AF3 is modeling the 5000pF from Tone to ground, right?Amp doesn't have a bright cap. It has a tone control with 500pF and 5000pF caps.
In this case, it DOES affect the tone. It just isn’t noticeable at the default value of the 4999pF.In my humble and unsolicited opinion, there should be a global UI/UX design principle for the III that a usable control should never be present in cases where it will never affect the tone.