What Makes an AC100 Clean/High Headroom?

[Frost]

Hi there,

I'm fascinated right now with some of the clean clips I've heard of Vox AC100s. Looking at the myriad of schematics of them I'm perplexed with what makes them such clean amps? I know they have a 12AU7 in both V1 and the PI. However in my experience with substituting a 12AU7 in the V1 and PI of a "Top Boost"-like EL84 amp it did not make it that much cleaner but you had to crank the volume waaaaaaay up to get a similar level of volume as with 12AX7s due to the gain loss. Is this the case with AC100s too? I would almost expect a linear volume pot to help this aspect out. Is the clean/high headroom coming from the power amp?

Many thanks,

Heiko

[Roe]

its largely the power amp, although these amps will breakup and compress

[JD0x0]

Mainly because you're going from cathode biased EL84's which are easy to drive into clipping, to fixed biased EL34's at a much higher voltage.

[Stevem]

As posted in the above it’s in the design of the amp.
Interestingly enough if you have a 200 watt Marshall Major for instance and you swap out the 12AU7 PI tube for a higher gain but lower current drive 12AX7 you loose about 85 watts rms of the amps 200!