The Presence Control

The presence control has an interesting history. Early musical instrument amplifiers typically had three or more inputs, often labeled, "Guitar," "Microphone," and "Acc." Whether that meant "accessory" or "accordion" is a matter of speculation. The presence control affected overtone frequencies in the human voice range, making the voice more "present," hence the name.

But the presence control is not part of the tone stack. Instead, it controls the amount of negative feedback in the output stage. Negative feedback controls the accuracy of the output stage's reaction to the signal sent to it by the prior stages in the amplifier. With no negative feedback, the output stage tends to overshoot and exaggerate the high midrange and sharp, percussive sounds. With too much negative feedback, the amp sounds slow and mellow; pick attack is muted.

The Billm presence control lets you adjust presence from a brighter, louder tone that puts an aggressive edge on your pick attack to a more laid-back sound that's ideal for jazz or a warm, mellow tone that swallows pick attack and lets the note bloom out.

	The Billm presence control is mounted on the faceplate, just below the Fat switch. It fits nicely here and doesn't interfere with other controls. All Billm presence controls now have a pull-up switch. On amplifiers where the Clean Boost is installed, the pull-up switch turns on the Clean Boost. On green board models it also makes the amp brighter. If no Clean Boost is installed, the green board switch just controls the brightness.
	The knob is the correct style to blend with the chickenhead knobs, but I was unable to find a suitably "classic" style with a line or an indicator dot. So I buy them in bulk and mill a flat-bottomed hole in each one and put in a drop of white paint. I made an angled shaft jig for the milling machine so the bit wouldn't skitter off the curve on the top of the knob.
	Neatness counts. This is a green board presence control. All signal connections are made with Mogami low-noise coaxial cable, the same stuff that's used in professional mixer consoles. The bright cap and standard-tone caps are attached to the side of the switch. The bright switch is unnecessary on the cream board. The other side of the switch can be used for the Clean Boost.
	Here's how the Blues Junior handles a square wave (all frequencies in the audio range at once), with no presence control. This is pretty good performance for a guitar amplifier. The rise time (left side of the line indicates how it handles high frequencies.
	Here's what happens when you crank the presence control all the way. It exaggerates the amplifier's response to high frequencies and it overshoots. If you like incisive pick attack, this is it.
	When you roll the presence control all the way back it softens treble response and causes the amp to undershoot. The tone is very mellow and controlled, with no "edge" to the notes.
	With all three tone controls all the way up, the red line shows the frequency response. It's actually fairly linear for a guitar amp, with a little scoop at 500 Hz and a little bump at 2.5K Hz. The blue line is the additional boost you get from the presence control. You can hear the additional loudness in the 1KHz to 4KHz range in the form of cutting power. 5 or 6dB makes a noticeable difference.

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