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 stock value for presence is at about the 1 o’clock position on the control; you can cut or boost from there.
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.
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.
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.
This is great mod if you’re a jazz player. Turning the presence way down gives you that dark ‘Wes’ sound without killing your treble response. Highly recommended.
I’d like to add my two cents regarding this mod. My BJr is used for everything from Jazz to Country to Blues to Rock and the presence control has been a wonderful way to broaden the amp’s versatility. When Bill suggested this mod I wasn’t sure that it was going to be worthwhile but one listen later I became a believer.
can you still get the same stock bjr sound once the presence is installed?
Yes. The presence control makes one of the resistors in the negative feedback circuit variable. When the knob is set at around 1:00 or 2:00, it’s the stock amount of resistance. So you can vary the presence higher or lower.
hi bill..is this presence control working on the same principle as on the blues deluxe/deville or is it a more like an attack control on say the boss compressor sustainer?
It works the same way as the presence control in the HRDeluxe/Deville. But as you can see from the scope photos, you hear it in the highs and feel it in the attack.
Bill,
I have the presence control on my BJ that you did the work on, would the same presence work on the Pro Junior? Or maybe a switchable tone stack bypass for the PJ?
Thanks,
Mike(too)
The negative feedback loop (where the presence works) is different on the PJ. It would have to be engineered for proper performance. But the simplest thing would be to replace R27 with a 100K pot so you can go from stock to no NFB.
You could do a tone lift for the PJ, but since it’s a tweed-style tone control, you wouldn’t get the big volume boost that you do on a blackface-style stack. And if you play at high volume, the tone control is essentially out of the circuit anyway. The higher you go, the less effect it has.
Can the reverb knob be used in place for a new presence control.
I don’t use reverb.
It’s the wrong value for a presence control, but you could switch to a lower-resistance pot. You’d have to cut traces and run wire to the reverb control, but there’s no reason why it wouldn’t work.