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.
in my own observations.. and conclusion.. presence knob sharpens and smoothens your sound.. and it adds the scratchy sound ..
If there’s a scratchy sound, there’s a problem with the control or the wiring. It should work silently if all is well.
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.
hi Bill, is the presence control mod a difficult mod or one of the easier ones?
If you can drill a clean hole in the faceplate, it’s pretty easy, just two wires to solder. But if you have doubts about the drilling, don’t do it.
Just did this mod. I play mostly jazz (Eastman AR803CE-15D). This rig used to start to boom a little right around 200Hz, which is where acoustic feedback first kicks in. I have found that turning the presence control all the way down (less feedback) improves the tone a lot and makes the rig much less prone to do this. The clean boost more than compensates for any loss in volume while keeping the tone clean.
I added the presence control mod, the main mod package and the master vol tapered pot..
My amp went from a box-like tonality to singing.. I think this is the best bang for the buck if you have a BluesJr (cream board) I have experienced. I actually am considering purchasing more mods – but I am not sure if I can get my amp to sound better.. it sounds pretty damn good.
I have a few Marshalls, a Randall, Gibson and a vintage Deluxe .. I prefer my BluesJr with the mods now to all of them – the little amp sings.
I finally got this one installed. Everything went well, but I find that the control really only has an effect at the very end of its range. While it does seem to get very bright and crisp at the far end, I’m not really hearing any sort of jazzy muffled tone at the other end. Is this pretty much the expected behavior?
thanks!
It should darken the tone as well, but the treble control can override it.
I recently installed basic mod kit, recap kit, T020 trans, twinstack mod, input jack, & presence control. Am extremely impressed & happy with the improvement in sound and sensitivity, but am experiencing same effect as DaveG regarding presence control, from 7 o-clock to 3 o-clock no difference in tone whatsoever, presence kicks in only from 3 to 5 o-clock. Have tried every variation of treble, mid, & bass constrols with no difference in presence control response. What think?
I’m hoping to find a different taper for the control. I’m researching options.
I got the basic mod kit and presence control kit today and installed it. basic mod kit was good but the presence control is kind of a dissapointment for me. I experience the same problem as Eric and DaveG: nothing happens in sound from 7 o-clock until 3. It’s more of a three way switch 1.(7 to 3 o-clock) nothing 2.(3 to 4 o-clock) little more presence 3.(5 o-clock) Too much presence.
I have read about good jazz sound on this site and some great feedback on the presence control. That’s why I’m dissapointed.
I measured the resistance on the pot and found 2.4 ohm when pot is in highest level (5 o-clock). When I turn the knob down the resistance seems to go zero all the way down to 7 o-clock.
Is it broken?
What is the small resistor soldered on the pot?
I really hope there is a solution for this.
BR Robert
The presence control with the switch is only available in 10K and 50K values. 25K linear would be ideal. The resistor reduces the value of the 50K pot to about 25K but it also makes it nonlinear. All of the tones are available, but they get compressed. Perhaps I’ll switch back to 10K.
Thank you Bill for the answer. What do you think is the reason some people experience different behaviour? Do we have different versions of the pot?
There are both 10K and 50K (with the resistor that reduces it to 25K) versions out there. Some people find the 50K to work well, others don’t. I’m switching back to the 10K.