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 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.

presknob_dot

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.

pres_71

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.

pres_101

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.

pres_11

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.

pres_vs_12_mod1

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.


53 Comments

  1. robert says:

    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.

  2. Mark S. says:

    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.

  3. mike says:

    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?

    • bill says:

      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.

  4. Miketoo says:

    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)

    • bill says:

      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.

  5. Lance says:

    Can the reverb knob be used in place for a new presence control.
    I don’t use reverb.

    • bill says:

      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.

  6. Mike says:

    hi Bill, is the presence control mod a difficult mod or one of the easier ones?

    • bill says:

      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.

  7. John says:

    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.

  8. Mark says:

    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.

  9. DaveG says:

    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!

  10. Eric says:

    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?

  11. Robert M says:

    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

    • bill says:

      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.

      • Robert M says:

        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?

        • bill says:

          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.

          • dennis sipe says:

            Bill,
            I have standby switch mod, and the basic mods for my son’s cream BJ. Also did twin reverb mod. I’m going to get the clean boost and a new volume knob and new capacitors. I have two questions: Do you like the dryer sound of a cream board with the extra headroom mod and do you think it is a bad idea to use only a stereo foot switch to engage and disengage the clean boost. I’m kind of lost in the thread about the pot variations on the prescience knob. Is the prescience control working for most people just fine?
            thanks
            dennis

          • bill says:

            The vast majority of the resistors on the Blues Junior are 1/4 watt. The plate resistors are 1/2 watt–slightly larger and fatter. Compare your resistor to the ones on the board. Most pedals use 1/4 watt and 1/8 watt resistors.

  12. captainbackfire says:

    Hi Bill,

    I have a new NOS and will do your mods soon. I have a serious idea though with this particular prescence mod. Can I possibly make the mod without drilling anywhere on the amp? Basically I want a “lively” tone just like the bright jangly characteristic of the Vibro King so maybe I can set it permanently on the max prescence setting and just use the treble control. Tell me what you think. Thanks alot!

  13. racktones says:

    HI Bill,

    I don’t use the Fat switch on the control panel, I only use it via footpedal. Is it possible to remove that switch and put the presence knob more or less in its place? – save cluttering my faceplate.

  14. Alex_Under says:

    Hello Bill.
    I just received the Basic Mods and the presence control which is now a Push/Pull.
    I’m not going to use a clean boost so I thought that there could be a way to make use of the Push/Pull ability otherwise.
    I thought that it may be possible to use the extracted R25 from the board and solder it on the upper pins of the push/pull and rearrange the cables as shown in the picture.

    http://imageshack.us/a/img684/2026/presencemod.png

    What I’m hopping to achieve is that when I pull the switch I will have the presence effect like stock and when I push I can use it to dial in the presence.
    I think that my diagram is correct. My only consideration is if there is a slight chance of damaging anything in the amp. I mean, the tiny milliseconds that the switch will need to change positions, is there a chance that I’ll have a break in the circuit that could be critical to the amps health.
    I guess to make the question simpler, Can the amp survive without the R25 (which will actually take place only for a msec).

    You could say that I don’t need that and that I can find the sweetspot on the dial which will give me the same amount of resistance as the R25. But I want to try my idea, if it’s harmless.
    What do you say?

    • bill says:

      Yes, that will work. The presence control merely controls the tone of the negative feedback loop. Interrupting it during switching will not harm the amp in any way.

      Pulling a resistor out of the board intact is difficult, however, since it has to be free of solder and what little remains has to be molten as you straighten the leads. Otherwise, it will peel up the solder pad. I would recommend getting a fresh 7.5K resistor and clipping the existing resistor, then desoldering the stubs.

      As you say, it’s harmless to try. But the 7.5K spot on the control rotation is at about the 1:00 or 2:00 position, and presence isn’t one of those “big difference” controls unless you’ve got it set to full cut or full bright.

  15. zifir says:

    Hello Bill,

    In terms of presence pot issue I have been using a 10K pot instead of R26 (greenboard Rev.B) for couple years. However my question is about the the R27 aka NFB resistor. As we know that blackface fenders have 680 Ohm NFB resistor which is way smaller value resistor compared to that of in Blues junior, 75 Kohm. Actually I quite dislike fuzzy and harsh preamp distortion in BJr and always willing to get enough clean sound from amp and leave the clipping job to an overdrive pedal. Most people mention changing NFB to 56K or 47K to achieve the Bassman-like tones. Thus I am curious that is it possible to lower the NFB and cleaner the tone? and what value will be the best for BJr?
    thanks

    • bill says:

      Not all NFB circuits are comparable. You can try different values for R27, but it’s not going to affect the preamp distortion at all. You’d be better off doing the basic mods kit to warm up the bottom end. Cooling off the bias and upgrading the output transformer will do more for clean headroom than the NFB resistor.

  16. bluedevil says:

    Hello Bill,

    I have purchased an early-made BJr w/greenboard which is previously modded with your basic kit. In terms of the presence issue, I know that R27 works as a NFB which is widely used in order to control presence in many designs but some people also mention R26. Which one would be more effective swapping R26 or R27? Thanks

  17. Dr. Bob says:

    Adding the Presence control mod has really sweetened up the clean tones from my ~2004 BJ (cream PCB). I had always found the tone too strident (harsh, thin, etc.) for my taste when Volume is set low to avoid preamp distortion. I did figure out that this was an output stage issue (not a preamp issue) but didn’t know how to improve matters. Then I re-read Bill’s excellent web site and blogs, and was reminded that the Presence control modifies tone and dynamics of the OUTPUT stage. Bingo! I still find the “stock” setting (~1:00) on my amp a bit harsh for my ears, but with the knob at about 10:30 the amp sounds so much sweeter! And smooth and jazzy all the way down to ~7:00. And the “in your face” tones are always on tap, just turn the knob clockwise past ~noon. Thanks, Bill.

  18. Coen Bijpost says:

    Hi Bill,

    I’ve got a limited edition Surf Green BJr with cream white chickenheads. The black presence and sparkle controls would really mess up the look, I’m afraid. Can I just replace the knob, any special size needed? You don’t happen to have any in a more whiteish color that you can swap in on request, do you?

    Cheers,

    Coen

    • bill says:

      I don’t stock any other colors. It’s a standard, 1/4 inch/6.3mm round shaft. So any small knob will do. You should be able to find some locally. If not, online stores such as smallbearelec.com or tubesandmore.com have small knobs in a variety of colors.

  19. jim streets says:

    Hi Bill

    Super-interested in the presence mod (although I might wait for my warranty to expire before I mess with my amp). Are there any compatibility issues with the 240v UK spec Blues Junior iii?

    Cheers

    JS

  20. jim streets says:

    Great, thanks Bill!

    I’d also be interested in the aux speaker jack in order to connect a cab in addition to the internal speaker. As it’s in parallel and will make two 8 ohm speakers into a 4 ohm load, would that not be a problem for the 8 ohm amp output? Also, would it have any effect on headroom/volume and/or tone?

    Thanks for your help!

    • bill says:

      The amp can drive a 4 ohm load without problems. It does not affect headroom or tone. The maximum increase in volume you can get from a second speaker with the same efficiency is 3dB. But the sound stage is broader and the low end sounds fuller because you’re moving more air.

  21. jim streets says:

    Thanks Bill! I’ve had itchy feet about the ohm mismatch as Fender told me it might cause damage, but I trust you know what your doing, or you’d have lost a lot of angry customers!

  22. harrypt says:

    Hi Bill,
    I just installed the presence control along with all the basic mods. Everything went well and it sounds great, a marked improvement. However my presence control doesn’t seem to be removed from the circuit when I pull out the knob. Tone is adjustable whether switch is in or out. Given the simplicity of this mod, seems there is no explanation other than a bad switch, no? Thanks for your advice.

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