A while back I replaced my very tired Fender SuperTwin Reverb amp with a 70 Twin Reverb which has been blackfaced, recapped and retubed. It sounds much better of course, but I find I miss the presence control the SuperTwin had. Is there a pedal out there that I can put in front of the Twin to get the same effect, or is this something that has to be integrated into the amp?
My take on a presence knob is that it's kind of like a "loudness" knob on a stereo which adds both high and low end and, on a stereo, is essential for lower volumes. On a guitar amp I think it's typically a way to add upper mids to fill out the sound. I don't know of a presence pedal, but you could use an eq pedal to do pretty much the same thing.
Try a really good EQ pedal. Might I suggest:
That is an outstanding EQ pedal. Presence is usually an upper mid-range frequency control. Not quite high frequency, not quite mid-frequency, but some place in between. Very easy to sweep a range of frequencies on the ParaEQ to find the sweet spots and then boost them.
"Presence" comes in different flavours.
The classic form comes as negative feedback from the speaker side of the output transformer. This results in taming harmonic distortion added by the power tubes and transformer. The more feedback, the cleaner the output. I have a 59 Princeton that uses a 22k feedback resistor from the speaker side. When I lift that resistor's connection (i.e., NO feedback possible), it gets real easy to achieve icepick-through-the-forehead treble.
But such negative feedback control is rarely, if ever, used in an all-or-none manner. Mostly, there is some feedback, and the presence control simply sets a minimum and maximum amount of feedback applied. The way that many came to utilise it was to apply negative post-transformer feedback in a frequency-dependent way. That is, there is more feedback applied for the upper mids and treble than for the low end. That makes sense since harmonic distortion consists of added high-frequency content. The frequency location of where the presence control kicks in can be varied. And many amps which do not currently have a presence control can be adapted to use one, much as I did with my Princeton (which only has a volume and tone pot).
Of course, such a control is interactive with the actual output signal. If there is anything about your penultimate signal that produces more distortion from the output tubes or transformer, then there is more opportunity for negative feedback to have a greater impact. On my 59 Bassman, if you turn the treble control down and limit your volume, the presence control has minimal impact.
There is a big difference between this sort of presence knob, and the "presence" knob on my little Fender SK-20, which is essentially an upper treble tone control, and has absolutely nothing to do with feedback, because it occurs well upstream and because the SK-20 is not a tube amp hence HAS not output transformer. Just because something is legended a certain way on the amp chassis does not mean it does the same thing or operates the same way.
If a person wants the kind of extra bite that they have come to expect from a true tube-amp presence control, there are a few possibilities to consider. Ian has noted one: that of a parametric EQ (or semi-parametric) that lets you zone in on the upper treble and "boost the bite".
A second way is to use what is sometimes referred to as an "exciter" pedal/effect. The earliest exciters were produced (and licensed to studios) by Aphex. These were the rack units that would make acoustic guitars ridiculously crisp-sounding, and add some snap to a snare or other acoustic instrument. Many companies have come to produce floor-pedal version of these, including Boss ( http://www.bossarea.com/loadpage.asp...boxes/EH-2.xml ) and DOD FX85 ( http://filters.muziq.be/model/dod/fx/fx85 ), and of course Aphex has continued to refine their own models, and included one in their current series of floor pedals.
An exciter essentially increases the treble content in an interesting way, or more correctly several ways. One fairly standard way is to separate the treble by means of a steep filter, and boost the bejeezus out of that part of the signal so that it clips and increases the amount of harmonic content. That added ahrmonic content is then blended back in with the clean signal. Because it is only distorted/clipped upper treble, you won't hear it as fuzz, but hear it as simply more edge to the otherwise clean signal.
I posted a design called the "Woody" a couple years back, which was intended to be an analog acoustic simulator, and it did a fair job I must say. It used the exciter idea to add the crispness associated with acoustic guitars. I adapted the idea from a circuit that had originally appeared in Electronic Musician magazine back when they actually had DIY projects. here's that circuit: http://courses.cit.cornell.edu/bionb...s/image014.jpg
The way that such exciters work, however, demands that there actually BE some bite in the signal to begin with nd that the reproducing rig be capable of producing it. One of the many reasons why they never seemed to catch on is because guys would come into a music store and plug into one with a guitar that used HB rather than SC pickups, and listen to it through an amp with dull-sounding 12" speakers. They would scratch their heads, wonder if the damn thing worked, and walk out in frustration.
I must emphasize that what an exciter does is different, not only in process but in tone, than what you get by using a decent EQ and cranking the high end.
I'm not sure that we're talking about the same thing. On the SuperTwin, I always left the Midrange control at 5 (flat) amd the five band EQ flat as well. When the Presence knob was at 1, there was a sense of standing right in front of the speaker, and I could use the Reverb at 8 or 10. When the Presence knob was at 10 (my preferred setting) there was a sense of distance and ambience that made the sound much "larger", and a Reverb setting of 4 was quite adequate. While the Twin Reverb is sweet and clean, I miss the sound of an amp with a presence control. I've heard a lot of negatives about the old SuperTwin Reverb amps, but they did have a quality that I can't get with the Twin Reverb.
You're right. We're NOT talking about the same thing.
I just called up the schematic for the Super Twin, and I see that the "Presence" control is essentially part of the graphic EQ circuit. Here's the schematic: http://www.schematicheaven.com/fende...180w_schem.pdf
The normal path for a "traditional" presence control would be the 820 ohm resistor you see in the upper right hand corner, returning from the speaker side of the transformer, and connecting to ground via a 100 ohm resistor. What gets called the preesence control on this amp, however, is on the lower left hand side, to the left of the 2300hz slider for the EQ. Different beast entirely, and essentially a high treble control.
There are a couple of things yo might consider. One is to alter the "bright" cap value in one of the channels, to let a different range of treble through. The other is to alter the value of the treble cap in the tonestack, from 250pf to some other value, to alter the functioning of the tonestack in the preferred direction.
Note that the controls do NOT boost and cut. Rather, they ONLY cut, but in a way that can result in a tone that is essentially flat-ish. By making the 250pf cap larger in value, that will let more lower-treble/upper-mids through as you turn up the treble control. Making it smaller in value, will mean that it lets through only the upper sheen, if you will. Your normal tone settings will certainly not continue to work, if you make such changes, but the overall tonal balance you are aiming for may will be more attainable, albeit with the treble control set a little higher (or lower, as the case may be) than usual.