How do great "touch dynamics" happen? Is it a matter of equipment or of skill? There's no short answer to this question. There's no simple formula. It's a matter of the whole being greater than the sum of the parts.
Everything matters. The more range you have in your picking-hand attack, the more you're going to be able to take advantage of whatever touch dynamics the rig has.
Tube amps certainly have an advantage. Some of the modellers - the Vox in particular - have a discernible response to touch dynamics. If you want great touch dynamics, though, you're more likely to find that in a rig that uses a tube amp. Which is not to say that all tube amps have great touch dynamics; they don't.
Some players believe that "Class A" tube amplifiers have inherently better response to touch dynamics. This a rather broad generalization that has some elements of truth. However, there may be other factors that make a difference.
First, let's distinguish between amp classes (determined by bias level) and biasing method. Marketing of guitar amps has all but obliterated this distinction. For all practical purposes, a "Class A" guitar amp is cathode-biased (i.e. using a resistor in the cathode circuit of the output stage to develop the bias voltage) while a "Class A/B" guitar amp grounds the output stage cathodes and applies a fixed negative voltage to the grids.
Why is this important? In a cathode-biased amp the bias voltage increases as you play louder; this causes the amp to compress and distort in a way that you don't get from an amp having a fixed-bias output stage. So cathode bias is one way to get touch dynamics out of a tube amp, but not the only way.
Some players believe that certain types of power tubes (EL-84, 6V6, etc.) produce a better response to touch dynamics. Again, there may be a grain of truth behind this belief. While there are some characteristics that you can associate with certain tube types, touch dynamics is more a matter of circuit design and component selection.
Output stage bias does have an effect upon touch dynamics. This is an easy experiment to perform on most amps. The amount that a bias change affects touch dynamics depends upon the amp.There are a number of factors which contribute to touch dynamics. Some are more important than others. I'd rank them like this:
- It starts with the player. The more range in your picking dynamics, the better the response you'll get out of the rest of the rig.
- The amp is important. I'll say a lot more about this later, because this is where you encounter the most variables.
- The speakers have a big influence depending upon how much power they'll take before starting to break up and how they sound both at the onset of breakup and as you pour even more power into them.
- The guitar and pickups make a difference, too. The output level of the pickups determines how hard the amp will be pushed. The tonal balance (with contributions from both the guitar and the pickups) will also affect the touch dynamics. So will pickup height, polepiece adjustment, and string gauge.
OK, back to the amp... There are a lot of things that can contribute to touch dynamics. Keep in mind that presence of any of these items doesn't necessarily mean that an amp will have good touch dynamics, any more than the absence means that an amp won't have good touch dynamics. It's a matter of how all the pieces fit together. These are listed in no particular order:
- Cathode bias. See discussion above.
- Absence of negative feedback around the output stage will help to give the amp a broader range of response to touch. An amp which has NFB will run the power stage clean right up to the point where the NFB loop can't keep up, then it'll get dirty. A power stage without NFB will make a more gradual transition from clean to dirty.
- Preamp topology makes a difference. Most Fender-derived amps have the tone stack between the first and second preamp stages. That, coupled with the mid-scoop inherent in the tone stack, helps to keep the preamp from distorting. Most of the "meat" of a guitar's signal is in the mids. Putting the tone stack after the second preamp stage (like a Bassman does) or using a simple treble-cut tone stack lets you get some preamp distortion that may vary with touch.
- Having too many preamp stages make the preamp the primary locus of distortion generation. This can detrct from touch dynamics. Preamp distortion is more like a "brick wall" instead of the gradual-onset distortion you get from an amp in which every stage of the amp contributes to distortion. In most cases adding a third preamp stage (except for the reverb-mixing stage in Fender amps) shifts the amp's emphasis toward preamp-generated distortion.
- Simplicity. Channel-switching amps compromise all tones in order to get their variety. A channel-switcher tends to run its power stage clean in order that the amp doesn't sound muddy on the high-gain channels. The downside is that the clean channel ends up sounding sterile or, if the designer tries to compensate using preamp distortion, has poor response to touch dynamics on the clean channel.
- Iron. Yes, the transformers -- especially the output transformer -- make a difference. Brand loyalty notwithstanding, you can't tell much by looking at a transformer beyond the general rule that a larger, heavier transformer will tend to have better low- and high-frequency response and less compression. Also, smaller transformers will exhibit a progressive reduction of low- and high-frequency response as power levels increase; this can be used to good advantage to tame harsh distortion products as the amp is pushed harder.
It's quite difficult to predict how an amp will behave by looking at its design. You're better off playing a bunch of amps and then learning about what design features distinguish one from another.
As far a good combinations, here are a couple I have found:
- A Vibro-King pushed by vintage-output humbuckers. This is my favorite.
- A Blackstone MOSFET Overdrive in front of just about any decent tube amp. The Blackstone provides the touch dynamic response, while the amp gives you familiar-sounding EQ. I've seen a few folks suggest that you can plug a Blackstone straight into a PA; I think that sounds terrible.
Again, though, it all starts with the player... I've learned to coax usable touch dynamics out of a wide variety of amps.