Touch Sensitivity

[MikeT]

What is it in the design/construction of an amp that gives the "touch sensitivity" only found in the highest quality amps? I realize it is probably the interaction of many things, but of you had to quantify them in tangible terms like capacitance or voltage or lead dress or components or materials or layout what would they be?

Mike T

[Analog Assassin]

I think it's a combination of things. A lot of it is in "straddling" the line between the sustain of just enough compression, yet having enough headroom in there to keep the right amount of dynamics. So that way when you play soft, it sounds soft, and when you dig in, it screams.

[Circuit1]

Mike,

This is probably one of my favorite subjects . I'll bite. I think touch sensitivity is at least as important as to what the amplifier is capable of producing audibly. Amps are musical instruments that should feel good as well as sound good. If I am reading you correctly, you want some basic techniques to get good "feel factor".

One of the first things I usually go for is the Speaker choice. Seriously, it is what does the most actual physical work moving the air. Personally, I love AlNiCo Blues for lower powered amps [<25 watts]. They have a thickness and bright "presence" that never kills the ears. The imaging tends to be very forward and "real". Plus, they do one thing exceedingly well--driven by the right amplifier, they are extremely responsive and controllable. This has to do a lot with the damping factor as well as the overall EQ'ing of the amp in question. These particular speakers are just beautiful when driven by a nice pair of 6V6's or EL-84's at optimum operating points in particular. If the OT is chosen for maximum dynamics and efficient power transfer, the Blues simply want to play themselves. For instance, when I play my PRS Standard 24 on the Neck PU through my amps, I am able to dampen the note to inaudibility and then use a moderate vibrato with my left hand to bring the note back and have it sustain just so. The amplifier in question is a medium gain style with most of the clipping and compression happening at the cathode follower, phase inverter, and power amp.

Next, I would look at the Output Transformer. As I mentioned above, it does make a huge difference not just tonally, but also with feel factor. A good hunk of Iron should be able to handle the power dissipated as well as efficiently transfer power from the modulate power supply to the speaker load [via the output tubes]. I used to really use a lot of big "HiFi" Iron in my guitar amps, and have been tending towards smaller, paper units instead for teh lower power ranges. I am finding far more dynamics and sensitivity with the physically smaller Iron in a 20 watt power amp range. Five years ago, I probably would have placed 80 watt Iron in a 20 watt amp. Now, as long as it's the right Iron [I like Heyboer lately], the big sound, tightness, and feel can be more easily controlled if damping and efficient power transfer is attended to.

Next is overall circuit path design. Some style phase inverters feel nicer to me than others. I like highly tweaked Long Tail Pair or Schmitt Style phase inverters. I messed around with the Cathodyne or Split Load a bunch and could never quite get it to give it up for me. I don't think I discovered the right output circuit to make this style really get me off personally. Maybe I'll keep going with them, but I haven't found "the one" yet. It's really because they don't offer any gain. I might try some buffers with the Cathodyne though at some point...but I like to stay in AB1 operation--especially with 6V6's and EL-84's. Stick with the LTP for the best dynamics, gain, and sensitivity in my opinion. Tweaking your tail resistor is a surefire way to get a great amount of static balance in the power section. Tweaking the bias [common cathode] resistor is the best way to adjust the operating point of your phase inverter. This will increase/decrease headroom in your power amplifier. I think a medium amount of headroom is optimal. Too much and the amp won't give it up. Too little and the amp is mushy and inarticulate. That was always my problem with the 1974 18 Watt Marshall. Too much mush and inarticulate. If you mess with the phase inverter and power supply in these amps, they feel factor gets magnified a lot.

Power supply--super important. All of your associated circuitry modulates the power supply. It truly is the sound the amplifier as well as the speaker. While the speaker does the actual physical air movement, the power supply is the electrical equivalent. It is what is "moved" by the associated circuitry. That's how I see it. A truly well designed power supply in a guitar amp might be too good. I have yet to feel a good regulated PS that works. What we want for incredible feel factor is the right power supply--operated safely--and with as much character as possible. Personally, I find that an amp designed with a good vacuum tube rectifier offers the best feel and sound available. Also, you can easily swap in a solid state rectifier if that's what you prefer now that there are coppercaps and the like. Again, a medium stiff power supply seems to be the best for feel. Too stiff and the amp doesn't breathe. Too loosey goosey and the amp is difficult to play. Somewhere in between is best for me.

I hope this helps out. What works for me might not work for you. But I feel these are the most important issues for good feel factor. It's really about efficient signal transfer. Also, don't forget that your solder joints matter a lot too. Bad solder joints make a good design on paper a true piece of junk in reality. A lot of people don't realize that oxidizing the joint will lead to crappy signal transfer and unreliability. That's a whole other rant though. Good soldering technique builds truly great amps vs. average amps.

[Steve Conner]

I say touch sensitivity all comes down to how the duty cycle of the clipped signal (in an overdriven amp stage) is modulated by the amplitude of the original signal. Things that affect this tend to be coupling capacitor values, gain structure (ie, do you have a few stages with high gain or a lot of stages with low gain), DC bias points of the stages, and any EQ that comes before stages that get overdriven. Changing tube types can have a big effect since it tends to change both stage gain and DC bias point.

I think you can improve the "Touch sensitivity" of solid-state amps by bunging a diode somewhere before the main overdriven stage, to rectify the incoming signal and "pump up" one of the coupling caps on loud signals. I'm sure many designs do this already. (Tube and JFET based amps do it naturally with their grid-cathode and gate-source diodes.)

[kg]

What is it in the design/construction of an amp that gives the "touch sensitivity" only found in the highest quality amps? I realize it is probably the interaction of many things, but of you had to quantify them in tangible terms like capacitance or voltage or lead dress or components or materials or layout what would they be?

Mike T

imo and ime, headroom is basically what it boils down to.

make sure that rather large signals may be passed, relatively unfettered, through the preamp. ie, start with a hifi amp.

then slowly start adding in more gain until it clips. make the clipping occur evenly, across a few different stages. do not try to make any single stage the point at which all the distortion occurs.

[Circuit1]

To add to what Ken wrote:

The gain structure and headroom certainly has a lot to do with touch sensitivity. Taking into account a gradual clipping/distortion of the signal instead of having it all in one place is a great way to put it. It's interesting to note that so many people building clones of existing designs have such a hard time getting them to mimic the original. It boils down to every detail. Every detail needs to be addressed to get a great instrument that can really be held to it's own standard. You just need to get in there and get your hands dirty, make a bunch of really good mistakes, and carry on.

BTW, great to see you hanging around kg! I always look forward to hearing your take on all things amp related . Any cool new designs cropping up?

[MikeT]

Thanks all you guys for your input. I just finished one of those Epi Valve Jr heads with the Mercury
Magnetic transformer kit, and I'm using it through a 1/12 cab with a Celestion 12" Professional 80 watt speaker, and it is hard to believe it is 5 watts. Anyway, it has some touch sensitive qualities, more so than my other amps (main ones being two champ chasis/cabs that I converted to (1) 6g2 with an 8" Alnico and (1) 5e3 with a Kendrick reverb kit and a 10" Alnico- they both use diodes as a rectifier. I also use a Fender Blues Jr that I put a cathode follower in the free triode along with some tone stack tweaks and I biased the EL84s to run cooler. I also use Vintage 30 in it.) and it is fun to play. It is single ended so obviously there is no phase inverter, with one gain stage and the other triode as the driver. The signal path is almost as simple as you can get with no tone control. It sounds really, really good. I'm recording with it soon. But I'm wondering if the choke in the power supply has anything to do with the touch sensitivity. If so, I'll put chokes in my two rebuilt Champs and maybe the Blues Jr. I probably will anyway but hey, it would be nice to know in advance!

[MikeT]

Steve,

You are reading me right, amps are musical instruments that should feel right as well as sound right. For speakers, I use Ceramics, mostly Celestion, and Alnicos, Weber or Jensen. Smaller amps, like 15 or so watt 6G2 (dual 6v6s) I rebuit in a Champ cab/chassis I use an 8" Weber Alnico. Another Champ cab/chassis that I rebuilt as a 5E3 and cut it for a 10", I use a Jensen Alnico. For my Epi Valve Junior (one EK84) head that I installed the Mercury Magnetics kit in, I use a Celestion 12, I think it's 80 watt in a Marshall 1/12 extension cab. I tried a Jensen 35 watt 12" Alnico but the Celestion had much more headroom and a much smoother breakup. That thing smokes for 5 watts. For my blues Junior (dual EL84s) that I put a cathode follower in the unused triode and a few other tweaks I use a Vintage 30. For my Reverend Hellhound (dual 6l6s) I use another one of those Celestion 80s (Mojo was selling them a few years ago and I bought 2). I guess my point here is that I am conscious of trying to find the best speaker for the job.

As for the OT, I've been using Mojo BF Deluxe OTs for all my 6v6 push-pull applications, my own amps and jobs I've done. I don't know who makes them for Mojo, but I've been using them for over 10 years and they sound good and have held up well. They are not oversized and seem to saturate well when pushed.

As for the circuit path design and the phase inverters, I use cathodyne in the rebuilt Champs, primarily because it is a really tight fit and a long tail takes considerably more real estate. But I like the Cathodyne because it doesn't add anything. I tried putting a long tail in a SF Princeton a few years ago, per Gerald Weber's article in VG, and I didn't like it. It added gain definately, but it also seemed to add a "honk" that I did not like. But it is long tailed in the Blues Junior and Reverend, and they are both fine. I don't use cascaded gain stages in any of my own amps, although I puit an MV after the PI in the 5F6-A. So circuit paths are simple.

As for power supplies, the PTs are the stock champ/Princeton PTs in the Champs and stock in the others. I use diodes in the Champs and the Blues Junior and Reverend come stock with diodes too. I also have a BF Bassman head that I rebult as a 5F6-A with a tube rectifier (long tailed PI). And I also have a 66 Princeton with a tube rectifier (cathodyne PI).

I've messed around carbon comp resisters in some places in the circuit, Orange Drops, Mallory 150s. I even tried Hovland and Auricaps. And I've tried lowering and raising the B+ to the gain stages. And also the cap-resister network on the cathodes of the triode gain stages have been tried in many variations.

All this stuff allows me to shape the sound to some extent, but the touch sensitivity hardly changes. Elusive, man.

So my next shot will be adding chokes to the power supplies of my rebuilt Champs.

Mike

[MikeT]

Jason,

Sorry man...sent my response to Steve that was supposed to go to you...

[Circuit1]

No problem . the choke will definitley give you different feel. It can be very subtle though, or it could be quite significant. It depends on what the resistance of the choke is as well as it's inductance. I always found that I like the "right" resistor in my power supplies as opposed to the choke. The choke tends towards more hifi. As a starting point, in my EL-34 heads, I use a 470 Ohm wirewound 25 watter as my initial dropping resistor. In my new combo amp, I am using 300 Ohm wirewounds. It depends on the output stage/tubes I am designing for and what I want from the amp. FWIW, I have found more tightness, fatness, and a nicer distortion with an "ethereal" smoothness about it using resistors on my PS.

Regarding the definition of touch sensitivity: the amp should have a slightly springy feel and "want" to almost play itself. This is totally different than a high gain amp that is very easy to "sear" on--thus hiding notes or creating a fuzzy circle around each note--for lack of better terms. In otherwords, the amp should posess a high degree of responsiveness--lower the volume knob, and the amp cleans up, while still being "loud". Change a tone control on the amp or guitar, the amp should easily change it's character. In otherwords, the amp could easily go from sounding like the sexiest 18 Watt 1974 circuit to a clean, thunky AC-15--or my GR 1.6 for example . Basically, this amounts to dialing in the exact amount of gain and frequency response throughout the preamp, getting the power supply to "resonate" in sync with the assiciated circuitry, and finding the proper OT and Speaker since these are the final links in your chain. As I said before, every detail matters--a lot.

It really boils down to your personal playing style and taste. I figure if I build amplifiers that I personally lust after, eventually other players will also hear/play them and likely feel the same way about them The thing is that it does take a lot of practice, a lot of failure, and some "good" mistakes here and there. Try and try again--fail better next time .