Touch-Sensitivity: Myth or Reality?

[Littlewyan]

I know NFB stops you having a smooth transition into distortion but you can have great touch sensitivity on a Marshall JTM45 which uses a LOT of NFB. Never understood that.

[pdf64]

Ha, I was thinking that as I was writing it.
It must be the exception that proves the rule.
Bear in mind that higher settings of presence will significantly reduce the degree of NFB in the frequency region where we're most sensitive.
And that weird screen grid arrangement, which seems to favour low order harmonics / smooth rounding off of the wave.
And the KT66, which are so smooth anyway, with a somewhat different overdrive character compared to 6L6 (which they may be expected to be similar too), EL34 etc.

[JD0x0]

I know NFB stops you having a smooth transition into distortion but you can have great touch sensitivity on a Marshall JTM45 which uses a LOT of NFB. Never understood that.

Hiwatts as well. Maybe because the NFB is mainly affecting the poweramp and most of the OD comes from the preamp?

To me, touch sensitivity is a balance between dynamics and compression, along with some other juju that I can't really explain. IMO, you want enough compression, that your sound isn't spikey and out of control, yet enough dynamics that you can still control volume with your fingers, especially clean. On an overdriven touch sensitive amp, your picking dynamics should be able to go from 'clean' to 'heavy OD' with mere dynamics from your fingers. Chords and single notes. I think a touch sensitive circuit should also be easy to coax into that endless sustaining feedback, as well. One, because you should be able to coax that feedback out with your fingers, and to me, that's part of touch sensitivity, but it also increases note sustain, on very dynamic amps, without adding undesirable compression that kills your dynamics.
This is why I like the Dumble circuits, personally, because they both have the touch sensitivity and the endless feedback, along with a pleasant (to me) voicing.

[LeeMo]

Mr. 335 himself says it best:
http://mr335.tv/?channel=rig&videofile= ... /tweaktone

[Littlewyan]

That explains a lot actually. As when I had my NFB set to 27K Resistor on the 8Ohm tap I found the amp got very smooth when I turned the Presence up to 10. I now have it on the 8Ohm Tap with a 47K Resistor as I prefer it which is even smoother still. With the presence at 5 though its a very hard/stiff sounding amp. Not nasty but not to my taste. Presence always above 8 .

[pdf64]

As I only ever use my JTM45 type amp with the presence up full, I devised a simple re-arrangement of the control such that at one end it was full presence, at the other it's open loop.
There being little point wasting front panel real estate on a control that never got tweaked.
As open loop provides a bit of a gain boost, it seems to make most sense with the presence boost at the CCW track end.
It's a fun control to twizzle now!
http://s963.photobucket.com/user/pdf64/ ... ort=2&o=16

[boots]

I just changed the 27k to a 47k to the 8 ohm tap on my JTM45/50/5f6A I was trying to debug. I got the idea from a comment on that thread from pdf64 (thanks!). I found that I liked the sound better with the 47k - more low end and very smooth. The 27k gave me a little more treble, more bite, slightly more note articulation. I ended up liking the 47k better.

I also don't care for the presence being set at less than 9 or 10 - the amp has no character to my ears at less than that.

All that being said, I find the JTM style amp to be very touch sensitive, especially with the volume(s) set around half to 2/3 of max. I am using a tube rectifier, by the way, which probably contributes to touch sensitivity.

My most very favorite amp in terms of touch sensitivity, though, would have to be the tweed Champ. Probably most single-ended, cathode biased amps are the same way. Set the volume somewhere around 1/2 to 2/3 and find the sweet spot with the guitar volume, and you're in for a treat!

[The New Steve H]

I rarely turn the presence down past 80% on any of my amps.

[Littlewyan]

Thats an awesome control! I must admit I feel that way. With the JTM50 I'm always turning the bass down to 0, middle on 10 and treble on 9. So really I'd probably be better off with the later tonestack and split cathode configuration. Although at the moment it is very touch sensitive and so much fun to play! So I'm not sure I want to change anything.

I must admit boots that I am tempted to build a little single ended Champ. Nice little amp to mess around with at home. Maybe one day.

[boots]

The Champ is really one of the sweetest, most pure, organic sounding amps out there. And so easy to build. Every amp builder should have one!