Re: New tube amps

On Mon, Dec 25, 2017 at 10:25:23PM +0000, Gordonjcp wrote:
> On Mon, Dec 25, 2017 at 09:50:49PM +0000, Will Godfrey wrote:
> > On Mon, 25 Dec 2017 21:34:58 +0000
> > Gordonjcp <[email protected]> wrote:
> > 
> > >On Sun, Dec 24, 2017 at 11:31:39AM +0100, David Kastrup wrote:
> > >> 
> > >> I mean, "staggering number of analog components that effectively
> > >> reproduce the dynamic character of a traditional tube amp"?  The dynamic
> > >> character of a traditional tube amp comes about by a staggeringly
> > >> _small_ number of components.  A typical full-scale tube amp has fewer  
> > >
> > >... which you can accurately mimic with about a dozen assorted resistors
> > >and diodes, and an opamp, to the point that no-one can tell the
> > >difference.
> > >
> > 
> > I'd like to see a schematic of such a design... as I'm sure a great many other
> > people would. You seem to be an expert on the matter so perhaps you could knock
> > one up. Seeing as it's only a dozen or so components it shouldn't take long.
> 
> http://gjcp.net/solidstatevalve.jpg
> 
> Here you go.  Quick sketch, really looks for about a +12/-12 supply rail
> and all diodes are 1N4148, or whatever small-signal silicon you have
> knocking about.
> 
> Adjust the gain and offset to suit - more offset will make it more
> assymetric and give you a more "secondish" tone but be careful, too much
> sounds harsh and unpleasant.
> 
> You'll want suitable equalisation before and after the circuit because
> frankly anything above the 7th harmonic is going to be dissonant.
> You'll also want a very powerful and clean power amp that can drive a
> good-quality speaker in a cabinet suitable for the instrument you're
> playing - I recommend either a 12" or a pair of 10" speakers in an
> open-backed box for a guitar.  If you're close-miking, stuff a couple of
> pillows in the back to bring the bass up a bit.

Just to reply to my own post, too, I would advise sticking with
tried-and-true MOSFET power amps here, although there's no reason to
suggest that those modern high-efficiency Class D amps would be in any
way unsatisfactory.  I've just never tried them.

It should be noted that the circuit in the photo is not a complete
preamp but just the distortion shaper stage, and you probably want a
stage of gain and some EQ before that.  You can replace the buffer amp
with a gain stage if you find it doesn't drive your PA module hard
enough but ensure you never push the PA into clipping, or at least not
severely.  Or, I guess, don't let me tell you what to do, if it sounds
good with the clipper clipping *and* the PA clipping, go with it.

The resistor values are low because you want a fair whack of current
into the clipper diodes.  You could use a FET opamp and larger
resistors, but you'd hang around too long in the diode's non-linear
regions.  This might sound good to you, so if you feel like trying it,
do so.

-- 
Gordonjcp
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