Space charge, was Re: amps/tone

Actually, there's a very good reason the old tube amps sound so good.  Tubes
have a softening effect.  It's because of something known in engineering
circles as "space charge".  I'll give a very basic explanation.  Bear with
me and you'll understand.  Questions entertained after the lecture.

Vacuum tubes are comprised of three elements - cathode, grid, and anode (or
plate).  They amplify by passing electricity from cathode to anode and
controlling it by putting the signal to be amplified on the grid.  This
small signal causes a much larger (but matching) variation in the
electricity flowing from cathode to plate.

Think of this like a faucet.  The water main is the cathode, and the sink is
the anode.  By turning the faucet (grid), we cause the flow to vary.

When the electrons hit the plate, they hit hard, and cause other electrons
to "boil off", creating an electron cloud.  This is called "space charge".
This varies with the signal.  ERlectrons are negatively charged, and repel
each other.  Therefore the electron cloud resists changes in the signal, and
especially the higher frequency changes.  As we drive the tube into
distortion, space charge becomes even more pronounced, and it smooths the
harsh distortion products.

The end effect is that tubes "mellow" the sound, even at "clean" volume, and
make overdriven distortion sound a LOT better.

Theoretically this is a form of distortion (even at clean volume settings),
but it's a very PLEASING form.

With solid state amps, we don't have space charge, and the distortion has a
"fingernails on the blackboard" quality to it.  Ironically, the harmonic
content of solid state distortion is remarkably similar to fingernails on
the blackboard.

The amps of Little Walters day were all tube.  It's just a coincidence of
technology.  Any amp he used would have the benefits of space charge.

Modern amplifier designs have improved on this - or they've just built amps
so large that they never distort.

- -IronMan Mike Curtis Band *Southland Blues
Magazine TU 8pm Starboard Attitude/Redondo
Santa Monica 3rd St Promenade, various times - email my cellphone (2 lines

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