Tubes Tubes

[Don Tillman]

   From: "Grant Richter" <grichter at execpc.com>
   Date: Mon, 13 Dec 1999 08:46:46 -0600

   Would someone be kind enough to explain why tube
   circuitry is preferable for audio over solid state?
   I realize this has probably been beat to death
   on ault.audio.hifi but is there a short explanation?

The tube transfer curve is somewhat linear in that the harmonic
distortion is almost entirely first and second harmonics.  And
clipping is gentle.  The is very different from transistors.

And tubes are bulky, expensive, produce heat and consume filament
current, so any practical tube designer has a serious incentive to use
as few stages as possible.

So tube amplifiers tend to use very few stages of mostly linear
devices with very little feedback.  Pretty much the opposite design
philosphy is used for transistor, and especially opamp, amps.

For example: The original Fender Bassman amp had only four active
stages between the input and the output.  The Vox AC-30 uses no
feedback.

By comparison, the transistorized Fender Ultra Chorus uses 7 opamps
and a number of transistors between the input and the output.  Each of
those opamps has about 7 transistors in its signal chain, so that's a
total of about 56 active stages between the input and the output.
(And the Ultra Chorus sounds pretty crappy.)

There's a lot more...

Tubes only come in one polarity, and since gain stages invert, the
even harmonic distortion of one stage tends to cancel the distortion
in the next stage.

Compare this to the exponential distortion of an NPN transitor stage
follwed by the exponential distortion of a PNP transistor stage adding
up to, I guess you'd call it a hyper-exponential curve.

Adding feedback reduces the harmonic distortion (like they told you in
school) but also has a side effect of changing the distortion
components (which they didn't tell you!) up to higher harmonics.

So tube amps and transistor amps are really nothing alike.

I think there's a lot transistor amp designers can learn from tube
amps.

  -- Don