A fat mid-range and warm liquid tone ?

[bbruhns at erols.com]

Tubes are kind of natural for some things that transistors need complex circuitry to do.  Bob Carver demonstrated distortion circuitry that could make a good solid state amplifier sound indistinguishable from highly respected tube amplifiers.  If nothing is 'wrong' with tube and solid-state hi-fi amps, 99% of the difference is linearity.

One of my friends showed me two solid state amps from reputable hi-fi firms, and they really performed nicely into eight ohm resistors.  But he connected them to his speakers, and it was a whole other story.  One amp produced horrible distorted treble, the other amp sounded great.  I did not have equipment to check, but I'd bet a year's pay that the problem was instability into the odd resonant impedance of the crossover and tweeter.  Certainly those impedance curves would be much worse than the ones that give us the "20 dB return loss" that we like for RF gear.  More like 3 dB, I would guess.  Result: sometimes instability and resulting bad sound.

Tube designs don't use usually cascade gain stages like solid state designs usually do.  The general result is more stability.  Not always, but usually.  And tube designers often want minimal negative feedback, relying instead on the relatively low 'plate resistance' of triode tubes.  This is really a negative feedback effect inherent in the vacuum tube itself, and it is inherently stable.

With any luck, a solid state amp won't go triggered-parasitic on your music, in which case it will sound good.  But if it clips...  Wham, square waves, with strong high-order harmonics.  But tubes hit saturation with a little more sonic grace (soggily), curving flat, instead of an instantaneous sharp change in direction - generating less high-order nastiness.  Yeah, it sounds better when it clips that way.

So build a soft clipper into a good hi-fi solid-state amp...  And maybe some bass resonance comparable to the result of a tube's soggy control of amplitude versus the impedance peak of a woofer at resonance...  and the sound is the same (nice).  Go with 
what works!
 
With musical instrument amplifiers, musicians like the effect of soggy grid bias regulation in push-pull tube amps.  It adds a fuzzy compression to the soggy clipping when they deliberately overdrive the amp.  Hey, I mean no insult - I like it too!  And it is the natural result of simple, cheap, capacitive coupling to the control grids of class A-B push-pull tetrode or pentode output tubes that require the usual negative grid bias :)

None of the above is really 'hi-fi' - it just sounds better!  That's really all that matters.

Maybe one tube makes a better mush of the sound than another.  Fine!

   Bob, WA3WDR

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