From: balon@cup.hp.com (Tom Balon)
Subject: Re: Why tubes? (was Re: Building effects units for gu
Date: Tue, 6 Oct 1992 20:13:13 GMT
Distribution: na
Organization: Hewlett-Packard, Cupertino, CA

In article <evh5150.718335083@camelot> evh5150@camelot.bradley.edu (Andrew Eckhart) writes:
>In <TILL.92Sep30092630@acid-rain.lucid.com> till@acid-rain.lucid.com (Don Tillman) writes:
>
>
>>A single common-emmiter bipolar transistor stage actually has more 2nd
>>harmonic than 3rd, a push-pull tube output stage is almost entirely odd
>>order distortion, and early transistor amps had a lot of even order
>>distortion.  (I believe the source of this misconception is an incorrectly
>>applied electrical engineering class example having to do with the Fourier
>>analysis of the exponential transfer function of an idealized single
>>bipolar transistor stage.)
>
>Well here it is... and I quote from the book Guitar Gear, a Guitar Player
>book, edited by John Brosh, Quill Publishing, copyright 1985 pages 194, and 207:
>
>"When a transistor is clipped, the harmonic content has a harsher sound
>than a tube would have. Transistor distortion has a lot of third-order
>harmonics compared to the tube's strong second-order harmonic content."   
>BUT..."The pleasing, smoother sound of tube distortion can be simulated with
>some success by a properly designed FET circuit."
>
>Similarly...(p. 207) "When the transistor amp clips, it produces more
>odd-order harmonics (and in its worst case can sound hollow and dry),
>whereas tube distortion produces even order harmonics. Therefore, in many
>cases tube distortion sounds warmer."
>
>And yes... I'm an EE.

	Here's an interesting fact for those who prefer power-stage distortion
	over preamp generated distortion. While I agree, an overdriven
	single tube stage will add more even order harmonics, a push-pull 
	output stage will not. 

	A push-pull transformer coupled amp (like the majority of guitar amps),
	differ from single-ended amps, like the Fender Champ or Bronco that
	have a single 6V6, in that a push-pull output stage:

	  * eliminates DC saturation in the output transformer
	      this allows the output transformer to be smaller in size

	  * generates no even order harmonics , in fact it will cancel
	     any *generated*.

	  * offers higher efficiency for the same % distortion, (ie. more 
	        output power)

	If the tubes are matched, a push-pull transfomer coupled
	amplifier will introduce no even harmonics. However, if the input
	is distorted and contains even harmonics, these harmonics, and any
	odd harmonics that exist, will be amplified and appear in the output.
	Any even harmonics caused by distortion in the output tubes is
	cancelled. In fact, a push-pull output stage is more likely to
	introduce odd-order harmonics. Since the push-pull stage will cancel
	any even-order distortion introduced, the distortion that is generated
	usually has strong 3rd and 5th order harmonics. 

	So I believe it's really a question of where the amp is generating 
	the distortion, if it's in the preamp, the sound you hear will have
	stronger even-order harmonics, if it's from a push-pull power stage
	design, it's more likely odd-order. If the amp is distorting in both
	the preamp and power stage, you may have no *dominant* harmonics. 

	Regards,
	 Tom 	
	(TAS Tube Amp Service)
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