Communiqué

Tubes vs. Transistors: One Response

Back in June, I wrote about the tube vs. transistor amplifier controversy that continues to this day. http://electronicdesign.com/Blogs/Communique/tabid/731/entryid/170/Default.aspx  that  I thank you all for responding.  I thought the response from Ray Mack was a good summary of the situation.  I am repeating his commentary here.

Lou
You hit the points pretty well.  For a guitar amplifier tubes absolutely make a difference.  The key is that they distort differently than transistors.  For some reason, musicians long ago decided that the amplifier+distortion was part of the instrument to get fuzz, feedback (that long A feedback in the Beetles tune), and other effects where the tubes, transformers and speakers all combine with the pickup and strings to create a system with a zillion different variables.  No wonder a DSP would be hard pressed to mimic a tube guitar amplifier.  The bottom line is that tubes tend to have odd harmonic distortion because the output stages are always true class B (AB actually) and fully balanced [cancelling even harmonics] where transistor amplifiers tend to be quasi-class B, frequently with two NPN in the same circuit rather than trying to balance with a PNP and NPN (which has its own limitations).  The transfer function for transistors has pretty much all of the harmonics and the higher level odd harmonics have much higher coefficients.


At very low volume those higher level terms add almost nothing for either type amplifier, but when you run at volume level 10, those higher level terms add bunches of harmonics and especially the even numbers that the tubes and transformer tend to cancel out.  Transistors also tend to have larger coefficients for the higher terms than tubes.

It is the surround sound/HiFi systems where people are just plain nuts!!!  A High Fidelity system should be exactly that: high fidelity.  That means that the system should be designed with lots of negative feedback and *ALWAYS* operated below the level where the high order terms start to become an appreciable part of the output.  If your transistor amplifier sounds like crap, it is your fault, not that of the transistors: it means your amplifier doesn't have enough head room for the desired volume or that it was just plain designed wrong.   I suspect that most systems also use speakers that are not matched to the amplifier properly.  You mentioned damping as a part of the issue.  It is probably true that transistors always color (that means distort) the output slightly differently than tubes, but you and I with our ~60 year old ears won't hear it where *maybe* a 20 year old with absolutely perfect pitch might be able to hear a slight difference.  There aren't many of those 20 year olds around and most of them are not the ones buying tube amps.

Ray
W5IFS

Thanks, Ray, for the elucidation.  And so the discussion continues.  Look for more coverage in ED on this subject in the near future.

And for more guitar and audio breadth, take a look at Mat Dirjish's entry on the famous fuzz box in his latesst blog. http://electronicdesign.com/Blogs/ComponentConnection/tabid/734/entryid/218/Default.aspx

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