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previous: Andy Cap coupled stages take a while ... -- 9/11/2003 5:37 PM View Thread

Re: How would YOU build a clean machine?

9/12/2003 9:14 AM
steve conner
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Re: How would YOU build a clean machine?
quote:
"are we talking about a few cycles of signal, or is it more on the order of a second or more?"
 
 
It depends on the RC time constant of the grid leak resistor and coupling cap, as well as the cathode resistor and bypass cap.  
 
On Pa and hi-fi amps this can indeed be a few tenths of a second. This causes a horrible farty sounding distortion (the dreaded blocking distortion) and the signal can sometimes even vanish completely for a split second.  
 
Hence guitar amps are designed with smaller coupling/bypass caps to make them recover much quicker. The shifts in bias point of the stage caused by grid current are still there, but now they track the signal quickly.  
 
The effect is to change the mark/space ratio of the output, hence the harmonic content of the distortion, according to the severity of the overdrive. Coincidentally, the harmonics vs. overdrive characteristic of a tube stage is almost identical to that of acoustic instruments, which also generate more overtones as they are blown/hit/strummed harder. This is why overdriven tube amps sound so darn good... But I'm sure you knew that already :)  
 
http://www.milbert.com/tstxt.htm