specifics of tube overdrive

[pchorman]

Can anyone explain exactly where in the signal chain of most tube amps (in what stage) and how (i.e. by saturation, cut-off or both) the heavy distortion is produced? this excludes speaker distortion.

I can see this taking place solely in a preamp, but I can also see a preamp overdriving the power amp section where the clipping can take place. Seems like either will work from an audio electronics point of of view, so what's actually done?

thanks

 

[virtual.ray]

Depends on the amp.In a Fender type circuit(black face & earlier),there's only 1volume knob,so it's not really possible to overdrive the preamp seperately from the power amp,which is why most people who use this style of amp also have their favorite pedals to get the grind going.On the other hand,in an amp that has a "master volume" or "gain" control,the idea is to shape the tone with the preamp controls and then amplify that w/the power tubes.On paper,this allows for a maxed out distortion sound without getting evicted.In practice,however,most people end up concluding that the particular character of the power tubes breaking up is more to their liking,because it's usually more responsive to the nuances of the player's style of picking,etc.Cheers!