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12AT7 PI: Shallow Load Line (large plate R's)

5/26/2005 8:41 PM
Jack
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12AT7 PI: Shallow Load Line (large plate R's)
Hi guys,  
 
Wondering about the sonic results of using the usual 100K or so plate R's with a 12AT7 PI. (This was done in the older Super Reverbs, Dual Showman, and Bassman amps.) From a design standpoint, you get a very "shallow" or almost horizontal load line, and relatively very low plate current thru the tube. It seems to me the tube is being run near cutoff all the time, down in the area where the grid bias voltage lines start to curve a lot.  
 
I've seen (and had) a few amps configured like this, and did get pretty decent sound out of them. I can't seem to wrap my wits around what the tube is actually doing down in the "curved line" region... is it like in current starvation down there? It makes sense to avoid this area due to non-linearities, no? And yet the amps sound fine. Is it that the tubes are just so damn forgiving, or is the bias set-point result so subtle that you can get away with biasing the tube just about anywhere (aside from clipping, that is)?  
 
KO'C in one of his books mentions IIRC that the tone of a preamp tube is sweeter if it has more current thru it. Also that the transconductance is higher at higher currents, which is figurable from the tube curves. Wouldn't you go for both the sweet and uSiemens in all designs?  
 
All clues seem to point to using the highest current you can in preamp design within the clipping requirements of the stage. So... what's the deal with the starving 12AT7's?  
 
TIA!  
Jack

 
Replies:
Jack Anyone? -- 5/29/2005 11:07 AM
Ray Ivers "harmonic-removal" filter region -- 5/30/2005 7:15 AM