SvErD said:I bet it's this one, plays very nice by the way even though it's not textbook........
http://www.diyaudio.com/forums/showthread.php?postid=131443#post131443
I put together a breadboard of this (used half of a 6SN7) and it is a very interesting effects box, but not even vaguely an accurate preamp. It choked when asked to swing more than 3V peak, and at that level, it had over 10% distortion. Gain of less than 2, but the distortion is so bad that it's difficult to assign a number. All of this is predictable from the datasheet curves which show the plate resistance as a function of current- it gets VERY steep at currents lower than 5mA (this circuit runs about 1.1-1.2mA). The tiny plate load means that the linearity is dominated by the plate resistance. The mu also becomes strongly nonlinear in this current and voltage range.
My generator source Z is quite low- for something fed from a potentiometer, expect the distortion to go higher since the tube is biased very near the region where grid current distortion begins.
There are tubes suitable for low voltage use. 6J5/6SN7 are not one of them.
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