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RE: GM20 questions

Posted by Ralph (M) on June 7, 2010 at 20:50:27

In Reply to: RE: GM20 questions posted by cpotl on June 7, 2010 at 14:15:21:

Actually that sounds right to me. You want the bias current to go down after warmup with the 6C33. It has a sort of thermal runaway characteristic. Also, you really don't want the correct bias near the middle of the pot, you want it near the *end* of the pot's range, so that if the amp is improperly adjusted, its a little harder to toast the tubes.

This circuit is very similar to one that I published in Positive Feedback about 16-17 years ago. The 50K pot with the bypass resistors is about the only way you can set something like that up in the cathodes of the input tube. We used to do something like this in our early amps made before 1985 or so. It is responsible for distortion- so you are actually better off using a coupling cap scenario between the input and the driver section despite the additional coloration of the caps.

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