Quote:
Originally Posted by Jaybird
I thought about this and decided to try some other value NFB resistor. First I used a 100k , because the squeal was so intense, I figured the 47k must be way off if what you told me is the case. There was still squeal, but not so immediate or scary sounding. Next I used a 1 meg resistor (overkill?) to just determine if anything stopped the squeal. No squeal, but no noticable difference in the amps sound. So now I think I will hook up a pot and see if I can dial in something. To your ears, just how does the NFB change the tone or sound of the amp, what should I be looking for?
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Again, something is not quite right with your amp. I believe the .1uF caps have made it unstable, and on the edge of parasitic oscillation. When you add the NFB resistor, it should not cause any problems, yet it does.
I can't say for sure without the amp in front of me, but my guess is that the .1uF cap installed between the second gain stage and the grid of the EL84 is causing the problem.
Until you get your amp under control, there is no point experimenting with NFB.
NFB smooths and evens out the frequency response. Early amps didn't have it at all. Tweeds acquired it, and blackface amps have more of it.
Less, or no NFB (high value resistor or none at all) makes the amp sound "raw". More NFB (lower value resistor) makes the amp sound more refined, with a more even response.
A common NFB resistor value for tweeds is 56K (some NFB). A common value for blackface amps is 820 ohms (a lot of NFB).