Optimize global NFB w/trimmer?

[Pete]

Looking for input before I try this. Lower the 4k7 FB resistor and add trimmer in series.
Q1) What would be a good range of values?
Q2) Any reason to adjust the 390 (tail) resistor?

Amp is a 2/6L6, standard PI (not BM)

[ayan]

Looking for input before I try this. Lower the 4k7 FB resistor and add trimmer in series.
Q1) What would be a good range of values?
Q2) Any reason to adjust the 390 (tail) resistor?

Amp is a 2/6L6, standard PI (not BM)

If you tweak the 390, the effect of the presence control will change unless you change the 1uF cap as well. I would probably change the 4K7 alone and see if you like more (doubt it) or less overall feedback.

Cheers,

Gil

[tele_player]

I've done it on Fenders - I just added a pot (I don't recall what, maybe 10K) in series with the existing resistor. Cranking up the pot reduces the NFB, and increases power amp distortion.

[ayan]

I've done it on Fenders - I just added a pot (I don't recall what, maybe 10K) in series with the existing resistor. Cranking up the pot reduces the NFB, and increases power amp distortion.

If you do that, you will get a lot of crackling as you turn the pot, which is nothing to get worried about and quite normal. If you want to avoid the noise, use a large coupling cap before the pot, and that will block the DC and get rid of the noise.

Gil

[Pete]

Thanks guys, I guess I'll just try a 10k pot. I wouldn't mind a little less power amp distortion but certainly not more.

I've done it on Fenders - I just added a pot (I don't recall what, maybe 10K) in series with the existing resistor. Cranking up the pot reduces the NFB, and increases power amp distortion.