Bias and Neg Feedback question

[SteveS]

Hi folks,
I just wanted to start a thread to help educate me on the sonic impact of tweaking these pots. I guess that the neg feedback pot is the one I am most interested in.

I need a little aural education here - what _should_ be the impact of adjusting these pots to the sound as a way to help me understand what I am hearing (if that makes any sense). This all stems from the fact that I have made some many changes to my system since I started doing the diy stuff about 2 years ago that I have way too many variables at play and I need to settle things down to get a handle on them.

For reference I am using all Hammond Transformers on a Rev B board, with EL84 tubes from Anthony Welsh (Russia Miltype 6Nxxx) and a JJ 12AX7 with Mullard 12AT7.

I know describing sound is a bit like trying to describe taste in words, but a general guideline I think might be useful to help me find that sweet spot - or more specifically notice those boundary settings between which the sweet spot exists.

thanks - SteveS

[Shannon Parks]

I guess that the neg feedback pot is the one I am most interested in.

Wow - I was just thinking about that little pot while in the shower this morning. Odd, I know, but it must be a sign. Basically it was a stream of thoughts I had with people wanting to tweak their sound with fancy caps, $2 resistors, NOS Mullards, etc., when tweaking circuit values will <always> make a pronounced changes (sometimes bad, hehe).

The stock value is at about 20dB of negative feedback. This was common in the push-pull ultralinear power amps of Dynaco and Eico, probably most others. But 20dB of NF requires that you know your circuit well and have good quality outputs (which of course they did), or you will be pushing the limits of stability. We'll here's the beauty: by increasing the value of the resistor we can drop the NF to 17dB. Distortion goes up a negligble amount (remember, they wanted the best specs for their manuals back then, too). We end up with 3dB more gain in the amp circuit, a boon for those low output sources - though the max output remains the same. You may find the sound more pleasing to your ear - subjective area.

I've found that the ST35 is a very stable little amp. I'd be wary of adding more NF, though, pushing the stability margin. But increasing the NF pot by 10K (16 ohm tap) or 5K (8 ohm tap) can be a neat experiment. You may just leave it there.

Shannon