NFB circuit - preferences?

Gunner · Post by **Gunner** » Sat Oct 06, 2007 3:41 pm

Since I installed the Larry/Rockstah PPIMV in my JMP 1987 there is need for some small changes in the circuit. On low volume This PPPIMV seems so bring out the treble freqs in a better way than the PPIMV I used before. Great! Still I sometimes feel the amp is not clear enough in the highest freqs at full volume, at least not compared to my JTM45 clone. I miss some shimmer on the top but I know I got it somewhere inside the amp.

The treble freqs are so important to get the best tone from your Marshall IMO. So I started some fine tweaking.

After changing some caps values in the preamp I paid attention to the NFB circuit. In my amp I've used the "original" (according to early schemes) 47k resistor but connected to 16 ohm tap (a lot of NFB, but not as much as in my JTM45 clone). Now I changed to 100k and felt the tone became slightly clearer, a little bit more cutting edge on full PPIMV volume, but still warm. Of course the Prescense control acts a little bit different now. Thinking about moving NFB to 8 ohm tap, but I'm not sure what differences I could expect.

There's of course nothing right or wrong here. Just your preferences can tell. And the "sound" from your OT is one important thing here. But I'm still interested in your opinon and experiences.
- What NFB circuit do you prefer in a 1987-circuit, and why?
- What do you think are the differences between for example 100k/16 ohm tap and 47k/8 ohm tap (and other combinations)
- Any other interesting thoughts on the important NFB in a Marshall?

My prefered sound is a classic plexi sound, not the EVH-ish type of sound. My amp has (for the moment) rather low filtering (32 in preamp and PI, 47 to the power tubes, and 50+50 as first caps), split cathode V1 (820/22 + 820/0.68uF), 820/4.7uF on V2 cathode, 56k/250pf tonestack an 0.1uF after PI. A Marstran OT. B+ 440V.

/G

jbzoso2002 · Post by **jbzoso2002** » Sat Oct 06, 2007 5:33 pm

check this out

http://marshall.redpt.com/clay/marshall ... s_101.html

jimmy

neikeel · Post by **neikeel** » Mon Oct 08, 2007 5:35 am

I have PPIMV on two 50watters.

I have an amp with 47k onto 8ohm (PPIMV Mk2)and one 47k onto speaker jack (effectively 16ohm with my cabs - with the latest Fischer/Novosibir/Rockstah version). I prefer the 47k onto the 8ohm tap and am planning on changing the other amp to the 8ohm tap.

Gunner · Post by **Gunner** » Mon Oct 08, 2007 5:55 am

jbzoso2002 wrote:check this out

http://marshall.redpt.com/clay/marshall ... s_101.html

jimmy

I've read that article before, but thanks for reminding me

Interesting information!
Did some testing this weekend. Changed first from the 47k-16ohm set up to 100k-16 ohm. I liked that, a little bit clearer in the treble freqs.
Then I moved the connection to 8 ohm tap, but now I felt the tone became hard and a little bit harsh, lost some shimmering on the top. Mabye the original 47k-8 ohm is the best solution (and probably no coincidence of course...).
I guess this is a never ending twaeking...
/G

Coondog · Post by **Coondog** » Fri Oct 19, 2007 11:48 pm

That link has some cool info on presence circuits . The Resonance control described at the bottom could easily be added to the NFB circuit without the pot . just replace the pot with a resistor for the desired effect .
some 18watt guys do this to keep the bottom big when they add neg feedback .