understanding NFB

[briane]

I am tryiing to understand the generic capabilities of nfb, and how to adjust an amp to taste in respect to nfb.

The question stems from a SUNN 100s I just built. Works great when I disconnect the nfb, oscillates like crazy when I hook up the nfb. This amp is built exactly to schematic as per

http://www.schematicheaven.com/newamps/sunn_100s.pdf

using the November 1967 version, the second one in the doc, the simple one without reverb or jfets.

The amp is crazy loud (paint peeling), as well as a little harsh, that hopefully getting the nfb working correctly would fix the harshness.

The lowest volume setting on the amp makes the walls shake. Talk about kt88 in PP!

the sunn 100s feeds the 16 ohm speaker tap, through a 1k resistor, which then goes back to the 6an8 pre-amp cathode through a 680 ohm resistor, voltage divided with a 47 ohm to ground.

So the question is....would upping this 1k off the 16ohm tap reduce nfb? anyone care to wager a guess as to what that resitor should be? 10k, 47k? Is there anything else that might help get the nfb usable?

thanks....

[ontariomaximus]

Sounds like you have positive feedback happening. Try swapping your primary wires then applying the nfb

[dartanion]

Yup, swap the OT primaries around and it should be all good.

the article on Aiken's site for the low down on NFB.