2nd stage local feedback

[txbluesboy]

Can someone please help me understand the local feedback on the second stage of the ODS? What is its effect sonically? What frequencies are affected? I've read KOCs explaination in TUT6, but its still about as clear as mud. HELP PLEASE!

[heisthl]

Your making it too difficult, whatever you feed the grid shows up out of phase on the plate, feed that back to the grid and in thoery it cancels. I've never worked out the exact properties of the .05/ 44m RC filter in this arrangement but if you A-B in and out of circuit you can hear the frequencies that are affected. A nice side effect is the larger the signal the more cancellation so it has a compression effect. When in doubt add a switch to ground at the junction of the 22m resistors.

[butwhatif]

I hear it as a slight bass boost, depending on the hipass of the circuit -it cancels highs . I don't really like the effect, and don't typically put it into
modded amps, but i think the other 'use' of it is to partially alleviate a
microphonic V1 tube, or fet input, and help keep parasitic highs and r-f out of the first two stages. Some players used the fet input for piezo pu's, and it helped to take the highs off those.

[Guitarman18]

I've found that if the input vol is at max (for most effect) when on a switch it has a pretty dramatic effect. = Compression when switched in but when switched out of circuit it can provide hints of raw Marshallesque tone. Certainly useful for anyone wanting tonal veriety from their build.

Cheers, Paul.

[txbluesboy]

Thanks, I do see in a graph in the TUT6 discussion of the circuit that gain starts to taper off above about 1.5k hertz, so the bass boost effect makes sense. The reason I ask is that I built a TW Liverpool a year or so ago that I never liked, Changed it to a Rocket preamp, which I liked, but then built a proper Rocket, so I changed to a pentode (EF 86) preamp that was OK but not as good as a 15watt version I had built. Then I decided to put an ODS clean channel, rock/mid boost-off/bright-off TMB, with the feedback circuit-----Its now my favorite amp, Lots of chime and shimmer. It does have more bass than the Rocket however. I was thinking that maybe it was the feedback circuit that gave what I call shimmer, but it sounds like this is not the source. Whatever it is I like it. Thanks again for you comments.