Master Volume within Feedback Loop

Feedback - if done scientifically - depends on the "open loop gain" and the "feedback fraction." The feedback fraction in this design is constant while the open loop gain is variable.

In a guitar amp, if you don't hear a problem, you probably won't have a problem, but you also probably will get different results with different master volume settings. It'll be hard to tease that apart from the changes in breakup etc. you'd get at various MV settings anyway.

If you use a scope more than you use your ears you'll see evidence. Otherwise possibly not so much.

 

What I didn’t like about the MV within NFB set up:

1. NFB all but disappeared at anything less than max MV.

2. Changing the MV changed the NFB.

If you are fine with these, then there is no problem.

My amp worked fine, but I felt like these annoyances were big enough to go back to a pre-driver MV.

 

One issue is that the nfb signal gets lower and lower as you lower the MV because the signal to the output stage is being attenuated by the MV. This causes the amp to get a bit looser and grittier sounding when you start cranking the gain and lowering the MV. I installed them on several non-MV Marshall type circuits and always removed them after a short time.

Installing a pot that allows you to alter the nfb level as you lower the MV does help a bit though and allows you to dial in a sweeter overdrive sound.