rayma you totally right, original design was using feedback loop, but after try differents values all produce a remarkable hum so no feedback loop installed, and I forget remove it the empty loop in the draw.
so that means that this circuit is only amp with an attenuator ? so can install the outside
preamp to the actual amp inputs and just control the volume in the outside preamp ?
In the power transformer is a 70v tap , but not shure if could interference with the others 280v that are using. PTm photo