This is one of the most insane modifications we've ever done or even thought about, at least in my opinion. I told the owner years ago that we could install a serial FX loop in his Dual Terror, no problem.
Then as I was about to get started, I realized that the two channels switched in separate post phase inverter master volumes via relays, and that the phase inverter itself is an integral clipping stage -- so there is no place to "tap" the signal post preamps, as it has already been split for the push pull power amp.
So, we took the 2 opposing phase signals in the power amp, attenuated/buffered/mixed them via a "Broskie Cathode Follower" circuit, which is kind of like a buffered differential amp with up to unity gain. We just adapted it for use with HV depletion mode MOSFETs instead of tubes. The buffered single output signal is the "send. " On the return side we re-amped and re-split the signal with more HV FETs, using adapted "tube" (LTPI) technology. We were careful to preserve all impedances and EQ characteristics in the amp and the loop is 100% transparent.
