Hi all,

I was given a pair of output transformers from a Pilot 602 receiver. I want to build them into a small stereo amp. I have a question about the OPT secondary taps.
The configuration of the OPT secondary taps looks unusual to me; the 4 ohm winding is grounded and the feedback is taken from the 16 ohm tap. Should I just ground the common (black) and use the 4 ohm tap for a 4 ohm load?

Thanks.

Was common for a while to use this connection to create a speaker-level center channel (it's essentially free...). Can be a nuisance if you want to look at the output with a scope! Yes, you can ground the common terminal.

The feedback network and "Zobel" (R-C) network can be used as a starting point in your amp, since they were chosen for that transformer.

I went through that years ago when reusing a pair of transformers from an amplifier that originally had similar secondary connections for center channel. If you change the ground point in your new design to the Common lead of the secondary and then take feedback from a point above that, it will likely be out of phase to what you need. I solved that issue by swapping the brown and blue plate leads.

The feedback is not taken from the 16 ohm tap, it's taken from the black wire used as the speaker common. Basically the 4 ohm tap and the common are used for feedback with the phase reversed.

Unless you want to duplicate the original circuit and keep the center channel speaker option you can re-wire the transformer the traditional way by grounding the black wire (speaker common GND) and using the 4 - 8 - 16 ohms output taps.
The global negative feedback can be taken either at the 16 or 8 ohms taps with the appropriate NFB RC network (R68/C58 on your schematic) which must be modified to keep the same negative feedback factor.
If the phase is incorrect (unstable or oscillating amp) just reverse the two primary plate wires.