Sunn Model T 90s reissue with a Mercury-made output transformer intended for the original 70s Model T. It’s a huge transformer (on the right in the picture, larger than the amp’s power transformer). You can see the old one on the right side in this picture. Some extra volume, LOTS of extra bandwidth. The new OT’s 2-ohm secondary tap and ultralinear primary taps are unused in this amp, and there is no lineout winding, so the XLR output is gone. It also has a slightly lower primary impedance, at 2.2k, vs. the old one’s 2.35k. The main change is in the character of the sound at really high volumes, where there’s quite a bit more clarity.
KT66s will no longer fit because the clearance on the OT end is too small. I had all but given up on them anyway for touring applications, because they seemed to stress the heater supply and the bias wasn’t stable enough for touring applications. I’ll still use 2 of them for recording at half-power, but for touring it’s 6L6GCs. Cheaper, cooler-running, more stable, higher headroom.
I also recently realized that the presence circuit on these amps is wired to remove upper-mids and treble when rolled down, rather than to add sparkle when turned up. Flat response corresponds to about 9.5 on the control, compared to a lot of amps where the minimum setting is neutral and higher settings add treble. This circuit’s corner frequency is 650Hz (really low!) and with the presence below 9, significant extra negative feedback is added above the corner frequency to the inverting input of the phase inverter. This is quite different from the other Fender amps that use this same power amp configuration and stock OT, which typically have no presence control or one with a more center-neutral setting. I think this explains a lot of why this amp has a reputation for being “dark” even though it’s a very typical Fender circuit. Keeping the presence down will change how much perceptible headroom the amp has too; you’ll think it’s breaking up (and farting out) a lot earlier on the volume dial.
For more info on this circuit, look here.