I recently completed a mod to my Deluxe Reverb Reissue to add a cathode bias mode. Overall it made the tone brighter and a little more raw, but didn’t make that much difference so I switched it back and forgot to take an internal picture in the complete state, but you can see part of the mod here:
I changed to a ClassicTone power transform which has a low power 300V primary along with the regular 330V. I used a 4PDT switch to switch between high power/standard bias, and low power cathode bias. Using the low power mode puts it inline with a Tweed Deluxe, so I used the same cathode resistor (250?) and bypass cap (25µF). I put the 4PDT switch in fuse slot, moved the fuse and power over and removed the standby switch. I disconnected the middle pin of the bias pot to do the mod, along with the respective pins on the power tubes, so no modification to the PCB was needed. I did need to drill extra holes though to fit the ClassicTone transformer, which is vintage spec.
Rough demo here, no comparison, just for posterity:
