A PDF schematic of this mod is here....
The guitar used for these sound samples is a Lone Star Strat modded with Bill Lawrence L280/L280/L290 "single coil" style noise-cancelling pickups. The speaker is one of a pair of Mesa-Boogie MS-12 150 Watt "Black Shadow" speakers--the original type Black Shadow from the early '80s. These speakers reside in a pair of homebuilt 1x12" rear-ported cabinets. I recorded this with an AKG D3900 microphone right next to the speaker grille, using no EQ. The amp is tubed with EH EL34s, a Tung Sol 12AT7 in the PI, a JJ/Tesla ECC83S in the loop buffer/recovery slot, another ECC83S in the drive channel's second/third gain stages, and an old ('80s) EI 12AX7A in the input/CF stage.
The rhythm guitar is going through the clean channel, the clean channel volume is set to 5, the bright switch is on, the neck pickup is selected, and the guitar volume is full up. The lead guitar is through the overdrive channel, the gain is set to 4, the deep switch is off, the neck pickup is again selected, and the guitar volume is rolled back to about 7.
The guitar is going through the overdrive channel, the gain is set to 5, the deep switch is off, the bridge pickup is selected, and the guitar volume is full up. Harmonic feedback is easily attainable at this gain setting (indeed, it requires effort to avoid it...), even though the volume at which I recorded this clip is moderate, i.e., loud bedroom/basement practice volume.