Hardware docs: Quad Amplifier Board (1413) photos

Four photos of the 1413 Rev. 1: component side with the bolt-on Pyramid
PB-150P, solder side (etch NT 1-0A 3695), and both faces of the Pyramid
module (inputs/level pots, speaker terminals/fuse/power). Photo index
updated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -139,8 +139,8 @@ chip's programmed signature even reads `U7`.
 
 ## Photo index ([`photos/`](photos/))
 
-Original Signal filenames preserved here for provenance (all taken
-2026-07-15).
+Original Signal filenames preserved here for provenance (taken 2026-07-15;
+amplifier board 2026-07-19).
 
 | File | Original | Shows |
 |------|----------|-------|
@@ -153,6 +153,10 @@ Original Signal filenames preserved here for provenance (all taken
 | `buttons-board-1401-component-side.jpeg` | `signal-2026-07-15-093051.jpeg` | Buttons board: 2× CD74HCT165E, 74HCT00AP, 2× UCN5821A-type drivers, 74HCT164N, 8× lamp/button connectors, "P/N 1401 REV. 0" |
 | `rio-board-1407-solder-side.jpeg` | `signal-2026-07-15-093058.jpeg` | RIO board solder side, PCB etch NT 3-0 4996 |
 | `rio-board-1407-component-side.jpeg` | `signal-2026-07-15-093105.jpeg` | RIO board: TMP68HC11 QFP, GAL20V8A ("5764" sticker), AM27C512 EPROM, LH52B256 SRAM, 5× HCTL-2016 + OPT1–5, J11–J18 pod ports, status LEDs, "P/N 1407 REV. 3" |
+| `amp-board-1413-component-side.jpeg` | `signal-2026-07-19-134932.jpeg` | Quad Amplifier Board full view: Pyramid PB-150P module, corner crossovers, "P/N 1413 REV. 1", J5 pinout silkscreen |
+| `amp-board-1413-solder-side.jpeg` | `signal-2026-07-19-134926.jpeg` | Amplifier board solder side, PCB etch NT 1-0A 3695 |
+| `amp-board-1413-pyramid-inputs.jpeg` | `signal-2026-07-19-134921.jpeg` | Pyramid input face: HIGH INPUT connectors, LEVEL pots, LOW INPUT RCAs; front-corner crossovers |
+| `amp-board-1413-pyramid-outputs.jpeg` | `signal-2026-07-19-134915.jpeg` | Pyramid output face: SPEAKERS terminals 1CH/L–4CH/R, blade FUSE, B+12V/GND/REMOTE; check LEDs, J1–J4 pinout silkscreen |
 
 ## Cross-references