GuitarPCB presents Rock Hard Dual Combo project page

June 1, 2026 · 1 sources

GuitarPCB has published a project page for the ROCK HARD Dual Combo, a build that combines the PlexAmp and Boost circuits with built-in order switching. The page is dated June 2026 and also notes a new DC Hub, described as available now for easier DC routing to both circuits and designed for the 2.1mm low profile jack.

The maker describes the project as a way "to explore the evolution of hard rock tone from the mid-1970s through the early 1980s." According to GuitarPCB, the PlexAmp side delivers "authentic British amplifier-style crunch with exceptional touch sensitivity and dynamic guitar-volume cleanup," and used alone it captures the open, responsive character the maker associates with classic hard rock recordings. The Boost is presented as a gain-staging tool rather than a simple volume boost: placed before the PlexAmp it is said to increase sustain, harmonic complexity, note bloom and natural compression by driving the input stage harder; placed after, it is said to add punch, fullness, tonal richness and output level while preserving the PlexAmp character. The maker credits the same front-end preamp principle to rigs including Angus Young's Schaffer-Vega wireless system and Ritchie Blackmore's Aiwa TP-1011 tape recorder.

The parts list gives the Boost controls as BOOST C500K and the PlexAmp controls as GAIN A500K, BASS C25K, TREB B10K and VOL B100K. The Boost uses a TL071 (IC1) and the PlexAmp a 4558 (IC1). Diodes listed are 1N5817, 1N914 and 1N4001 parts across D1 to D5. The Boost calls for a 22M resistor at R1; the page says two 10M resistors in series (20M) can substitute "with no noticeable difference in performance," and that original KOA Speer CF1/4C226J 22M carbon film resistors are available from Mouser and DigiKey. A 10uF or 15uF may be used at C4 of the Preamp Boost "without affecting tone." A C20K or W-taper pot may replace the C25K bass pot.

Build notes call for a 3PDT toggle switch, On/On, solder lug version with a short bat, for order switching on the dual wiring board PCB, and say both nuts should be removed from each 3PDT foot switch for the best height match. Current limiting resistors may be 1k8 for bright or 4k7 for dim status LEDs, and a jumper goes on the unused LED pads. The page lists a 1590BB2 drill template and points to a Tayda drilled enclosure through the shop page; with that enclosure, both status LEDs must be installed on the wiring board. No price is given.