Advertisement
- Cosmic Ampworks
Ground wire
Figure 24. Drill two holes near the first preamp tube: Drill hole 3 right next to the tube socket solder connections of pins 4 and 5;
the location is not particularly critical, but make sure not to damage the trace running from pin 6 of the tube (anode) to R7 (load
resistor). Drill hole 4 just above the tube socket solder connections of pin 9.
7.
Optional: install a presence and negative feedback control (5k and 1M potmeters, B = linear tapers) and
modify the phase inverter:
a. Mount the included 10k resistor diagonally to the circuit board by soldering one leg into the rear
hole of (the removed) R28; solder the other leg into the front hole of R8 (Figure 25).
b. Take about 20 cm of double stranded wire. Tear the two strands about 2 cm apart on one end.
Solder one strand into the front hole of R28. Solder the other strand into the rear hole of wire
bridge J12 (Figure 25). `
c. On the other side of the double stranded wire, pull the strands apart for some 4 cm and remove
some 5mm of insulation from the ends. Solder the end that runs from the rear hole of J12 to the left
lug of the 5k potmeter (Figure 26). Solder the other strand to the right lug. Solder the legs of the
0.1uF capacitor to the right and center (wiper) lugs of the 5k potmeter. Solder one leg of the 120k
resistor to the left lug of the 5k potmeter and the other leg to the left lug of negative feedback
control (1M potmeter, B = linear taper, Figure 26). The run some 10 cm of wire from the center tap of
the 1M potmeter to P3, on the left rear of the board (Figure 27).
d. Get a Dremel with a grinding or sanding attachment, a rotary drill with a small drill bit or a file.
Underneath the board, carefully interrupt the PCB traces on either side of the front leg of C15
(Figure 28), by grinding right through it, at about 5 mm distance from the soldering point. Then
restore the ground connection for parts to the right of C15 with a wire bridge (Figure 29).
e. Cut a length of single stranded wire about of 4.5 cm. Underneath the board, solder one end of it
into the rear hole of J12 (Figure 28); solder the other end to the front leg of C15 (Figure 28; also
underneath the board). Be careful not to heat the leg of C15 for too long with the soldering iron,
because it is very short and the heat might damage the capacitor!
© 2020 -
www.cosmicampworks.com
- The Ultimate Harley Benton GA15 Mod Kit- Version 1.4 - 200127
®
Hole 4
21
Hole 3
Dedicated to the Advancement of Stellar Tone!
Hole 2
Advertisement
