- Cosmic Ampworks
®
- The Ultimate Harley Benton GA15 Mod Kit- Version 1.4 - 200127
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20 Dedicated to the Advancement of Stellar Tone!
j.
Run a wire from the L7912 output (-12V DC) to the second preamp tube (V3) heater filaments:
i.
Cut some 12-13cm of wire and solder one end to the front hole of (the removed) wire bridge
J20 (Figure 18
–
see “heater wire”).
ii.
(If, for some reason, the L7912 got damaged and you need to use the L7812 instead, carry
out steps 6a.-g. again, but substitute L7812 for L7912 in the text. Then, solder the 12-13 cm
wire mentioned under the previous point to the left hole of the (removed) wire bridge J10
instead).
iii.
Bend the wire, so it runs some 4 cm above the board and lead it through hole 1 (Figures 21
and 22). Then to solder it to the tube socket connection of pins 4 and 5 (Figures 21, 22, 2);
k.
Solder a wire between the heaters of V3 and V2:
i.
Lead some 5 cm of wire through hole 2 (Figures 22, 24), then solder it to the tube socket
connection of pin 9, V3 (Figure 21)
ii.
Bend the wire, so it runs a couple of centimeters above the board and then lead it through
hole 3, then solder it to the tube socket connection of pins 4 and 5, V2 (Figure 23).
l.
Lastly, close the DC heater circuit by connecting the other heater connection of the first (leftmost)
preamp tube V2 to ground:
i.
Cut some 1-12 cm of wire and lead one end through hole 4 (figures 25 and 26), then solder
it to the tube socket connection
of V2’s pin 9
(Figure 23)
ii.
Bend the wire around the edge of the circuit board and solder the other end to ground; the
underside of the ground wire that runs to the chassis (on the large trace) is a suitable
place (Figures 23, 24).
Figure 23. Drill two holes near the first preamp tube: Drill hole 3 right next to the tube socket solder connections of pins 4 and 5;
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. Make sure not to accidentally damage the wire bridge on top of the board! Ideally, also
interrupt the circuit board trace in go, because the heater traces spread hum throughout the circuit in their stock layout.
3
9
8
1
2
4
5
6
7
(hole - ignore)
Hole 3
Hole 4 (to ground)
Ground wire (from hole 4)
