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- Cosmic Ampworks
Front hole of R58
Figure 29. Re-establish ground connection for components to the right of C15 with a wire between the front hole of (the former) R58 and
the rear hole of the removed connector SCN1. The ground connection will be needed for the new volume control (see further down).
8. Optional: Rewire the power tube heater supply. It is quite likely that mod 6 (DC power supply for the
preamp heater filaments and rearrange the preamp heater wiring) already resulted in a very quiet
amplifier. When the mods were developed, power and preamp tubes were rerouted all at once, so it is
difficult to verify what the effect is of only rerouting the power amp heater wiring. Normally, in a push-pull
power amp, most of the hum in the opposing tubes cancels out.
a. First cut the two circuit board traces that conduct 6.3 V AC to the power tube heaters (Figure 30)
with a Dremel tool with grinding attachment, with a rotary drill or with a simple file.
b. Then, drill three holes in the circuit board:
i. Drill holes 1 and 2 close to the spade connectors that connect the board to the 6.3V DC
power supply (Figures 30, 31)
ii. Drill hole 3 close to the 2
c. Twist two lengths of single stranded wire (Figure 31). Run (solder) one strand from pin no. 4 of one
EL84 output tube to pin 4 of the other. With the other strand, do the same for pins no. 5 of the two
output tubes. By mixing up the wires, i.e. accidentally connecting pin 4 to pin 5, the heater current
in both tubes will be out-of-phase, hence any hum caused will no longer be cancelled out by the
push-pull stage. If it is necessary to use wires of identical color, use a marker to distinguish them.
Make sure that there is no contact between the wires near hole 3!
© 2020 -
www.cosmicampworks.com
- The Ultimate Harley Benton GA15 Mod Kit- Version 1.4 - 200127
®
power tube, near the circuit board screw hole (Figures 30, 31)
nd
24
Rear hole of (former)
connector SCN1
Dedicated to the Advancement of Stellar Tone!
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