Test Equipment
Load Box
The following load box can be used between an amplifier and a speaker to reduce the
volume of the amp and still get distortion. The values are set for a speaker impedance of 8
ohms and an amplifier wattage of no more than 20W.
I've listed several different resistor value sets to choose from based on how much signal
power you want making its way to the speaker.
I recommend building everything inside a small metal box. it is important to mount the
resistors firmly against the sides of the box. When the load box is in operation, the resistors
will get very hot and the heat will be properly dissipated when in contact with the metal sides.
To improve this heat transfer, the resistors can be epoxied to the sides of the box.
Power to
Speaker
Approximate
R1
R1
R2
R2
(15W source)
Attenuation
value
wattage
value
wattage
10 watts
- 2db
1.5 ohms
5W
36 ohms
5W
7.3 watts
- 3db
2.4 ohms
10W
18 ohms
10W
5.2 watts
- 5db
3.3 ohms
15W
12 ohms
10W
3.8 watts
- 6db
4.0 ohms
15W
8 ohms
10W
1.9 watts
- 9db
5.1 ohms
25W
4.7 ohms
10W
0.9 watts
- 12db
6.2 ohms
25W
3.9 ohms
5W
These resistors can be wire wound ceramic cast and / or chassis mount and can be sourced
from various electronic suppliers such as:
Mouser.com; Jameco.com; Digikey.com
Appendix E
TubeDepot.com 45
E
