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| VOLUME 6 | JULY 2005 | FREE |
By Terry Downs
Maintenance of audio amplifiers often requires observing an oscilloscope waveform with the amplifier running at maximum output. A good example of this type of maintenance is the adjustment of the bias of the final amplifier tubes in a guitar amp. Using a sine-wave test signal as input to the amplifier, the output tube bias may be adjusted so that the output signal has no crossover distortion. A sine wave at maximum amplifier output is hard on your hearing and your speaker. The dummy load can be used in the place of the speaker, allowing your speaker AND you hearing to be protected. This can be accomplished with simple high-power resistors. I have included a parts list with links to the manufacturers' websites to obtain the parts, along with a printable layout template for marking the hole locations in the chassis.
Item |
Qty |
Description |
1 |
1 |
Chassis, 9.5" x 5" Aluminum (Bud Corp.AC-403) |
2 |
8 |
Resistor, 8-ohms, 50W, 1% (Dale RH-50 8R000) |
3 |
2 |
Binding Post, Dual, (Radio Shack 274-718) |
4 |
16 |
Screw, Pan Head, #4-40 x 0.5" |
5 |
16 |
Washer, Split Lock, #4 |
6 |
16 |
Nut, Plain, #4-40 |
The dummy load simply contains power resistors. It is two independent 8-ohm loads rated at 200 Watts each. The Dale RH-50 series 8-ohm wirewound power resistors are used. 4 resistors are used for each load. (2) 8-ohm resistors are connected in parallel, then connected in series with another paralleled network to make the total 8-ohm load.
The holes in the chassis can be easily laid out by printing the Dummy Load Layout PDF file. Cut out the 9.5" x 5" rectangle and tape it onto the top of the chassis as shown below.
Use a scribe to accurately poke a hole in the center of the cross marks to make a indentation into the chassis top. Do this for the 16 resistor mounting holes and the 4 binding post holes.
Remove the paper template. I recommend using an automatic center punch to make the scribed indentations larger to eliminate walking of the drill bit. It works best to have something under the chassis top to keep it firm when center punching. Otherwise, the flimsy chassis top will absorb the impact of the center punch.
Drill the holes next. The resistor mounting screws are #4-40 and require 0.125" (1/8") diameter clearance hole. The binding post holes require a 0.5" (1/2") diameter clearance hole.
Wire the resistors according the layout on the template. Use 16 or 18AWG bus wire to connect the resistors. It saves the labor of stripping wires. You may find it easier to snake the buss wire through the resistor terminals by passing it through the hole in the corner of the chassis. Make sure you cut off the ends of the bus wire after soldering.
Use 18AWG or larger insulated wire to connect the binding posts to the resistor networks as shown below.
Use an ohmmeter to verify the resistance at the terminals is 8 ohms.
The 2 8-ohm dummy loads can be configured into a 4-ohm, 400W dummy load connecting the two load in parallel. Connect the two red posts together and connect both black posts together.
Below is an example of a sine-wave tube amplifier output with the bias adjusted so that no crossover distortion is present.
Revised 01/20/2007 - Hyperlinked parts in partslist had stale links.
T's Technical Notes Index
©2005, 2006, 2007 Terry Downs Music™
