UralTone Hot Mama 2.0 dummy load – Building instructions

UralTone Hot Mama 2.0 Load Box - Assembly Instructions

UralTone Hot Mama is an adjustable load box designed for guitar and bass tube amplifiers. With the load box, you can open up the amplifier more at a reasonable volume - part of the power is converted into heat energy in the load.

Hot Mama is an updated version of the old 1.2 version. The new kit is based on the same proven circuit, with improvements including a reduced size and easier assembly.

Follow the parts list made for the series during assembly. We occasionally update the circuits of the series, and over time there may be improvements or changes, for example, according to the availability of parts. Possible deviations from the image series in this guide are therefore possible. In case of changes, we strive to add notes to the text of this guide. Note the correct impedance of the device being built in the parts list: 4, 8, or 16 ohms.

Note: The model assembled in the image series of the guide is built for an eight-ohm load. If you are assembling a four or 16-ohm version, the 50W power resistors shown in the pictures are not the correct values.

Before starting the assembly, it is good to check the version number of the parts list from the links below. If the documentation of an older version is provided in the kit, make sure before starting which guide is best to follow for the assembly.

UralTone Hot Mama 2.0 4OHM - BOM - bill of materials

UralTone Hot Mama 2.0 8OHM - BOM - bill of materials

UralTone Hot Mama 2.0 16OHM - BOM - bill of materials

Always first check that all parts are included in the delivery. If you find any deficiencies, contact us via email or through the "contact us" form on the pages. Note that components may look slightly different than in the photos. Also note that the values of power resistors may be rounded to the nearest whole number, e.g., 6.8ohm = 7ohm.

We strive to keep the models of components the same, so sometimes due to poor availability, we have to change the manufacturer or model of a part. So if a part is slightly different in color, don't be alarmed, each component has the necessary values marked on the side. Read any notes on the parts list carefully.

Start the assembly by marking the drill holes on the front panel. Use the front plate as a guide for marking the holes. The height of the holes from the open edge of the enclosure (from the bottom to the center of the hole in the image above) is 15.5 mm. It is advisable to start drilling with a small bit to ensure accuracy. Then, drill the hole to the specified size.

The hole for the line out potentiometer is 7 mm in diameter, and for the rotary switch, it is 9 mm.

Two front plates are provided, one is mounted at the front of the enclosure, and the other is reversed and mounted at the back.

Mark the positions for the Speaker out and Speaker in holes on the back panel. Drill a 9.5 mm (or 10 mm) hole for the Speaker out jack and attach the panel to the enclosure with the jack.

Drill the holes for the Speaker in and Line out jacks to the same 9.5 mm size with the panel installed.

Attach the remaining two jacks to the enclosure.

Hot Mama 2.0 circuit board pair. The power resistors of Hot Mama are placed between two circuit boards. Each board has a resistor part number (e.g., R4). Each part number corresponds to a component value given in the bill of materials (BOM). The corners of the circuit boards are marked with letters A-D. These should be aligned with each other.

Turn the rotary switch guide pin to the side. In the picture, this is visible on the right.

Solder a 0.6 W resistor to the board on the front panel side.

Press the rotary switch and potentiometer into the circuit board on the front panel side. Do not solder these yet.

Attach the circuit board with components to the enclosure. Rotate the board if necessary so that it is perpendicular to the bottom of the enclosure. When the position is correct, solder the potentiometer and switch in place. After soldering, both the potentiometer and the switch are slightly tilted relative to the circuit board.

Remove the circuit board on the front panel side from the enclosure. Lay the power resistors R1, R2, R3, R4, R5, R6, R8 on the board on the front panel side (upper in the picture). When the resistors are in place, push the circuit board on the back panel side into place.

Lay the upper row of power resistors R10, R11, R12, R13, R7, R9 on the board on the front panel side.

Solder the legs of the resistors on the top row of the circuit board on the back panel side (lower).

Slide the package back into the enclosure so that the front panel parts are attached in place.

Solder the outermost resistors on the top row from the front panel side.

Check that the back panel is perpendicular and solder the outermost resistors on the top edge. Before soldering, press the resistors down so that they press against the lower resistors, with the lower one pressing against the bottom of the enclosure.

The remaining soldering is easier to do when the circuit is removed from the enclosure.

Keep the circuit on a flat surface with the resistor mounting flanges facing down. This way, the resistors are on the same level and conduct heat to the aluminum device enclosure as they warm up. Before soldering, it is advisable to press down the stacked resistors so that the upper layer presses against the lower layer of resistors.

Cut five jumper wires from a single-strand cable. Strip the conductor enough so that the wire can be inserted into place by first pushing it over from one end and then backing it into place from the other end.

Ensure that the conductors do not touch the resistors that heat up during use. If the insulation of the conductor melts through, the conductor will ground through the metal body of the resistor to the signal ground.

Solder the jumper wire to the REACTIVE LOAD pads.

Attach the circuit to the enclosure with the nuts of the front panel potentiometer and rotary switch. At the same time, install the panel in place.

Drill a 3 mm hole in the enclosure through the R5 mounting flange hole.

Make a countersink with a 6 mm drill bit so that the countersunk screw is flush with the cover.

Secure the circuit with a nut. This way, the resistor pack does not rely on the front panel components.

Wire the jacks as shown in the picture according to the markings on the circuit board and panel. The pads marked - on the circuit board are soldered to the grounding lug of the jacks.

Finally, push the knobs onto the potentiometers. The dummy load is ready for use!

Instructions and notes.

A load box is always designed for both the speaker impedance used and, in the case of tube amplifiers, also for the output impedance selection. For a tube amplifier, especially one with higher power, selecting the correct output impedance is important. A load box acts as an attenuator between the amplifier and the speaker, ensuring that the impedance seen by the amplifier remains the same regardless of the level of attenuation. If the speaker and load box impedances are not the same, the impedance changes from the speaker element's impedance at 0 dB attenuation to the load box's impedance in the load box position. For example, a 16-ohm speaker and a 4-ohm load of the Hot Mama "seen" by amplifier changes from 16 ohms at zero attenuation to 4 ohms with the load box. The load box should always be selected to match the impedance of the speaker and the amplifier's output.

The Hot Mama load box is an excellent tool to "reduce" the amplifier moderately so that it can be driven more easily to power stage distortion. When the load box is used at "playing volumes," a somewhat large amplifier can be reduced to function better in ensemble playing.

However, bedroom musicians should note that a load box does not achieve extreme results and that the amplifier and speaker largely work in tandem. Although the amplifier's power is reduced with the load box, the speaker's sensitivity remains the same. The volume levels desired in rehearsal spaces/stages and at home, which are pleasing to housemates or neighbors, are often very far apart. A sensitive 12" guitar element that produces an average of 100 dB sound pressure with 1 W power still plays loudly, even if the amplifier's power is reduced from one hundred watts to one. The Hot Mama is not suitable for such use and often does not meet the builder's expectations.

Scaling down an amplifier is always a compromise in terms of sound, and the larger the change, the greater the impact on the sound. The speaker element, with its distortion and nonlinearities in movement, is part of the sound outcome. If the element is not fed the expected amount of power, it will not sound the same as when played loudly. For this reason, Hot Mama's attenuations are implemented in such a way that the attenuation is limited and does not alter the outcome too much.