UralTone 18W 1974X - build guide

The UralTone 18W 1974X is our version of the original Marshall amplifier. The construction method is the same as with our other amplifier kits. The UralTone 18W has two channels, one with tremolo. The tried-and-tested EL84 pair serves as the power stage. The UralTone 1974X 18W chassis shares the same dimensions as the JTM45 and AC15 amplifiers, and this amplifier can also be installed in a Marshall head enclosure.

In this article, the stages of construction are outlined. Note that not all details are reviewed, and the architects' idea is mainly to familiarise the builder with its main features. The assembly is done by using the layout picture and BOM. The pictorial guide works as additional help to understand the assembly process and as well the correct order of it.

  1. UralTone 18W guitar tube amp - DIY kit
    UralTone 18W guitar tube amp - DIY kit
    €495.00
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    SKU: 950-18W

UralTone 18W construction kit - Layout

UralTone 18W construction kit - BOM - Kit parts list

Soldering instructions

Identifying components

First, always check that all parts are included in the delivery. If you find any deficiencies, please get in touch with us by e-mail. Please note that the components may look slightly different from the photos. Also, note that the voltages of the parts may be higher than what is written in the list. We try to keep the models of the components the same, but sometimes, we have to change the manufacturer or model of a part due to poor availability. So, if a part is a slightly different color, don't panic; each component has the necessary values ​​marked on the side of the component. Read the notes on the part list carefully.

Building the amplifier involves three stages. Installation of the parts to be attached to the chassis and their incomplete wiring, laying of components and soldering of the PCB board, and installation of the PCB to the chassis and its wiring. You can do the first two in the order that suits you. However, we recommend that you follow the order of the images below.

  • Attach all parts to be mounted on the chassis. The layout picture and parts list indicate which screw and nut are used to fasten each part. Fasten the small and light parts first. This makes handling the device easy. Significantly protruding and heavy components, such as transformers, are installed last.
  • Note the mounting direction of the supports. With the smaller noval sockets, the empty pin is directed to the center of the case. The control pin groove in the Octal base's middle is directed toward the back plate.
  • Do not install the impedance selection switch or the anode voltage fuse yet. These are obstacles in the way of installation.

  • A close-up view of the tube socket positions and their direction. Pins 1 and 9 are installed toward where the PCB will be mounted.

  • Pay attention to the installation direction of the jacks.

  • In some series, the indicator light has pre-installed wires, different from the picture.

  • Install the mains transformer. Push the transformer into the mounting hole as it is and fasten it to the frame with large star washers and nuts.
  • Pay attention to the installation direction and how the wires come out of the package.

Tip: The shortened/cut wires can be reused during construction. The set comes with enough connecting wires, but by recycling the wires of the transformers, you can use more colors and wires of the same thickness.

  • Install the output transformer.
  • Note the installation direction; the wires of the primary winding come through the hole that is on the side of the mains transformer.

Installation status from the side of the cover

  • Solder the rotary switch to the impedance selector PCB. The installation direction of the rotary switch is printed on the component side of the board. The installation can be made easier by attaching the switch to the frame first to stay in place when soldering.
  • Solder the wires coming from the output transformer to the PCB and the solder lug of the speaker connector.
  • Solder the blue and brown wires of the primary side to the anode pins of the output tubes.
  • Note: the picture shows the heater wires installed on the second output tube - do not solder these yet.

The red wire pair and the pink wire are shortened and insulated with a heat shrink sleeve. 

  • Tip: Keep the cut red wires; they are convenient for making the stand-by wiring for the switch on the front panel. 

(Sufficient wire is supplied with the set, but the extra wires of the transformers are often of high quality and suitable for wiring the kit, so these can be used elsewhere.)< /p>

Next, we solder the wires of the main transformer that are connected to parts attached to the chassis. Solder the wires of the mains transformer on the side of the back panel to the base of the rectifier.

  • Solder the purple wires of the anode voltage of the mains transformer to pins 4 and 6 of the rectifier tube base. 
  • The gray and orange wires of the five-volt heater are soldered to pins 2 and 8. Solder the wires to the lower holes.
  • The red wire put aside in the previous section is soldered to the base of the rectifier.
  • The yellow wire of the center pin of the anode voltage is left unsoldered.
  • Also, leave the green wires of the heater that have already been connected in the picture unsoldered.

Solder the wires on the front panel side of the mains transformer and the wires of the mains switch and stand-by switch.

      • A ring connector is soldered to the central output wire of the heaters coming from between the green wires. This will later be attached to a hole in the chassis for this connection.
      • The blue mains power wires are brought to the middle pins of the mains switch. The wires of the indicator light go to the same pins.
      • The blue and brown wires go from the lower pins of the mains switch to the IEC connector on the back panel.
      • The red wire from the rectifier is soldered to the stand-by switch. Use both sides of the switch by bridging the pins.
      • The red wire from the lower pin of the stand-by switch goes to the fuse connector on the back panel
      • The red wire from the middle pins of the stand-by switch goes to a + lug of the multi-cap capacitor (with the red wire center tap of the output transformer)

      • Solder the brown and blue wires from the mains switch to the IEC connector.
      • Solder the yellow-green shield wire to the middle pin of the IEC connector. Leave the wire long. If the IEC connector comes off, the yellow-green wire should be the last wire to which it will be attached. The wire makes a loop before the ring connector to the left in the picture. The ring connector uses a star washer between the connector and the chassis.
      • DO NOT USE THE YELLOW-GREEN WIRE FOR OTHER CONNECTIONS. Yellow-Green is reserved for only the protective ground connection.
      • Install the fuse connector. Solder the red wire from the rectifier socket to one lug and the wire from the stand-by switch's lower pin to the other.

Continue assembly by wiring the heaters. This step can always be done one tube socket at a time, twist a sufficient length of wire, and fit the pair of wires in place before soldering. If the jacks and the impedance selection switch on the back plate are in the way, these can be temporarily removed. 

      • The bridge of pins 4 and 5 of the 12AX7 tubes can be soldered separately or strip the wire enough so the end can be folded over the pins.
      • Move the wires coming between pins 4 and 5 to the side when soldering, i.e. press the wires Pin 5 to the side and solder pin 4. This way the insulation won't melt.
      • When the tubes are wired according to the picture, measure the resistance in terms of the one on the lower right in the picture and between pins 4 and 5 and the resistance between these pins and the frame. Make sure there are no short circuits here. 
      • After the wiring is measured for infinite resistance, solder the green cables of the mains transformer to the base.

A close-up of the wiring of the heater lugs with the base of the 12AX7 tube.

The parts to be installed in the chassis are now installed and the wiring done within it is ready. The potentiometers and jacks on the front panel are not wired yet. 

Make sure that all soldering is done according to the layout picture. Some pictures are missing, for example, the wire between the multi-tap capacitor ground and the chassis, which is attached to the same point as the center pin of the heater wires. 

Now the chassis can be moved aside, the second step - laying out the components of the main PCB begins. Building tips:

      • The points on the board where more wires will be soldered later can be left unsoldered at this stage. This way, you don't have to install the wires later on already soldered lugs, etc.
      • Lay out the parts by grouping them by type and height. Doing the lower parts first makes soldering easier because the parts stay on the board's surface when it is turned over for soldering. Typically, installation keeps the work organized. Always mark the installed part (R1, C1, etc.) on the parts list.

NOTE: Changes have been made to the PCB after the prototype phase. The pictures below give an idea of the construction, follow the part placement of the layout picture during construction. At the end of the article, there are pictures of the newer PCB.

When mounting components, you should start with the smaller components so that these remain on the surface of the board when the board is turned over. To begin with, it is enough to fold the legs of the components inwards. You can already solder the points where no more parts or wires will be installed and shorten the legs of the components here. Even though the part values ​​are printed on the PCB, always verify any changes from the parts list. Regarding the color codes of the parts marked in the layout picture or the part number in the parts list - the numerical values are always correct. The link to the latest parts list can be found at the beginning of the article.

Fold the legs of the components so that the parts stay on the board when it is turned over for soldering.

Solder those components to the board where there will be no wires.

Cut the extra wires short. The points where there is no solder can also be cut to the final size so that the part remains just attached to the plate.

Detail from the previous point.

When the board is ready for laying out the parts, start by soldering the wires to the ends of the tubes, the front plate, and the other points.

In the prototype in the picture, the wires are strung through the board. This setup is not the most practical. Solder the wire directly to the pad from the top side of the PCB.

The signal path capacitors are installed between the potentiometers and the PCB.

Run the wires on the surface of the bottom (roof) of the chassis. The metal frame acts as a protective ground. Keep the wires far apart and try to make the crossings at a 90° angle. This way, the wires do not run parallel, and the signal cannot be connected capacitively.

Another detail about the wiring.

The amplifier installation work is complete. Next, following the layout image, we check that all the wiring has been run and the parts have been laid out correctly. Go through the connection wire by wire, part by part, and mark the checked part on the layout picture. When everything is correct, measure with a multimeter as well as the integrity of the grounding and rule out short circuits in the power source:

      • Measure using the resistance range of the multimeter: One probe to any part of the chassis frame and the other to the negative lug of the multi-tap capacitor, the negative terminals of the capacitors on the PCB, the grounds of the potentiometers / black wires and the bodies of the jacks. The results should read zero ohms. 
      • Measure in the resistance range of the multimeter: one at a time, measure the resistance over the positive poles of the multi-tap capacitor and the chassis and the large 500V electrolytic capacitors on the PCB. In this measurement, the resistance is small at first, but settles to several hundred kilo-ohms. Make sure to wait for the meter to settle. A small resistance, zero or only a few kilo-ohms can be a sign of a possible short circuit, which should be looked for before connecting power.

When the measurements and checks have been done successfully, you can connect the power to the device. First, install the fuses and the rectifier tube in place, the other tubes are left uninstalled. Measure the voltage from the ground and positive poles of the multi-tap capacitor and the capacitors of the PCB. The unloaded voltage is approximately the same at all positive poles and at this stage well above the results of the measurement points marked in the layout image. If there is no voltage on the capacitors, the device smokes, sparks, or does something unexpected, turn off the power. 

Before troubleshooting, make sure by measuring that the capacitors of the power source are de-energized and the device is disconnected from the mains. The layout picture also shows the AC voltage measuring points. If necessary, these can be measured in the 1000Vac range of the multimeter if there are problems with the device. Of these, those marked with one arrow are measured relative to the ground, and those marked with two arrows are measured between the arrows.

Install the rest of the tubes and connect a speaker or an artificial load to the output. Turn the knobs to zero. Turn on the electricity. Let the amplifier settle down briefly and measure the voltages at the measuring points. At the same time, make sure that the amplifier does not do anything unexpected.

Measuring points A: 350V, B: 345V, etc. are marked in the layout picture. The values ​​of these measuring points are measured relative to the ground, i.e., the black measuring head of the multimeter is attached to the body of the device, and the measurements are made with the red one. It is advisable to even sit on your other hand while you make the measurements. Use the 1000Vdc setting as the measurement range and allow approx. 10% deviation from the given measurement results. 

The voltages at the measuring points are always affected by the dispersion of the tube parameters and the differences in current consumption. Another factor that changes the results is the magnitude of the mains voltage. This can vary between 220-240Vac, which is easily seen in the measurements. It is better to check the voltages more as a whole and allow them to slide in one direction or another. Large deviations in a single measuring point when the others are in place can reveal, for example, the wrong size of resistor on the PCB. You should also rely on your ears: Something is wrong if the amp sounds bad.

Note: If you measure voltages without tubes, the voltages with the mains transformer unloaded are always higher. Except for alternating current measurements of the mains transformer, always measure the voltages with the tubes.

The amplifier is ready when the measurements are done to an acceptable level. Connect the guitar, speaker and open the volume pot on the channel of your choice. 

Good luck and happy playing with your new amplifier!

PCB version

Compared to the previous pictures, the PCB has changed slightly. The placement and wiring of the parts are almost the same, but the board is slightly longer. The components are located on the board more in the direction of the mains transformer, which shortens the size of the wires going to the front panel and tube connections.

SUPPORT: Unlimited EMAIL support is included, but we cannot give support over the phone or in our store. The best way to contact us is to reply to the order confirmation email or just email to orders@uraltone.com.