ALL OCTAL SE TUBE AMP BUILD GUIDE

The UralTone All Octal SE Stereo amplifier is a traditional Single Ended stereo amplifier with one exception. It operates on a 12VDC (7A) power supply, so there is no 230VAC network input or transformer within the amplifier itself, making assembly safer for beginners. Inside the chassis, there are two boost regulator-type power supplies that elevate the incoming 12V voltage to about 300V, enabling the tubes to operate at optimal voltage and current levels.

When designing the structure, special consideration was given to beginner builders: there is very little wiring, the structure is spacious, and the components are large. Even the Switching PSU circuit is assembled with through-hole, traditional components.

Thanks to the modern power supply, the noise level is very low: with average speakers, the hum is totally inaudible.

The assembly of PSU modules is guided in a separate article, which can be found here!

  1. UralTone All Octal SE Stereo tube amp kit
    €389.00 €313.71

    The UralTone All Octal SE Stereo amplifier is a traditional Single Ended stereo amplifier with one exception: It works with a 12VDC (7A) power source. So the amplifier itself has no 230VAC mains connection or mains transformer, so assembling it is safer for beginners. Inside the chassis is two boost regulator-type power sources that boosts the incoming 12V voltage to 300V, so that the tubes operate at the optimal voltage level and currents.

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Preparations

First, ensure that all parts are included in the delivery. If anything is missing, contact us by replying to the order confirmation email or by sending a separate message to orders@uraltone.com.

Get to know the components and ensure that you recognize them and can find them on the parts list. You can sort the smaller parts on the table based on their designators (C1, C2, R1, R2, etc.).

Downloads and Links

A voltage chart for all UralTone kits can be found here:

Assembling the PCB

First, assemble the PCB. It's advisable to start with the smallest components.

Solder the resistors and other components according to the picture onto the board. Resistors do not have polarity, but it is good to install them facing the same way for readability.

Next, install the cathode capacitors. Note the polarity. The positive terminal is marked with a groove. Film capacitors (the small yellow and red ones shown in the pictures) do not have polarity.

Oil capacitors (paper in oil, or PIO) work either way, but the case is grounded through one of the terminals, so they must be installed as shown in the picture. In the image, the end whose lead is connected to the casing should be on the left.

On the other side of the board, install the tube sockets, switches, and DC connector. The tube sockets must be installed in the correct orientation. Be especially careful because a wrongly installed socket is difficult to desolder. Press the socket as close to the board as possible.

Solder the remaining components onto the board. However, do not install the LED yet; it will be installed during the cover attachment phase. The large filtering capacitors have polarity, so they should be soldered with the negative (negative side) facing the outer edge of the board. Be careful: incorrectly installed capacitors can explode upon startup.

At this stage, it's good to go through ALL the components and ensure that each one is in the right place and oriented correctly.

Power Supply Installation

Next, attach the output transformers and power supply boards to the main circuit board.

You can see the wiring for the output transformers in the circuit board image found at the beginning of the article. First, solder them in place. Cut the wires very short, but be careful not to make them too short.

If you have an adjustable power supply, it is good to test the PSU boards separately before installing them. If not, the power supplies can also be tested in the amplifier. For testing, it's advisable to attach the boards upside down. This way, you can better measure and use the switch.

First, install only one power supply onto the board and solder the cables between the boards. After this, you can test the functions of the power supply.

Turn the switches to the "on" position and place the assembly upside down on the table. Ensure that there are no component legs or anything else conductive on the table. Attach the power supply's DC jack connector, but do not yet plug the power cord into the wall. Make sure everything looks good and the power supply cables are definitely connected correctly. Ensure that the switches are set as shown in the picture and the trimmer is adjusted to the middle position.

Plug the 12VDC 7A (minimum 7A) power supply's power cord into the wall and watch for any smoke or other suspicious occurrences. Measure the voltage, for example, between the poles of capacitor C5 (which has not been installed). The voltage should be somewhere between 200-400V. Adjusting the trimmer to a lower setting should reduce the voltage.

Then install the power supply on the other side and perform the same test.

Now switch all the dip switches to the ON position and set the trimmer to the middle position. Install the tubes into their sockets and turn the power back on. Wait for the tubes to warm up for about 30-60 seconds and then measure the voltage at the terminals of capacitor C5 again. Adjust the voltage to 300VDC using the trimmers in the PSU board.

Turn off the power. Wait a moment and confirm by measuring that the voltage has dropped below 10V. Then, remove the tubes and continue with the assembly.

Front Panel Assembly

Attach the connectors and potentiometer to the front panel and solder the cables as shown in the picture.

The speaker connectors are high-quality and robust, meaning they have a lot of mass. Soldering them with hobbyist equipment can be challenging. Use the largest possible tip on your soldering iron and set the temperature to the maximum (around 450°C or similar). You can even use two different soldering irons if one is not powerful enough. Pre-tin the connector so that the solder properly adheres to the connector's soldering surface. Only after this, add the cable and more solder. You can use quite a lot of solder.

Finally, attach the front panel and circuit boards. The LED is installed at this stage to get the correct height for the LED. Solder the longest leg to the square pad. Flip the power supply boards and ensure that the transistors and diodes soldered underneath are folded as shown in the picture.

A plastic washer goes between the transformer and the board. A metal washer is placed between the screw head and the transformer bracket.

Place 15mm standoffs between the panel and the circuit board. Use 20mm standoffs between the power supply board and the main circuit board.

Solder the speaker connector wires to the board as shown in the first PCB picture (with text), place the tubes in their sockets, and voila, the device is complete except for the external casing! Adjust the feedback trimmers to the middle position. Connect the speakers and audio source to the connectors and take the first taste of how the amplifier sounds!

Measurements and Verifications

If anything suspicious happens after turning on the power, immediately shut off the power. It is convenient to use an extension cord with a switch, which serves as an emergency cut-off if needed.

Before using the amp further, it is good to measure the voltages from the test points. Voltages are measured against ground (black speaker connector).

At the TP1A1 B+ and TP1B1 B+ test points, the highest operating voltage, around 300V, should be present. Adjust the voltage between the channels to be the same using the power supply trimmers. 290-310V is an appropriate range.

The TP3A1_Cathode and TP3B1_Cathode are the test points for the cathodes of the output tubes. The voltage here should be about 12-20V, depending on the type of tube used. The voltage should be within 20% of the same between both channels. As the tubes age, the voltage difference may increase and the voltage may change. In that case, it's time to replace the tubes (actually, usually much earlier).

The feedback is adjusted according to taste, but always so that the volume level of both channels is the same. A good starting point is to connect a CD or other consistent audio source to the amplifier, or if you are using a computer or phone, adjust the volume to full or nearly full. Adjust the feedback so that the amplifier's input sensitivity is at an appropriate level, meaning the volume is about halfway in normal listening. From here, you can start adjusting the feedback by ear and compare the changes in sound quality. When you adjust the feedback such that the volume decreases, the feedback increases and the distortions decrease. However, this does not always mean better sound quality. It is a subjective experience and also depends on the synergy between the speaker and the amplifier. Trust your ears as few listen to music with the ears of meters or Facebook experts.