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A single-ended tube amplifier is a relatively simple amplifier design that uses a single vacuum tube to amplify the audio signal. These amplifiers can produce a warm and natural sound, and they are a popular choice for audiophiles and guitar players alike. This particular build involves a very simple, yet versatile and high-quality stereo amp.

This write-up provides a play-by-play rundown of building the SSE tube amp Nox Box Pro, a single-ended tube amp based on the Tubelab SSE circuit design and layout. It is important to note that this project involved working with high voltages, and if you decided to build your own tube amp, extreme caution should be taken at all times. I am not responsible for any injury and this post is not intended to serve as proper instructions for a build.

Why I built the amp

As a producer, I love to have a second pair of speakers to A/B my mixes and prefer having the richest sound possible. However, the cost constraints of a high-end single-ended stereo amplifier can be prohibitive. Falling into all these categories, I have always wanted to build my own amplifier.

I decided it was finally time when a good friend with minimal experience built his first tube amplifier and I heard how rich and defined it sounded. I have long considered myself an audiophile, but the clarity and spacial resolution that came out of his system breathed new life into songs I thought I knew quite well.

My audio requirements

I’m a biophysicist by trade and have soldered and studied my fair share of circuits. If I was going to build this myself, I wanted to cater it to my wants and needs. I had a few desires out of the amp I build.

1. Take a range of available tubes

Most amps are built around a single tube type and at a minimum I wanted an amp that would use 6L6 power tubes. I’ve had a fair share of American-stye guitar amps and have boxes of these tubes lying around. I also had a lot of 12AX7 preamp tubes. With the high prices of tubes theses days, I wanted to utilize this stockpile if possible.

2. Not a kit

I wasn’t sure I was ready for point-to-point wiring, but I knew I did not want a kit. I have already built guitar pedal kits, designed a few circuits, and repaired amps. However, I did want a reliable circuit to base the build off of so I had a reference point for testing voltages, knew the power output I could expect, and get a general idea of the sound it would produce.

3. Scalable

I wanted the option to start with cheap components and transformers, get the circuit up and running, then upgrade as I go. A quality tube amp build could cost into the thousands of dollars and I didn’t want to cough up that much cash up front. I needed a working circuit first. So scalability was essential. For this build I ordered all high-end components for the PCB board, but used a cheap enclosure, switches, terminals, etc. Once I confirmed it worked, I upgraded everything to high-end components. I ordered the majority of my components from:

• Mouser (electronic components supplier)
• Parts Express (nice variety of quality terminal plugs, switches, etc.)
• Amazon (odds and ends)
• Ebay (mostly for the tube sockets and extra tubes)
• EdcorUSA (custom build quality audio and power transformers)

4. An active community with help resources

If I got stuck on this build, I wanted to make sure that I would have some level of support somewhere. I can do my share of troubleshooting, but I did want the security of knowing other people have built this circuit.

The Tubelab SSE

After searching forums, chatting with friends, and researching all my available options, I came across the Tubelab website. This site is run by a gentleman named George who has designed a range of amplifiers and PCB boards. There is an extremely active community around his amps on the Tubelab diyAudio forum.

The Tubelab SSE takes the majority of popular power tubes – including EL34, 6L6, 6V6, and KT88’s. All without needing to be biased (although it’s always advised to bias the amp when using new tubes). George does not sell kits, just PCB boards and publishes many amp designs.

I knew this was the perfect board for the job and it was an excellent choice – I’m listening to it as I write this post. The board was of solid quality with large traces to ensure a strong connection between components. There is only one report of a board that could not be repaired. This board was sent back to George and turned out that the builder drilled holes through the traces to accomodate larger components. This is a known way to ruin connections on a PCB board and is not advised.

Getting Started

There are a range of tools and materials necessary to build a single-ended amplifier. In particular, there will be many odds and ends you don’t think of. Such as fasteners, screws, step bits for drilling out holes in the chassis, etc. Below is a short list of the common materials necessary for building this amp:

Tools and Materials:

• Soldering iron
• Solder
• Wire cutters and strippers
• Needle-nose pliers
• Multimeter
• Power transformer(s)
• Output transformer(s)
• Chassis
• Tubes
• Capacitors
• Resistors
• Power switch
• Input and output jacks
• Finishing supplies.

I. Choosing the Design

There are several different designs for single-ended tube amplifiers, and it is important to choose one that suits your needs. I spent a lot of time looking at kits, potential point-to-point circuits, and different PCB boards. I had a few needs I wanted met: First, I had a large box of perfectly good 6L6 power tubes and 12AX7 preamp tubes. I wanted a circuit compatible with these tubes to both save money and keep the types of tubes used by my various pieces of audio equipment consistent. Next, I wanted a customizable circuit was easily expandable and upgradeable.

I looked for schematics online and in books on tube amplifier design. This process took me a couple months, when I finally came across the website Tubelab.com. This site is run by George, an amazingly detailed, helpful, and brilliant tube amp circuit designer. One particular circuit, called the Tubelab Simple Single-End (SSE), was exactly what I was looking for. It is capable of running numerous types of power and preamp tubes with little to no modification of the circuit, has a very active online community with numerous forum posts and build reports, and a really detailed write up and walk through.

Step 2: Prepare the Chassis

The chassis is the metal frame that houses the components of the amplifier. Drill holes in the chassis for the tubes, jacks, and switches. The chassis shown below is the second chassis I built for this amp. On my initial build of this amp, I used a chassis that was far too large. I also damaged the top while drilling holes for the tubes.

Pro Tip: The easiest way to get these large holes in standard aluminum chassis are to use a step bit. Some also refer to them as tree bits. These can be purchased from Amazon or hardware stores. Get a range of sizes, and slowly work your way up in size. If you start a big hole with a big bit you’re going to destroy your chassis. I learned this the hard way.

Pro Tip 2: Always use a punch to mark your pilot holes. If you get a metal punch and give it a good wack with a hammer, your bit won’t dance around all over your chassis. This is true for wood as well.

Although it might seem counter intuitive to start with the chassis, you want to have a solid layout and design before building your amp. This lets you know which holes you need to drill, how long your wires should be, and how much clearance you’re working with.

I highly recommend drawing out the chassis and measuring your components before starting any build. An additional benefit to starting with the chassis is that it can take a long time to allow the finishes to cure and sanding is a pain. You can get the ball rolling on this aspect and let the coats of paint and lacquer rest while you work on the circuit itself.

After you finish the chassis you can mount the transformers and other components to the chassis, but I prefer to test all my circuits before doing a final install. I will often mount my hardware to get a gauge on what the final product will look like, but I don’t tighten anything in until everything is complete.

Step 3: Solder the Components

Once you have the circuit chosen, have ordered all your parts and supplies, and have started the chassis, it is time to solder the components. You should solder the components to the chassis according to the schematic. Use a high-wattage soldering iron and high-quality solder to ensure good connections. Take care not to overheat the components or the chassis.

Each amplifier will have a different bill of materials (BOM). I sourced the majority of my materials from Mouser. There were a few pieces I ordered from eBay and Amazon. I also had custom power and audio transformers built by Edcor USA.

Pro Tip 1: When soldering components it’s always wise to start with the resistors and other small components, then moving to the larger capacitors.

Pro Tip 2: Always use a socket for any transistors or IC circuits. This allows you to easily swap out different components if you fry something or need it for another circuit.

Step 4: Off board Wiring

This is arguable the most difficult step in building a circuit. Populating a PCB board is a bit like paint by numbers. However, the off board wiring is when you have to start figuring out +/-, organizing a birds nest of wires, and getting your soldering iron into difficult to reach places.

Step 4: Test the Voltages

Before applying power to the amplifier, use a multimeter to test the voltages across the tubes and transformers. Follow the schematic to ensure that the voltages are correct. If the voltages are not correct, double-check your wiring and component placement.

Step 5: Apply Power

Once you have verified that the voltages are correct, it is time to apply power to the amplifier. Connect the power transformer to the power switch and the input jack. Connect the output transformer to the output jack.

Step 6: Test the Amplifier

Always ensure that a proper fuse is installed prior to testing the amp. I also advise using a variac, a power source that allows you to slowly increase the AC power applied to the circuit. Another important testing tool that can be built for fairly cheap (or free if you have an extra lamp and light bulb around) is a current limiter. There are numerous layouts available on the web.

A current limiter runs the power through a high wattage lightbulb (100W+ is recommended) in series with the amplifier. If everything is running smoothly the lightbulb won’t draw any current and stay unlit or very dim. However, if there is a short in the circuit the lightbulb will draw the excess current and light up. This device protects the amp similar to a fuse, without blowing a ton of fuses during the testing phase of the build.

With power applied (preferably through a variac paired with a current limiter), turn on the amplifier and test it with a signal source. Make sure that the sound is clear and that there are no strange noises or hums. Use the multimeter to test the voltages across the tubes and transformers again to make sure that they are still correct.

Warnings:

• High voltage can be lethal. Always take extreme caution when working with high voltages. Wear rubber gloves and use one hand whenever possible.
• Double-check your wiring and component placement before applying power to the amplifier. A mistake can cause damage to the components or even start a fire.
• Make sure that the amplifier is properly grounded to prevent electric shock or noise in the signal.

Conclusion

Building a single-ended tube amplifier can be a rewarding project for audiophiles and guitar players. Follow step-by-step instructions from a reputable book or circuit designer carefully to ensure a successful build. Remember to take extreme caution when working with high voltages and always double-check your wiring and component placement. With proper testing and care, you can have a beautiful and functional amplifier that produces a warm and natural sound.