Workflow
Here you can find a collection of free tools and practices that I use.
Designing a tube amp may be an overwhelming task, especially if you're just starting out.
It's my first time building such a large project, but as an engineer I've learned that the best way to tackle a problem is to break it down into smaller, more manageable tasks.
This is the workflow I've come up with, and I think it's a good starting point for anyone who wants to build a tube amp.
It's presented as a list, but really it's more of a cycle, as you'll find yourself looping back to previous steps as you move from a "general overview" to a more detailed design.
Every time you loop back, you're refining your design, that means something changed. This means that you have to follow the next steps rigorously and update or review your previous work.
DON'T skip steps, or you'll regret it later.
0: Safety First
There's just one important topic that is absolutely necessary to learn (maybe after the Ohm's Law), and that's about safety.
I'll never stress this enough, but you'll be dealing with high voltages and currents, and you can get killed if you don't know what you're doing.
Be sure to follow everything you can find about the topic, even if you think of knowing it already.
Almost every source I quote in the resoure page covers safety topics appropriately, and I often get back to it.
Repetita iuvant: If you keep reviewing safety measures they will eventually become natural to you.
I'm in no position of teaching you about this subject, but here's a compendium of important things I learned (they don't replace proper training):
- NEVER handle a circuit without unplugging it first: Even when you think no current is flowing, there may be some residual charge in the capacitors.
- DON'T TRUST SWITCHES: I want to re-iterate on this. Don't trust on a switch being off to work on a circuit. If you live in EU and you're using a switched power strip, there's a good chance that only one of the two poles is switched.
This means that the other wire may be still live (depending on which way you plug it in the wall socket), and you'd get a nasty surprise if you're not isulated from ground.
- ALWAYS discharge the filter/reservoir capacitors before touching anything: You can do this by shorting the terminals with an appropriate high wattage resistor (don't short them directly or they will make a spark and possibly get damaged from the current surge / heat).
- BUILD yourself a current limiting device (see Uncle Doug's channel): This is a device that limits the current that can flow through your circuit, so that if you make a mistake you probably won't get killed.
While this doesn't allow you to touch anything, it has the extra advantage of somewhat protecting your components from damage, or at least from exploding with pyrotechnic effects that could scare you and make you touch something you shouldn't.
- Be aware that some devices like Variacs or toroidal transformers are not galvanically isolated, so basically all the (virtually infinite) power coming from the wall is connected to their output.
- I like to be extra safe in my designs, and I usually use double-switches and double fuses. Moreover, reservoir/filter caps are always in parallel with a discharge resistor to let them dissipate energy then powered off. Again, never trust them.
- If you want to be extra safe, use safety gloves when unsure. Always measure voltages before touching something.
- DON'T work on high voltage circuits when tired or distracted. If someone's talking to you, put down your tools and listen. Mistakes happen often when you're not looking or paying attention.
- DON'T work on high voltage circuits when you're alone. If you get shocked, you'll need someone to help you. Instruct them not to touch your body should an event occour, just to switch off the safety switch.
- Don't work near pets (or worse, children). They can't know the dangers.
- Always check what you're probing. Oscilloscopes also have their own ground path, and they become part of your circuit. Don't try probing rectifiers if you don't know what you're doing.
- Remember that also multimeters and can store charge, you can fry them or get shocked if used improperly.
- Bonus rule: NEVER open a microwave oven.
From now on, this is your mantra
- SWITCH OFF the amp
- UNPLUG the power cord
- DISCHARGE the capacitors
- CHECK mains voltages to be 0
- DO YOUR STUFF
- DOUBLE CHECK
- APPLY probes
- STEP AWAY.
- SWITCH ON
- (Wait for smoke)
- READ MEASUREMENTS, don't touch anything.
- REPEAT
I will not be responsible for any damage or injury you may incur while building your amp.
1: Requirements
Everything you do should be motivated. If you're building for yourself, it can relly be any reason, even a simple "I think it's a cool feature", but it must provide an outcome, either in terms of functionality, aesthetics, or quality.
I think it's important to keep a list of requirements, so you can have a clear idea of what you want to achieve and review it later.
Keep in mind that's it's not written in stone (as long as you don't prefer to do so), and you can always change it later on. During the design of my amp I changed my mind a lot and converged to something that's both useful and (maybe) achievable.
First Time
At the beginning It would be nice to set first a general overview of the kind of amp you want to build.
Ask youreself:
- What kind of tone is required for the music I wanna play with it?
- Will it be used for recording, live performances, or both?
- How many channels do I need?
- Till it be an amp in a box, a preamp, a combo, or a head?
- What usability features do I use most? (Effect loops, channel switching, etc.)
For example, I answered these question as:
- Almost all my favourite guitarists use a Mesa MkIIC+ or similar, and that's "my sound" too. I'd like also a medium-gain, "crunchy" tone for playing blues and rock.
- I need a medium power amp that's suitable for rehearsals and small gigs, as I already have a 400W rack amp for bigger venues.
- The quad could switch between 4 (+1) tones, but they are too interactive and sounds levels are a pain to adjust. I'd be happy with a clean, a medium gain and a high gain channel with independent controls. I'd also like to have plenty of tone shaping capabilities because I like to suffer.
- If I can afford to carry around a big refrigerator, I'd carry my rack. I want a good compromise between portability and usability. It will be a head, plus a separate 1x12" cabinet that I can leave home when not needed.
- I'd like it to be midi-switchable, with a parallel effects loop for just a delay and a reverb pedal.
You can find the long story on the requirements page.
Looping back
As you move on, you'll find yourself looping back to this step. When you have a new idea, something so cool you can't possibly leave it out, you'll have to ask yourself:
- Imagine yourself using your new amp. Can you REALLY figure out a situation in which you'll find the feature useful?
- What advantages does it bring?
- Does it conflicts with other requirements?
- Is the added complexity worth it?
- (If you have at least a physical design or prototype already) Would it FIT with the current design?
Don't just do this for your newest idea, do it for all the requirements.
This is the best way to make sure that you'll be satisfied with the final result, because all the features will be consistent with each other and you're only keeping those that you've found to be really useful consistently over time.
When you have made up your mind, you can move on to the next step.
2: Study, Study, Study.
Once you have a clear idea of what you want to achieve, it's time to do some research.
First Time
I found some resources and started to understand how a Tube Amp is made. Being driven from the desire to build your dream amp is a great incetive to learn.
Learning is a great way to find new ideas and possibly new requirements. For example, Merlin's book is full of tips and tricks that can help you introduce some features that you didn't think about before.
It also works the other way around: you may find that some features you thought were cool are actually not that useful, for example I got convinced that It's not worth having a standby switch in my design (more on this on Merlin's book).
This is the good time to get back to the requirements often and update them as much as you can, since it's virtually free to do so (you've spent the less amount of money and time possible).
They say to never stop learning. While it's true, I think that at some point you have to stop and start doing something. I've found myself often stuck in the "learning" phase, while you could also learn by doing and checking your results in a book.
Looping back
Just make sure you understand fully what you're trying to build in this iteration. Find schematics or articles from those that have built it before you, or just review the necessary theory and get to the next step.
3: Electrical Design and Simulation
This is a very important step, that will give you a lot of insight on how your amp will behave. You should always have an updated simulation of your amplifier or at least its modules.
Now, keep in mind that a simulation is just... a simulation. It means that It won't necessarily match reality, nor you can find accurate models for each components you're going to use, but they will give you a rough estimation.
Here the main tool I use is LTSpice, a free circuit simulator that's very powerful and easy to use. Be sure you understand how to use it, in particular it's useful to understand how to:
- How to run transient (time domain) and AC (frequency domain) simulation. Keep in mind that time domain simulation can be very slow or even wrong if your circuit is very complex or heavily non-linear, while frequency domain simulation often
does not take into account limitations of real components (like the maximum voltage swing of a tube and the subsequent distortion).
- How to use the .step directive to sweep a parameter and plot the results, to make rational decisions about your components.
- How to create a subcircuit and use it in your design. This is very useful to keep your schematics clean and readable and to "unit test" your circuits with different inputs and loads.
- How to import a SPICE model and use it in your design. This is useful to simulate real components, like tubes or transistors.
First Time
Always build small circuits as soon as you learn a new concept. This helps in understanding how each component affects the result. If you have plenty of time, you could also take real amp schematics and simulate them to see if they make sense.
Since my amp is based on the Mk II C+, I started by simulating the preamp section. This helped me in understanding how the different stages work and how they interact with each other. However, this is difficult to manage, so I switched to a modular design using subcircuits.
This way I could simulate each stage separately and then put them together.
Once you have a rough general scheme, you can start to refine it adding functionalities or considering alternatives (such as different tone stacks).
Looping back
Often in the component selection you may find that you have to change your design to accomodate for the limitations of the components (or the availability of them). This is the time to update your schematics and simulation and check that your new component is suitable for your amp.
4: Modules Definition and general Layout
From this point on, It's a good idea to split your design in modules and proceed with the following steps for each of them.
In general, I like to design on reverse from the signal path, that is I start from the power supply, then I add the power amp, and then up to the input stage.
You should keep a very approximate layout of how the amplifier will look like (both seen from the outside and inside). It doesn't have to be accurate, this will come later.
This involves starting to imagine how the final layout will be. Draw a sketch by of a chassis with the transformers and the tubes you need, following the signal path,
then add the input/output/controls where you want them to be and try to imagine how the wiring will be.
For example, in my design I identified the following modules:
- Preamp
- Tone Stacks
- Power Amp
- Power Supply
- Power Scaling and Bias Tracking circuit
- MIDI, Display and Channel Switching circuit
It's also the moment to start thinking about the two most important things in a tube amp (and the first two thing you will build): The ground scheme, and the heather supply. They will more or less dictate your layout and where you can place various components.
As ground scheme I try to follow the "multiple star" topology, that is each stage ground is connected directly to their filter cap ground, and then all the filter cap grounds are daisy-chained up to the reservoir cap near the rectifier.
The general rule is to avoid ground loops, that is every component has one and only one path to the capacitor that is providing current. This involves never using the chassis as ground, except for two points, the main ground (placed near the IEC socket) and the audio ground placed near the input jack.
For heaters, you can search for good heater layouts on the internet to see how they should look. The rule here is to keep them away from the signal path, since they carry a lot of current (and hum).
Defining a layout isn't the simplest of tasks, but a wrong layout can be the source of a lot of problems and loss of time and money, so it's better to spend some time on it.
The general rules that I've collected are:
- High AC current is nasty, so keep it away from the signal path.
- Keep the signal path as short as possible, especially in high-gain amps.
- Input tube is the most sensitive to noise, so keep it away from the power supply and the output tubes. I'm actually unsure if it can live near the output transformer, but keep it away from the main outputs.
- Transformers generate a lot of magnetic fields. You should keep it away from input tubes and sensitive components. Also, they output transformers should be kept away from the power transformer and at a 90° angle to reduce interaction as much as possible.
- Rectifier, Power Transformer and reservoir caps provide huge spikes of currents, so they should be kept as close as possible to each other and as away as possible from the signal path.
- Filter caps can be kept near the corresponding stage, as they are the ones providing current to it and provide some degree of shielding between stages.
The best way is to search for "amp guts" images for your favourite amps and see how they are laid out (and try to understand why).
5: Component Selection and Electrical Layout
This is the time to select the components you're going to use, starting from the schematics in LTSpice. This will be the most tedious task, but also the one that's most important for the final design.
The first thing to define your building technique (point-to-point, turret board, PCB, etc.) for every module of your amp. Every building technique has its own advantages and disadvantages, there're plenty of resources for it on the internet.
If you're building a simpler amp with a less features, you can go for a point-to-point or turret board for the whole amplifier. This has the advantage of being very easy to work with, and if laid out well it can be also very silent.
In general, point-to-point is optimum for keeping signal paths short and clean while allowing to experiment and modify the circuit on the fly. However, it can get messy very quickly.
Turret board forces you to keep your components in a grid, which is good for keeping things tidy, but it doesn't scale up well for complex circuits (turret boards are expensive and somewhat hard to find and build) and they force you to use mainly axial components.
Perforated boards are a good compromise between the two, they are cheap and they can be modified fairly easily, they can fit almost any component in a small space, due to this they require careful planning and layout.
PCBs are the best choice for complex circuits, they are cheap and easy to build (there are plenty of services that can make them for you), but once built you cannot modify them easily (you can at most replace components with similar ones).
If you plan to use mainly point-to-point and turret board, you can use DIY Layout Creator to design your layout. It's a free software that allows you to draw your layout easily before building them.
However, adding new components is quite difficult and it doesn't provide many complex features, so you better use it to plan your layout and experiment directly on the actual circuit.
I personally chose to use a mix of perforated boards and PCBs, since my amp is quite complex and I want to keep it tidy.
For this task, I use KiCad, a PCB design software.
It's not extremely practical to use compared to modern commercial software, but it's free and it's very powerful.
Its only big limitation is that while it allows you to draw a hierarchical schematics, it doesn't allow to do so have multiple PCB layouts in one project, so the best approach is to use a project for each different module.
The first thing to do is to create a new project for your module, then you have to replicate your schematics in the Schematics Editor. You can also have a spreadsheet to keep track of the components, quantities and costs.
For each component, you have to:
- Look for a suitable component in one of the major distributors (Mouser, DigiKey, Farnell, RS-Online, etc.). If the required component is not available, take the closest value and get back to the simulation step.
- If the distributor provides a CAD and Footprint, download them and add them to your KiCad Library (make a dedicated lib for your amp for portability). Remember to add the footprint in the footprint editor and also the 3D model, if available (set the .stp file in the footprint property window).
- If the distributor doesn't provide a CAD and Footprint, you have to make them yourself. This is a very tedious task, but it's worth it.
- Read the datasheet of the component, as many of them (like voltage regulators or op-amps) requires some external components to work properly. Also include them into the LTSpice schematics to avoid forgetting them between iterations.
- If you're not happy with how the component model looks like in the schematics, you can edit it in the symbol editor.
- Place the component and connect it to the rest of the circuit. Keep the schematics tidy and similar to how it looks on LTSpice.
- In general, leave the grid size as default as if you do it mid-way, components won't snap to the grid anymore.
You now have a complete schematics of your module, with all the components you need. This will be your main reference, so complement it with all the necessary information. You can also print it on paper for reference when building the actual circuit.
Now, give a good naming convention to all your components and make sure everyone has a unique name and a footprint. You can then click on the "Generate Netlist" button to generate a netlist of your circuit.
Now you can move on to the PCB Editor. It can be used either to design a PCB or a perfboard layout (by changing the grid size). Here you have simply to import the netlist you've just generated and place the components on the board.
You also have to draw solder tracers. Try to follow common some PCB layout guidelines:
- Keep power lines away from signal path.
- Select a good trace size for the current that will flow through it.
- Place terminals considering the general layout of the amp.
- Consider that components also have a height
- I use star grounding rather than ground planes, as I read many sources having problems with them, but I can't provide any evidence.
These are obviously true also for perfboards, moreover I'm not an expert in PCB design, so I suggest you to look for more information on the internet.
Just make sure they are targeted to audio stuff, as audio brings a whole lot of issues that often conflicts with requirements for other kinds of circuits.
Phew. Now here comes the fun part.
6: CAD Design
Ok, this may sound a bit overkill and not fun at all. If you live in an apartment like me and don't have the time to reach your workshop every day, you'll want to have a good idea of how your amp will look like before building it.
This is also a good way to check if your layout is good and if everything fits.
I tried to use FreeCAD for this task, but I found it to be too complex and not very intuitive. I then discovered SolidEdge, which is a free CAD software for students and hobbyists.
I just have to say that it's a very powerful software, and it's very easy to use. I made in 10 minutes the same 3D model that took me 2 hours in FreeCAD, and I'm no CAD expert.
The coolest part is that you can use it to model your chassis and components, and them assembly them together (using amazing tools that are super-intuitive).
All the work you've done in the previous steps will come in handy here, as you can use the 3D view of your components to place them in the chassis and see if they fit.
Moreover, you can use them to place the holes for the components in the chassis, so virtually you can have your custom chassis made for you by a metalworker.
I will expand this section in the future, as I'm still working on it.
7: Build
This is the moment you've been waiting for. You have all the information you need to build your amp. If you also have a CAD, you'll be also quite confident on how it will look like.
I suggest you to refrain from taking a chassis and start drilling holes in it. First build the modules that you're sure about, when you have tested them and you're sure they work, you can start preparing the chassis for placement.
As soon you have a chassis with holes, you can start laying out the things that you're sure about, starting from the IEC inlet, safety earth ground, power switch, the transformers, the tube sockets and finally layout the heater wires.
Then you can start mount the boards and wire them together.
I will make some posts about the building process, since I'm still in the design phase.