Welcome back for part 2 of our article on making your own DIY amp. (Be sure to check out Part 1 here)
In this installment we will be looking at the actual construction of the amp, from layout to power up. The warning I issued in the first article is even more important now. I am dealing with potentially lethal voltages in this project and have the required training and experience to do so safely. Please use extreme caution and do not work on live electrical equipment without proper training and safety considerations.
The work of building an amp can happen in several ways but I have established an order that works for me.
- Determine the layout
- Prepare the chassis for mounted components
- Install the chassis mounted components
- Install the transformers
- Wire the power supply components
- Wire the signal system components
The layout is something that looks very simple but requires a bit of thought. There a few cardinal rules that I recommend for a good layout. The first is to keep the power supply components away from the input and pre amp section. I typically have them at opposite ends of the chassis. The second is to orient the transformers correctly. Transformers work by magnetically coupling the primary and secondary windings through a process known as induction. There is no physical connection. A field is generated in the primary and induces a field in the secondary. That’s a very simplified explanation but for our purposes it should be enough.
The power transformer and output transformer are both working with these magnetic fields, but the power transformer field is much stronger and you don’t want it corrupting the field in the output transformer by induction. To prevent this unwanted coupling designers mount the power and output transformers at a 90 degree angle to each other. In figure 1 you can see a very typical layout with the transformers mounted at 90 degrees and located at opposite ends of the chassis. If you wanted to maintain these design goals with something more creative, you could do something like I’ve shown in figure 2.
There are other considerations when planning the layout. Location of the transformers will anchor the design and then you build around that. You want to determine where the sockets, jacks and controls will be located. You don’t want a rectifier mounted beside your pre amp. I like to think of design much like flowing water. It starts at one point and flows to the next but never doubles back. The pre amp socket comes first and on to the next stage until we get to the output tube. You should also think about the orientation of each socket because this will affect the layout of the heater wiring, high voltage and signal wiring.
Prepare the chassis
Once you’ve determined the location for the chassis mounted components you should mark the location of each and determine the size of hole that will be required. Use a bit of caution here, not all components of the same type are the same size. The sockets used in a Fender guitar amp are not the same size as those used by Marshall. I recommend that you get all the chassis mounted components before you start making holes in the chassis. At that point it’s easy to measure each component to be sure you’re drilling the right sized hole. For consistent results I like to mark the component locations and hole sizes directly on the chassis, using a fine Sharpie marker. Many chassis will come with a protective plastic film and I leave it on until I’m finished drilling the holes. Don’t worry if you draw on the chassis, a little alcohol and a paper towel will remove any marks.
There are a bunch of different ways to make holes in the chassis. The very best way to make socket holes is by using a special punch designed for metal. Greenlee makes several models, both manual and hydraulic. These specialty tools cost hundreds or thousands of dollars. If this is a hobby I’d suggest something more affordable. Switches, pots and jacks can be drilled by hand but I like to use a drill press. The sockets require holes that are much larger and for this job I use hole saws as seen in figure 3. These are not the cheap imports. They are carbide cutters designed for metal and they will last a long time, particularly if you use Aluminum for the chassis. You buy just the sizes you need. Visit a shop that specializes in tools and you should have several options to choose from. When using cutters like this I always use a drill press.
When it comes to the small holes I like to use uni-bits. These are specialized drills that handle more than 1 size of hole. They come in various sizes as seen in figure 4. You will also notice a small drill for creating pilot holes. Pilot holes act as reference marks and make everything work better. Another handy feature is that the various step sizes are usually marked directly on the bit. In figure 5 you can see that this bit will cut from 3/16” to ½” without changing your drill bit. This saves a lot of time when working on the drill press.
Here’s the chassis after I’ve completed the chassis preparation. All the holes for the components and their mounting hardware are cut and the plastic wrap has been removed. At his point I finish the chassis prep by taking a piece of metal and scraping off any sharp burrs around the holes and then wiping the entire chassis with alcohol and a paper towel to remove any oils that were left behind during manufacture. Voila, we have a chassis ready for assembly. Figure 6.
Install the components
Now that the chassis is ready to go we can install the chassis mounted components. The order of operations is flexible but I like to get all the sockets, jacks and switches installed so I can start to think about wiring the amp. I like to get it all roughed in although I may change or remove things later. In figure 7 you can see the input jack and preamp socket. They will be removed later and reinstalled with “O” rings for shock mounting.
Install the transformers
When I build, my choice is to install the transformers right after the chassis mounted components. This allows me to flip the chassis over for wiring while the weight of the transformers holds things steady. The most important things about transformer installation are the orientation of the primary and secondary wiring, protecting the transformer leads by installing rubber grommets and securing the transformers in place to prevent vibration. I like to use #10 stainless steel bolts for this power transformer and #8 stainless for the output transformer. For mounting nuts you can go a couple of ways. One is to use a nut, bolt, flat washer and lock washer, and the alternative is to use nuts with captive washers such as kepps nuts or nylock nuts. This eliminates the need for extra washers. You can see in figure 8 that I’ve used both types in this example. The transformers are oriented so that the leads are as close to their final destination as possible. I’ve marked all the sockets to clarify.
Wire the power supply components
The power supply is the first section you should wire up. In this amp the power supply is the foundation of the entire build. I like to think of it much like a human circulatory system. Voltage is generated, filtered, regulated and distributed throughout the amp. In all vacuum tube amps there are Direct Current (DC) supplies and Alternating Current (AC) supplies. The DC is the high voltage component that runs from the rectifier to all the tubes. The AC component is the 6.3 volt and often 5 volt supply that runs to the filaments of each tube and the actual input signal that runs from the input jack through the entire circuit to the output transformer. The signal circuit is variable AC that is coupled and decoupled to the high voltage DC circuit as it is amplified and moves through the circuit.
The AC heater supply is fixed and has no direct interaction with the DC circuit or the signal. In order to keep all these voltages working together smoothly we want to make sure that the DC voltage is cleanly filtered to eliminate any noise and make a clear path for the signal to travel on. The AC filament supply has to be routed in order to prevent noise from coupling with the AC signal.
In this build the DC is created using a rectifier to make pulsating DC. Then the DC passes through a filter capacitor, a choke and then another filter capacitor before it gets used. This first node is referred to as B+. That’s a left over term from the time when batteries were used to power amplifiers. After we have set the B+ node, additional nodes are created by dropping the B+ across a resistor and adding another filter cap. I have added a node for the screen supply, tone controls and the preamplifier. At each node the voltage is reduced and filtered again. In this design the B+ is in the 300 – 400 volt range and by the time we get to the pre amp it is down around 100 volts. The closer we get to the pre amp, the more important our voltage control and filtering become.
The filament supply is AC at around 6.3 volts and is used to heat the filaments inside each vacuum tube, with the exception of the rectifier tube. The rectifier I’ve used is a GZ34 and it requires 5 volts for its filament. When you spec the power transformer make sure you have all the voltage taps needed. I like rectifiers because they react more to circuit conditions than silicon diodes and under load will “sag” a bit and give the amp a more organic feel. Many companies don’t use them because they cost more or wish to avoid any sag. Since we have a socket for the rectifier, it can be removed and replaced with a diode module if we have an application where we want to minimize sag and distortion. Figure 9 shows the connection to the rectifier.
When running the heater wires it’s a good idea to run them as a twisted pair. Since there are 2 wires handling the AC one is technically high and the other low at the same time. By twisting the 2 wires together we can reduce noise as the effects of noise will be cancelled out. I typically make up a long string of twisted pair heater wires and cut them to the length required as I work through the circuit. I don’t try to minimize the wire length at this point, just get it in the ballpark in case it needs refinement later. Figure 10 shows the chassis with the primary power supply system installed. The DC for the pre amp and tone stack will be added later once I make a few boards.
Wiring the signal system components
I’ve drawn up a draft schematic to guide the wiring process. This is where the fun starts. Now that we’ve completed the foundation of the amp it’s time to install all the components that will carry the audio signal for amplification and filtering. Basically, a bunch of resistors and capacitors that will shape the sound of the amp. Now we have to figure out how to mount these parts. Most modern amps have printed circuit boards to hold the components. While this method is good for large production runs it doesn’t work so well for the home hobbyist doing a one-off project. There are a couple other methods that work very well. The oldest method is true point to point. There are no boards, the components are directly connected to each other and use small terminal strips for common connections. It’s an extremely rugged build using this method but it can be a bit daunting to beginners because the results can be confusing.
The other option is to build a component board or boards to hold the small parts and wire this sub assembly to the chassis mounted components. These boards were used on most classic amps of the 50’s and 60’s and feature a non-conductive fiber based board that has small metal eyelets embedded or vertical metal posts called turrets. There are tools available for building an eyelet board and they can also be purchased pre fitted with eyelets. For turret board construction you can purchase a board that’s pre drilled with a grid of holes, and you install the turrets where you need them for components. This is the method I chose for this project.
Since this is a unique creation I could not buy a pre made circuit card so I had to make my own. I bought a pre drilled fiberglass board, a supply of turrets and seating tool for around $20. The quality of these components is all over the place so I recommend buying them from a place that lets you see what you are buying. In this project you’ll notice that the bare board was drilled roughly and that makes it harder to install and seat the turrets.
I needed to make 3 boards. One for the remaining power supply components, the second to hold all the components for the tone controls and a third for the preamp section. These could all have been on the same board. Since I’ve made sure the chassis has lots of space I don’t have to cram everything on one board and went for a modular design. In figure 12 you can see the board, the turrets and the seating tool. You work on these things upside down unless you have a special jig to hold everything. A turret is inserted into a hole from the bottom. You take the seating tool, place it on the bottom of the turret and give it a smack with a hammer. This expands the base of the turret and locks it into the hole. It’s a good idea to do this work on a solid surface and use a piece of metal sheet to work on. Remember, I’m building this thing in my living room on the dinner table and maybe you are too.
After I created a board with rows of turrets I had to think about components and their layout. This is where a schematic is important. Have a look at figure 13, our working schematic. Once you’ve worked on a bunch of amps you’ll know what all the pins on each tube are supposed to connect to. For now I suggest that you look at the basing diagrams for the tubes you want to use. You can use a reprinted vacuum tube guide or many of the online resources available. I can’t stress the importance of doing some background reading if this is a hobby you really want to pursue.
I like to install the components in the order that the signal flows through the circuit. In figure 14 you will see how I arranged the components for the pre-amp board. Compare this picture to the schematic.
In figure 15 you will see how I arranged the components for the tone stack. Again, compare it to the schematic and you’ll get a good idea of how the circuit is flowing. You should orient all the components that require a ground connection in one direction. The grounds are very important and it’s best to localize them instead of running them all over the place. In this project all the power supply grounds are at one point, as are the grounds for the preamp and tone stack boards.
In figure 16 you can see how I located the pre amp and tone stack boards prior to wiring. Once the boards are mounted the components are connected to the chassis mounted controls and tube sockets as required.
To do this work you need to have average skills with hand tools and soldering irons. I use a 60 watt pencil type soldering iron with a variable heat setting. You don’t want to use a soldering gun to wire jacks and pots. The best use of soldering guns is for larger wires and ground connections when you need a lot of heat. All of the main ground connections in this amp design are mechanical. A bolt is attached directly to the chassis and the ground wires are connected to the bolt using ring type connectors that can be either soldered or crimp connected to the wires.
The transformers come with their own wires so they are simply cut to length and neatly bundled. When connecting the component boards to sockets and controls I like to use solid core wire or stranded wire that’s been tinned at the factory. Both of these types allow you to control the location of your wire runs and help with the overall lead dress. Moving a wire slightly can sometimes cause or cancel a hum, so when you find the right spot for the lead you want it to stay in place. Insulation is a matter of choice, as long as the insulation is rated for the voltage and current required you are good to go. Solid core wire with Teflon insulation is great for most applications and it won’t melt while you learn to solder. Cheap, stranded wire, with plastic insulation is the worst. Don’t use it unless there’s nothing else available, because you will make a mess of the project. Just sayin’.
In figure 17 you can see the completed amp in a condition that I call roughed in. All the components are installed and wired. The amp is not finished at this point because we have to commission it. Commissioning is the process of going through the designed assembly and performing tests to make sure everything went together correctly. The first tests will focus on general operation and safety. After that, listening to the amp becomes the most important test and final tuning will be done at the component level.
That’s it for now. In the final installment of this article we’ll look at TNT, or Testing & Tuning. I’ll provide some hard numbers for circuit voltages and check the power output. At the end I’ll fill in the blanks on the schematic and examine the finished product. Thanks for your attention and be sure to come back for part 3.