How To Build a Tube Amplifier
By Rob Robinette, edited 5/10/2017
Have comments or corrections? Email rob at: robinette at comcast dot net
This webpage is available in PDF form: How To Build a Tube Amplifier.pdf
For a first amplifier build you should consider a kit because it can be difficult to source all the correct components and shipping charges from multiple vendors can add up. Building a kit is also a great way to boost your understanding of electronic theory and tube amplification. I recommend Mojotone.com and BootHillAmps.com for complete or partial tube amp kits.
There's no better guitar amp to start with than the Fender 5F1 Champ. It has only 26 components but sounds fantastic. The circuit is so simple I use it in my How Tube Amps Work webpage. It is a single-ended amp with one power tube.
5F1 Champ Chassis
Control on top, Circuit Board inside, tubes on bottom: V1 Preamp Tube on right, V2 Power Tube in center, V3 Rectifier Tube on left. The Power Transformer and Output Transformer are attached to the other side of the chassis.
Another option for someone that has some electronic experience is the wonderful Fender 5E3 Deluxe amplifier. It has two power tubes in a push-pull configuration and is loud enough for many gigs.
I went from being a lousy solderer and having a casual knowledge of electronics (I couldn't even calculate the value of 3 parallel resistors) and having no idea how tubes worked to really understanding electronic amplification and tube theory and my soldering skills are now top notch after building an amp or two. Researching that first build motivated my learning and I have really enjoyed the entire process--and then of course there's the end result--having a nice self-built tube guitar amplifier to play through.
Opening a tube amp kit and seeing all those little parts can be overwhelming so I recommend you do an inventory of the big parts like transformers, chassis, switches, tube sockets, etc., then immediately begin assembling the circuit board. Getting all those little resistors and caps in place on the board will make your amp project seem much more manageable and it will help you identify missing components or circuit board layout problems. It sucks to come close to completing an amp and then realize you're short a capacitor and have to wait for delivery.
I also recommend you study factory built examples of the amp you plan to build so you know what your amp should look like. Also go to the tdpri.com DIY Amplifier Forum and look at similar amp builds.
WARNING: A tube amplifier chassis contains lethal high voltage even when unplugged--sometimes over 700 volts AC and 500 volts DC. If you have not been trained to work with high voltage then have an amp technician service your amp. Never touch the amplifier chassis with one hand while probing with the other hand because a lethal shock can run between your arms through your heart. Use just one hand when working on a powered amp. Learn how to drain the filter capacitors. See more tube amplifier safety info here.
This is the General Amplifier Build Sequence I Recommend
Assemble the circuit board with longer than needed lead wires.
Install the tube sockets and pilot light into the chassis. Match the tube socket pin orientation to your layout diagram.
Install the power transformer into the chassis. Use grommets in the chassis holes where wires pass through.
Wire the heater circuit.
Install pots, fuse, power, standby and ground switches and as much of their wiring as possible.
Install the output transformer and choke.
Install the power cord.
Install the circuit board.
Trim the circuit board wires to length and solder their connections.
Complete the Amplifier Startup Procedure.
Build or buy a cabinet.
Mount the speaker into the cab.
Finish the amp, install faceplate, knobs and insert the chassis into the cab.
Building an Amplifier
You need a good quality soldering iron with a clean, pre-tinned tip to successfully solder eyelets and turrets. Frequent tip steam cleanings using a damp sponge will make soldering easy and keep your solder joints looking good. For high quality solder joints you must use rosin flux on the eyelet/turret and the component leads because it cleans and removes oxidation so the solder can adhere for a good, long term connection. I just dab a little flux on the joint using a thin artist paint brush or you can use a refillable flux pen. You want good solder joints because chasing a cold solder joint can be a colossal pain in the butt. I'm a fan of the $99 Hakko FX888D soldering iron. Weller soldering irons are also well liked.
I have run into turrets that still have some sort of lubrication on them that can make soldering very difficult. If using flux on a turret doesn't make it easy to solder then you may need to wipe them down with rubbing alcohol to get them clean. When soldering turrets keep in mind they are pretty good heat sinks so you need to use a larger iron tip and preheat the turrets. I use a big fat chisel tip with turrets. Just apply iron heat to the turret for a second or two before making contact with the component leads. You need the keep the iron tip in contact with the turret and all the component leads to get a good, shiny solder joint. This will keep you from frying components as you wait for the turret to get hot enough to bond with the solder. Your iron's tip size & temperature should allow a good joint with 2 seconds or less contact with component leads.
Speaking of frying components, it's a good idea to use heat sink clamps on component leads--especially capacitor leads--to protect the component during soldering. Even just an alligator clip on the component lead between the soldering iron and component body will protect it from over heating. This is a pretty good soldering training video. Part 2 is about soldering turrets.
I like to use 60/40 .031" solder for most amp soldering.
Start your build with the circuit board itself, either eyelet, turret, B9A development board or even perf board. If you are making your own circuit boards I recommend you use #3 Eyelets or Turrets designed for 1/8" (0.125") thick circuit board. Making your own eyelet or turret board is surprisingly easy with the right board making tools.
I like to measure and record the resistance value of all resistors just before I place them on the circuit board. It can be impossible to measure some resistors once they are in circuit and it's a very common mistake to install a 470K resistor instead of the required 470 ohm. Measuring the resistors just before installation can save you from hours of frustrated troubleshooting. Also, knowing the exact resistance value can come in handy when measuring bias and troubleshooting.
Install the rest of the components to populate the board but don't start soldering until the components and lead wires are all in place and you're sure they are placed correctly. If you have access to an oscilloscope you can easily determine the outside foil lead and orient your non-polarized capacitors for least noise.
I recommend securing the big filter capacitors so they don't hang by their leads. You can use a mild glue (remember, someone will need to change these out someday) or drill two holes in the circuit board and wrap a big tie-wrap around the board and caps. Put the tie-wrap on before soldering the leads to keep from stressing the leads.
Ensure the negative terminals of the big electrolytic filter capacitors are connected to ground because they may explode and damage the output transformer if installed backwards. The exception to this rule are the filter capacitors in a fixed bias circuit. Since the bias circuit is dealing with negative voltage the filter caps are connected with their + terminals to ground. Most filter caps have an 'indention' on their positive end and arrows pointing to the negative end.
Nice straight component leads look good but they stress the component during expansion and contraction every time the amplifier warms and cools during a power cycle. Adding some bend to your component leads is the way the military and NASA demanded their circuit boards be built.
Mil Spec Turret Board Strain Relief
You need some bend in your component leads to allow for expansion and contraction during amplifier heat cycles.
If using turrets you should suspend your resistors above the board to aid in cooling.
You may need to install the circuit board hold-down bolts before covering them up with components.
Remember to leave the turret top holes available for leads that run from the turret to the tubes, controls and jacks. You can connect lead wires to the bottom turret hole for a cleaner look but be sure and use a circuit board bottom cover which will hold leads in place. With no bottom cover your leads may fall out when you solder components to the top of the turret.
The exception to the rule above is for components you know will have to be replaced in the future, like electrolytic filter capacitors, put their leads in the turret top hole to make unsoldering them easier. If you think you may mod your amp then place components you think you may swap out with their leads in the top turret holes to make replacement easier.
To attach component leads to turrets I use the "3/4 wrap around the turret" method which is old mil spec. Leave about 1/4 inch lead sticking out from the wrap so you can grab the lead with a pair of needle nose pliers to facilitate removal for a future repair.
22 or 24 gauge wire is fine for all the amp's wiring except maybe the heater wires. For heater wires even 22 gauge is plenty big but many people like to use 20 or 18 gauge but the thicker wire, the more difficult it is to fit it in the little tube socket pin holes.
For wire color I like to use yellow for the signal path, red for power, black for ground and cathodes and green for heaters. If two signal wires are next to one another I sometimes use orange wire to differentiate them.
With the circuit board fully populated but not soldered it's a good time to post a pic of the circuit board (front and back) on a guitar amp forum for others to review. I like the TDPRI Shock Brothers DIY Amps forum (I'm robrob). Be sure and take a photo of the back side of the board so you can review it in case you have to troubleshoot the amp at startup--you don't want to have to remove the circuit board to verify its backside wiring and solder joints.
Once you are sure your board is right insert the wires that run from the board to the tube sockets and pots with a couple of extra inches of length and solder everything together. When you're finished take a very close look at every solder joint--it's common for people to miss a joint or two and leave them unsoldered. If you have some dull or nasty looking joints use a solder sucker or solder wick to remove the solder. Put some flux on the joint and leads and resolder the joint.
If you make your own chassis I recommend using a step drill bit to drill the tube socket, jack and pot holes.
Mount the tube sockets and pilot light in the chassis and wire the heater circuit before installing the circuit board. If you plan to do Fender blackface style flying heater wires then save the heater wiring for last. I recommend you connect the 6.3V filament heater wires from the power transformer directly to the first power tube, then split two wires from there: a thin set of wires to the pilot light and one to the next tube. Try to keep the tightly twisted heater wires down against the chassis floor to reduce noise and hum (unless you're using flying heater wires).
Wire the heaters. I like to use a black and a green heater wire to help with keeping track of phase between the tube sockets. In push-pull amps the power tubes should be wired in phase for hum cancelation (black heater wire to both sockets' pin 2, green wire to both pin 7). For power tubes wired in parallel the heaters should be out of phase. I like to wire my preamp tubes out of phase to help minimize hum but this is not as critical as the power tubes. Many builders keep all the tubes in phase. Phase does not matter to the pilot light.
Install pots, fuse, power & standby switches, transformers and choke in the chassis and do as much wiring on these as you can before you mount the circuit board because it can get crowded in the chassis, especially Fender tweed chassis. It's a good idea to use washers on your output transformer and choke bolts to reinforce their mounting flanges. Their weight can cause their mount holes to rip if the amp is dropped. Don't forget to do the heater wires before you install the circuit board (unless doing flying heaters) and begin hooking up the other tube leads.
Install the power cord. The black hot wire usually connects to the fuse's center terminal. The white neutral wire connects to a power transformer primary wire. The green safety ground wire is bolted directly to the chassis using a star washer for a good electrical and mechanical connection. The safety ground wire needs to have more slack in it than the other power cord wires so if the cord is yanked out of the chassis the ground wire is the last to come loose.
USA Power Cord Wiring
Modern U.S. wall cords and sockets have a narrow blade for Hot (black wire 120v), a wide blade for Neutral (white wire ground), and a round or 'D' shaped prong for the chassis Safety Ground (green wire ground--the Neutral and Safety Ground are connected together at your home's main circuit breaker box). Power cord wire colors are sometimes non-standard so use a multimeter in continuity mode to identify the Hot, Neutral and Ground wires.
Bolt the circuit board into the chassis but make sure you have the transformers and choke installed and bolted down before the circuit board covers them up. Trim all the leads to length and solder them to the tube sockets and pots. You don't want the leads too short because you need to push them down to hug the floor of the chassis. You also may have to move them around for proper lead dress--lead separation and crossing them at 90 degrees when possible.
Pay particular attention to your preamp tube plate and grid wires. Keep them separated because they can interact with one another and cause hum and oscillation.
If you are doing flying heater wires install them last.
Post detailed pics for forum review before applying power.
If your amp has adjustable bias set the bias pot for max resistance for a cold, safe bias.
Don't insert the tubes until you power it up without tubes to check the power transformer output voltages.
I recommend you follow my Amp Startup Procedure. Following it can prevent damage from a miss-wired amp.
If you have a light bulb current limiter you should use it for the startup.
After startup remove the light bulb limiter and measure and set the bias. If your amp is cathode biased and the bias is measured above about 110% max dissipation you can install a larger value power tube cathode resistor to cool the bias.
Play the amp and sample the clean and overdrive tone. Try it with your usual pedals to make sure it behaves.
If your amp has problems see the Tube Amp Troubleshooting Guide.
Measure and record the following amp voltages for troubleshooting and future reference:
Wall Voltage AC because it affects all the amp's voltages
Power Transformer Output in volts AC
Heater Voltage should be between 5.7 to 6.9 volts AC, see this to lower the heater voltage.
Rectifier Heater Voltage should be between 4.5 and 5.5 volts AC.
These are the only AC measurements, everything below is DC
B+1 Measured at the first filter cap
B+2 Measured at the second filter cap
B+3 Measured at the third filter cap
B+4 Measured at the fourth filter cap (if applicable)
All the tube pin voltages (remember the heater pins are AC)
Finish the Amp
Install the faceplate, knobs, etc. Put the speaker in the cab, wire the speaker, then insert the chassis into the cab.
Don't forget to connect the speaker--you can damage a tube amp if you power it up with no speaker connected.
This is a reproduction of a standard tweed era tube chart that Fender glued to the inside of their amps:
Click the image to download the editable 5E3_Tube_Chart.doc
Here's a good "glue paper to wood" how to.
Placing your non-polarized capacitors with their outer foil connected to ground or the circuit's lowest impedance can reduce the amount of RFI (radio frequency interference) they pick up. If you have an oscilloscope it takes just a few seconds to determine which lead is the outer foil on non-polarized caps (caps with no + or - end markings). Some caps come marked with the supposed outer foil end but many times the marking is wrong so I recommend testing all your caps before installation. Doing this really can reduce RFI noise in an amplifier circuit.
Determine a Cap's Outer Foil Lead
Set the oscilloscope to its lowest millivolt setting (usually 2 to 5 millivolts) and 5 milliseconds
Clip the oscilloscope probe to one capacitor lead and the ground clip to the other.
Hold the cap between your fingers making sure not to touch either lead. Your body acts as an antenna to pick up RFI noise which is transmitted through your fingers to the cap.
Look at the scope and note the size of the waveform. The waveform is the noise transferred from your fingers into the cap.
Reverse the probe connections and repeat.
When the scope probe ground clip is connected to the outer foil lead the waveform size will be smallest. The smaller waveform represents less RFI noise getting into the cap.
Mark the ground probe end of the cap and connect it to the lowest impedance in the amp circuit.
Cap Hooked Up Backwards
Larger waveform so black ground alligator clip is on wrong (inner foil) lead.
Cap Hooked Up Right
Smaller waveform so ground clip is on outer foil lead. Mark that end of the cap. The oscilloscope is set to 2 millivolts and 5 milliseconds.
Cap Outer Foil Low Impedance Orientation
Note the "O" on the yellow coupling caps at mid right on the circuit board. That is the outer foil lead mark. Connect the outer foil lead to the lowest impedance which is usually the tube plate or ground connection. Most of the layouts on this website have cap outer foil markings like this 5E3 Deluxe layout.
This webpage is available in PDF form: How To Build a Tube Amplifier.pdf
By Rob Robinette