how to build a guitar amp

Tube amps sounds best when at full volume*. I have a nice 100 watt Mesa Boogie Dual Rectifier that, in a small room, will very quickly cause permanent hearing loss at full volume. I wanted an very low wattage amp that I could crank right up to "10" and still be at a nice easy volume to practice at. 5 watts sounded about right so I looked up some schematics for classic 5W amps. I had all the Fender schematics in Dave Funk's Tube Amp Workbook. This is a great book for learning about tube amp electronics in general. Also found a nice schematic for a Gibson GA-5. I decided to base my amp off the GA-5 simply because the schematic was easier to understand. I did use the Fender Champ schematics to clarify certain things (like pin numbers for the tubes).

If you're reading this because you're interested in building a guitar amp recognize that I have relatively little experience with tube electronics. I don't really know what I'm doing. If a more experienced person recommends something counter to something I've written here - I'd listen to said other person. However, my amp did work flawlessly the first time I plugged it in so I did something right.

* - why do tube amps sound best at full volume? Because the rectifier tube will saturate and then start to "sag". This adds a human-voice-like expressiveness to the sound. Oh, and the overdriven audio tubes will generate second, fourth and sixth order harmonics. This is why many guitar players believe that, for guitar amplification, tube amps sound better than solid-state amps.

Most of the electronics components (resistors, capacitors, transformers) I got from local electronics stores. The tubes I ordered from thetubestore.com. Don't forget to order the tube sockets. The chassis, eyelets, turrents and circuit board material I got from wattstubeaudio.com. In hindsight I should have ordered isolated input jacks from Watts as well. I didn't splurge on expensive transformers or tubes, I used Hammond transformers and Tung-Sol and Sovtek tubes. I wanted to hear the amp with ordinary components before deciding to spend the big bucks. No use blowing up an expensive transformer with a rookie mistake either. I used ordinary solid-core wire but I really want to try the Teflon-covered stuff sometime.

Parts list:

• chassis
• fuse holder
• power on/off indicator light
• hammond 1750C transformer (output)
• hammond 290AX transfomer (power)
• 2 1/4" input jacks (isolated preferred)
• tall bolts (from local hardware store) to mount the circuit boards off of the chassis
• the resistors and capacitors as prescribed by the GA-5 schematic. used electrolytic caps for all but the 0.02 MFD C2 and C3.
• Sovtek 5Y3, Tong-Sol 6V6GT and Ei 12AX7 tubes
• tube sockets and a tube shield for the 12AX7

I want to get 290BX so I can have 4, 8 and 16 ohm speaker connectors but it was sold out. I should have left more room for this larger transformer.

Total price including all the shipping costs: $385.

A surprising amount of time and effort goes into getting the chassis ready and all the parts mounted. I had ordered the 9x6.5x2.5 project box from Watts but they shipped me the 12.5x6.5x2.5 box. This was great, it would have been really tight with the smaller one.

Important! Before you start cutting any holes in your chassis think about the following things:

• Orientation of the power supply transformer has an effect on noise and hum. You need to get it oriented the right way.
• The hole for the three-prong electrical plug (NEVER use a two prong plug) should be close to the power supply transformer. As should the power switch, fuse holder and power light.
• The guitar input jack and volume control (and other stuff in the audio path) should be far away from the power supply stuff.
• What star grounding is. Read this article.
• Which capacitors are your "filter caps".
• What the pin numbers are for the tubes and which components connect to which pins.
• Which wires you'll want to twist together to reduce EM interference.

I use a Dremel and a drill press to cut holes in the chassis. If you don't have a drill press, borrow, buy or steal one. Drilling holes without one is torturous compared to drilling holes with one. Your next best friend is a "Unibit" (or equivalent). Use the Dremel with a metal cutting disk to cut out the square holes for the transformer(s) and three-prong-plug outlet. Eventually your chassis should look something like this:

Dont' forget to drill a hole for your star-grounding lug.

Now is a good time to layout the eyelet board for the primary circuitry. I did this on paper first.

Don't follow my layout, I didn't think about filter cap placement as well as I should have.

Use your drill press to drill holes in the nice 1/8" thick circuit board material. I used two boards - one that I installed turrents on that served to interface with the transformer leads. And one that I installed eyelets on to hold all the resistors and capacitors. I used the staker and a hammer to mount the eyelets, which worked just fine. The same technique didn't work well with the turrents, they're a bit loose still. Perhaps I should have ordered the drill press mountable staking tool base.

Now mount all the jacks, switches, boards, transformers, tube sockets, etc.

Time to print out your schematic(s), collect all your parts and plug in your soldering iron. It's very important to relax here. You don't want to make a wiring mistake so you want to get yourself in a very calm and collected state of mind. Go for a walk, meditate on your eternal soul, or whatever.

If you're anal-retentive like me you'll use your multimeter to check the values of all your resistors and capacitors to make sure they're OK.

The first thing I'd do is deal with the power supply. Cut the wires from the power transformers to a short, clean length and solder them to the power supply terminals/turrets board. Run the wires from the three-prong plug, twisting the hot and neutral together. Ground pin goes to your star ground lug. Proper way to wire the fuse holder in a three-prong situation is on the HOT lead.

Now you can place your resistors and capacitors on your audio circuit board. Run wires to the appropriate places. Remember to run your filter cap ground leads to a star ground and your audio-path grounds to a separate star ground, and then connect each star to the chassis star ground. You can see the stars I made in the bottom middle of this picture:

I didn't learn about star grounding until I was almost finished so I didn't do it in the nicest or fail-proof way. You can do better.

Triple check every connection in your schematic against your circuitry. I also checked the ohms of EVERY connection. Anything over a few ohms I resoldered.

Remember that turning it on and finding problems is not optimal - you a transformer might blow, a capacitor might explode, something could light on fire, you might blow the fuse (you bought lots right?), you might blow your speaker and you might put 700V to your guitar strings. Just plugging the amp into the wall for the first time, with the power switch off, should be done with care. I usually like to look at the circuitry when plugging it in so, in the case of exploding caps, I wear safety glasses when doing so. Here's my checklist:

1. Make sure the power switch is off, and nothing is actually connected to the amp (guitar or speakers).
2. Connect the power cord to the amp first (not to the wall). This way you don't have to be touching the chassis (which might have a voltage on it if something is wrong).
3. While looking at the circuitry plug the power cord into the wall.
4. Look and listen intently. Anything smoking? Any smells? Anything weird happens, unplug it immediately and start double-checking your wiring.
5. If all seems OK, the first thing I do is check that there isn't any voltage on the chassis. With a voltmeter check voltage across chassis and your ground pin.
6. Now is a good time to check that you got your hot and neutral leads correct and that the fuse hasn't blown.
7. Next step is to flick the power switch to ON. Safety glasses and a repeat of steps 4 & 5.
8. Let the tubes warm up for a few minutes. Watch for tube thermal runaway. If a tube starts to glow really brightly turn the amp off.
9. Check the tube heater voltages. Check the plate voltages.
10. All's good? Turn it off and plug in a guitar and a speaker. Put a knob on the volume pot (insulates you). Turn the volume to off.
11. Turn the amp on, don't hold the guitar. Let the tubes warm up. Any loud sound from the speaker and turn it off - don't want to blow the speaker.
12. With a voltmeter check that there is no voltage between the guitar strings and the chassis. Playing guitar with 700V on the strings is no fun.
13. Slowly turn the volume up. There will always be a bit of hiss from a tube amp but it shouldn't be loud.
14. Play guitar and enjoy. Remember that new electronic components tend to sound a bit "harsh" until they've been used for 10+ hours.

Rest of the pics are from turning it on for the first time. Next step is to get my carpenter-extraordinaire friend Gerald to build a beautiful box for it.