Designing A Guitar Tube Amp: Part 1

schematicIt seems like this has been one of my favorite topics on Tech-Tut. I really love to build tube guitar amps. It is proven over the many discussions that I take great enjoyment in this part of my hobby.
There have been many ups and downs, and I have temporarily given up many times, but I rarely give up for good. I know when to cut losses, but a hobby is something that is for me, and I can revisit it again at any time. As with the second revision of my 6DG6GT amp, it goes to prove that persistence finally paid off.
Here I am once again working on my next challenge. I want to design and build a low power, but higher power than a single-ended amp, push-pull guitar amp using the EL84 tube.
I want to document most of what I have done for this project. While I am building this project for a profitable venture, I want to leave the whole project as open source for those who might want to try to build their own.
As with any “tutorial” on my website, sometimes I tend to narrate more than give step-by-step instructions. That’s what leads me to my first installment in how I construct the EL84 guitar amp. I want to explain my research.
There are many links and books available to explain how to design and build tube amps, but my experience with them is that a second opinion or angle helps to solidify the learning process. I didn’t get where I am today from one source. I have read Instructables, webpage after website, and several crazy expensive books to help me wrap my head around the process. With that, I pass to you some of my most favorite links explaining this hobby.

Books

  • Valve Amplifiers by Morgan Jones
    ISBN-10: 0-7506-2337-3
    $60.00 USD
    This book has a lot of great info, but is more directed to HiFi.
  • Design and Construction of Tube Guitar Amplifiers by Robert C. Megantz
    ISBN-10: 0615291805
    $23.00 USD
    This title does a great job of showing many stages of a guitar amp. I would have liked more discussion about how the design process happened, but it has been a valuable tool.

Web Resources

  • Guitar Tube Amp by gmoon

    http://www.instructables.com/id/Guitar-Tube-Amp/

    I’ve mentioned this Instructable many times because this guy walked me through most of the processes. It’s a long read, and it will take many passes. From some of the rudiments of design to how to cover a cabinet, gmoon does a great job of documenting the process.

  • The Valve Wizard

    http://www.valvewizard.co.uk/index.html

    This website does a little better job at explaining the design process.

  • The Cooperative Tube Guitar Amp Project

    http://www.ax84.com

    For Single Ended amps, this site has a whole document of theory. There is also a forum with people who can help you along the way.

Do Your Research

Ask questions. Google stuff! Check datasheets. See what others are doing. It’s the age of information, and you can find almost anything on the web. There’s no reason why you shouldn’t. Step one in starting any project is doing research. Start with knowledge bases that are directly related to what you want to accomplish.

Tune in next time when I show you where I’m starting with my latest guitar amp build.

EL84 Tube Amp Revisited

…and the next tube amp build on the horizon of completion.
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I have been playing this amp quite a bit lately, and I keep checking the parts against my schematic. Today I noticed that the chassis wasn’t grounded, and the heaters were referenced to the output tube’s cathode voltage. I figured I would make a few changes.

I referenced the heaters to ground using two 180 ohm resistors. I also connected the ridiculous looking star ground to the chassis to keep someone from accidental death. I’d rather pop a fuse than pop a human being should something come loose in there.

This amp has a little hum to it, and I’ll check that out in Duncan’s power supply design program. Single ended amps need some stiff HT supplies to keep the 120Hz hum from moving the speaker. It won’t be anytime soon that I mess with that though.

I would like to point out that I dated the turret board “April 23, 2010.” I have learned some since then. One thing for sure is this: that thing is a wiring mess.

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This is my latest project amp that hasn’t quite made its debut on Tech-Tut.com. I call this “Upcycle” because it is made from quite a few pieces of trash. A chassis and toroid power transformer from a Backline 250 bass amp, some capacitors from a desktop PC power supply, tube sockets and transformers from the original 6DG6GT tube amp that I recently redesigned, feet and handle from some amp that was scrapped, and pots left over from some other experiments. Some of the caps were also from the old build. Many parts came from my inventory (which isn’t much), and I purchased a few items new to fill in the gaps.

I believe that I have only spent about $120 on this amp. The speaker was $60; I bought a 12″ Jensen speaker. I wish I could afford a Weber.

I’m getting better at this. Once I get some paint of the cabinet and get my logo embroidered on the black burlap, I’ll put a design blog post explaining how I built this thing.
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This is the dry fit of the chassis of “Upcycle” in the cabinet. This amp has a few more steps to go before it is complete.

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This is the revised 6DG6GT amp. It is a single tube output. You can see my logo on the speaker cabinet. This amp has a Weber Blue Pup 8″ speaker.

6DG6GT Tube Amp Design Update 6-12-2014

6DG6GT_Tone_StackI couldn’t stand it. I really didn’t like the tone stack that I included in the 6DG6GT amp. After reading the GMOON Instructable the other day, I found out that he used the Big Muff Pi tone circuit as shown in Duncan’s Tone Stack Calculator. I decided that I liked the simplicity of the Vox style tone stack. I’ve added this to the 1.5 version schematic. Check out the page dedicated to all of my tube amps here: Tube Amp Builds…all in one place. The image shows the new tone stack built on my existing turret board. Since this is a prototype, I suppose I can live with a few parts that appear to float in mid-air.

One thing to note is that I wired the bass pot backwards. This is because I really want all of my knobs to be at full clockwise for the twangy, overdriven sound. That’s pretty much where I’ll keep it especially since I usually play my guitar on the neck pickup which tends to lend to a darker sound.

6DG6GT Tube Amp Design Update 6-9-2014

6DG6GT Images and Descriptions

The amp appears to be complete. In case you missed it, check here for the final schematic: Tube Amp Builds…all in one place!