ng5921's DIY Amplifiers

[ng5921]

It happened - I've been bitten by the DIY bug.

This thread will be where I leave my thoughts and write down design decisions along my DIY amplifier journey. It will range from the most basic stuff, like buying equipment and setting up a workbench, or biasing a tube and experimenting with how it affects harmonic distortion, to higher level design decisions for a full amplifier build.

As some background, I have a degree in Electrical Engineering, so while circuits are not entirely foreign to me, it has been about 5 years since I completed my degree. I never ended up using my EE degree because I instead focused on data science and ended up in healthcare. I also have a long 20+ year participation in model aeronautics - designing, building, and flying radio-controlled airplanes (also some drones). I've always loved DIY projects.

Right now, I'm working through a couple of Blencowe's books, at the recommendation of DIY expert L0rdGwyn. Also doing a ton of reading online and watching youtube videos. I will compile the resources I use into a list on post #2 for others who want to follow along!

I only started on this journey about 1-2 months ago. I built a Bottlehead Crack to get a basic understanding of physical circuit layouts, and remind myself how to solder properly. My next project will be a simple EL34 speaker amplifier from Analog Ethos (https://www.analogethos.com/ae1). The designer has a wonderful guide that not only explains how to build the circuit, but goes through step by step and explains the function of every electrical element in the circuit. I also need an amplifier for my living room setup, and I am very curious to see how they sound off a tube amp.

So, first build of this thread will be my DIY AE1! I've already ordered the parts for the amplifier, and now need to start thinking about how I want to lay it out, and sourcing the chassis. I will likely use a Landfall Chassis and may send it to Front Panel Express for machining and engraving. There will also be some small mini projects like building a dummy load for testing an amplifier.

 

[ng5921]

Reserved for list of resources:

SAFETY (Must reads!):

• https://www.diyaudio.com/community/threads/safety-practices-general-and-ultra-high-voltage.30172/

Books:

• Fundamentals of tube circuits
  • Merlin Blencowe (https://www.valvewizard.co.uk/Book1.html)
    • Designing High-Fidelity Tube Preamps
    • Designing Power Supplies for Tube Amplifiers
  • RCA Radiotron manual
    • https://www.nmr.mgh.harvard.edu/~reese/RC10/rc10.pdf
• Building and layout of a tube amplifier
  • Morgan Jones
    • Building Valve Amplifiers (ISBN-13 978-0750656955)

Websites:

• Rob Robinette's website - https://robrobinette.com/How_Amps_Work.htm
• Pete Millett's DIY Website - http://www.pmillett.com/
• Blueglow electronics https://blueglowelectronics.wordpress.com/
• Merlin Blencowe's website https://www.valvewizard.co.uk/index.html
• Analog Ethos website - https://www.analogethos.com/post/tube-amp-explained-intro - an excellent beginner's guide for how a tube circuit works
• VTADiy - https://www.vtadiy.com/ - also has an excellent beginners guide to how a tube amp works, and very helpful loadline calculator
• AmpBooks - https://www.ampbooks.com/mobile/amplifier-calculators/ - a bunch of helpful calculators
• Tubes for Newbies - http://www.bonavolta.ch/hobby/en/audio/t_bas.htm
• HeadWize website - https://headwizememorial.wordpress.com/

Youtube Channels:

• Blueglow Electronics - https://www.youtube.com/@Blueglow - easily one of my favorite Youtube channels for valve amplifiers
• Uncle Doug - https://www.youtube.com/@UncleDoug

Guides:

• https://www.diyaudio.com/community/threads/heater-wiring-the-good-the-bad-and-the-ugly.211731/ - excellent resource for how to properly set up heater wires

Software:

• PSU Designer II https://www.duncanamps.com/psud2/

 

[ng5921]

Let's get started on the AE1! This is a 7 watt per channel amplifier that utilizes a 12AT7 input tube and two EL34 power tubes. It runs in Single ended Class A configuration. It uses diodes for power rectification, simplifying things a bit since we will not need a tube rectifier. So our final amplifier will have: 1 12AT7 input tube, 2 EL34 power tubes, 1 power transformer, 2 output transformers, and 1 choke.

While I will primarily use this as a speaker amplifier for my B&W DM602 S2 floor standing speakers, I am also going to add a headphone output, just for fun This headphone output will likely have a couple resistors to attenduate the signal by either -6dB or -12dB to reduce noise, and create an 8ohm load independent of the speakers for optimal impedance matching.

To begin, I want to reiterate that this is not my design - it was borrowed from Analog Ethos who is gracious enough to publish his design for free.
Schematic and build guide can be found here: https://www.analogethos.com/_files/ugd/c50b96_71db3288335041eea202a7a421d43acf.pdf

And the schematic which we will reference throughout this build:

Regarding parts, I bought essentially identical components as the parts list provided in the build document (linked above), except for the output transformers.

** Excel spreadsheet link to come **

I did not want to break the bank on this build, so I am still searcing for some suitable OPTs for the EL34 tubes. If I do buy a OPT with ultralinear (UL) tap, I will probably add a switch to the circuit to switch between triode and UL mode. According to the EL34 datasheet, this tube can pull between 70-100 milliamps of plate current, so our OPT will need to be able to handle that. Right now, I am considering this OPT: https://edcorusa.com/products/gxse15-5k-15w-5k-ohms-single-ended-guitar-tube-output-transformer?_pos=1&_fid=69f0c8e49&_ss=c

 

[theveterans]

Pretty good circuit! Wonder why you need a negative feedback though where you can just run no-feedback? Does this circuit become unstable without negative feedback, particularly when ultralinear tap is used?

Just a newb with this electronic stuff and mildly curious about its innerworkings

 

[ng5921]

Pretty good circuit! Wonder why you need a negative feedback though where you can just run no-feedback? Does this circuit become unstable without negative feedback, particularly when ultralinear tap is used?

Just a newb with this electronic stuff and mildly curious about its innerworkings

Very good question! Unfortunately I do not have an answer quite yet. Will do some reading up later today.

I think it could also be interesting to add a switch to turn negative feedback on/off.

 

[ng5921]

It's been a little while since my last update - I have mainly been collecting parts for drilling the chassis and building the amplifier.

Acquired a decent amount of necessary gear over the past few weeks like knockout punches, drill bits, and stepped drill bits. Now that I have everything, I should be able to at least drill pilot holes in the chassis this week, and next week I'll probably finish drilling and painting the chassis.

I had some free time today so I worked on the amplifier layout and I think I finally decided on the following:

I underestimated the size of the power transformer, so this is definitely going to be a tight squeeze! But I think it should be manageable for a first DIY project. This is a 8x10x2.5inch Hammond aluminum chassis (model number 1444-10825). Only cost around $25 so I'm not worried about messing it up. I opted for drilling out my own chassis just for the experience and to save some money instead of going with a Landfall chassis on this first amplifier.

A quick note about this power transformer - in general you want your magnetic fields of the transformers to be orthogonal to eachother, so that there is no induction from one transformer to another. The nice thing about a transformer that sits sideways is that its field is orthogonal to both output transformers (to the left and right of the giant transformer in the middle), allowing me to position the output transformers any direction I want. Due to size constraints, they will face in the direction shown, which looks a bit silly but it is just due to chassis size limitations.

I will use a combination of stepped drill bits and knockout punches to make all the holes in the chassis. Unfortunately the power transformer is going to take a good bit of work to get it to fit since it needs to drop through the chassis (similar to those used in many Bottlehead amps). I bought a metal hand nibbler for that part.

The larger octal sockets fit perfectly in a 1-3/16" knockout punch hole, and the smaller ones should fit in a 3/4" punch hole.

I will most likely paint the chassis black to keep it simple and clean.

Hopefully I will update later this week with a chassis with most of the drilling done!

 

[ng5921]

Pretty good circuit! Wonder why you need a negative feedback though where you can just run no-feedback? Does this circuit become unstable without negative feedback, particularly when ultralinear tap is used?

Just a newb with this electronic stuff and mildly curious about its innerworkings

Did some reading - this method of local feedback (also called schade feedback, or anode-follower) will provide some reduction in distortion, but it is not absolutely necessary for stability of the circuit. Global feedback is also an option, but from reading a bit in Blencowe's Preamp design book, it sounds like global feedback has a higher chance of entering oscillation than local feedback.

There is some confusing math that can be used to determine if an amplifier circuit will be unconditionally stable, which would allow you to apply any amount of feedback (global or local) and the system would never enter a state of oscillation even if you were to feedback the entire output signal. I'm not entirely sure how to calculate this yet.

For a better explanation, see Blencowe's Preamp design for Hifi section 1.24.

 

[ng5921]

Had some time today to sit down and think more about the amplifier layout and I made a couple changes:

With this new layout, the "power" aspect of the amplifier is entirely on the left side of the amp, while all signal/tube related items are on the right side. This should help avoid hum/noise in the amp by minimizing overlap of wires that carry power vs. signal.

I also swapped the position of the volume pot and XLR jack so that the volume pot is as close to the RCA inputs as possible while still being able to make a very short wire run to the input tube.

Just waiting for a few more tools to arrive and then I will start drilling the chassis!


I have also been sketching out plans for a "magic box" similar to that which Mark from Blueglow made in an effort to think ahead a bit to future amplifier design/build/testing.



This box will have a number of functions including a dummy load, BNC passthroughs (to the oscilloscope and Analog Discovery 2 unit), speaker passthroughs, and headphone outputs, as well as source outputs for the amplifier. It should simplify the test bench so that there are only 3 connections running from the amp to all my test equipment: one pair of RCA cables (amplifier input), and 2 pairs of output wires (or just one XLR/TRS connection in the case of testing a headphone amp). I'll post pictures of my schematic (if I can even call it that) once I clean it up a bit.

 

[ng5921]

Finally got some time today to drill the chassis!

Some of the larger cutouts are not the prettiest, since I had to use a metal nibbler to expand the cutouts. But they are not visible with components installed, so no big deal.

Currenly priming the chassis for a matte black finish!

 

[ng5921]

And finally - a completed chassis with components installed! (Minus one electrolytic capacitor which I forgot to drill holes for...)

It is FAR from perfect -- paint is being pulled off by some screws, I miscalculated how tight the inside would be. But learning tons as I go!

 

[G8torbyte]

And finally - a completed chassis with components installed! (Minus one electrolytic capacitor which I forgot to drill holes for...)

It is FAR from perfect -- paint is being pulled off by some screws, I miscalculated how tight the inside would be. But learning tons as I go!

Nice work! It is coming together very well.
Any chance you're bringing it to the local meet-up this weekend?

 

[ng5921]

Nice work! It is coming together very well.
Any chance you're bringing it to the local meet-up this weekend?

I could bring it! But it won’t be wired up and functional for at least a few weeks

 

[G8torbyte]

I could bring it! But it won’t be wired up and functional for at least a few weeks

As an engineer I like to do a lot of DIY stuff: home and appliance repair, PC building, basic auto repair etc but was always hesitant on electrical soldering. And I only took introductory classes in circuitry. It is fascinating and maybe something I may dig into later. Kudos to you on an awesome project!

 

[ng5921]

As an engineer I like to do a lot of DIY stuff: home and appliance repair, PC building, basic auto repair etc but was always hesitant on electrical soldering. And I only took introductory classes in circuitry. It is fascinating and maybe something I may dig into later. Kudos to you on an awesome project!

Take the leap! It is tons of fun. It isn't too hard to solder at a basic level.

 

[ng5921]

Decided to make some additions to the amplifier, while I still can drill more holes in the chassis.

1) I will be adding toggle switches for the OPTs to switch between 4 and 8 ohms
2) I will add a toggle switch to alternate between Pentode and triode mode on the output tubes.

Ordered the parts today, hopefully they will arrive by the weekend.

In other news, I got my workbench set up. I ended up splurging a bit on what is probably an end-game workbench for me, since I will also use it for my RC drones/airplanes/helicopters. Next step is to build a shelf to hold all my equipment.