In search of the perfect tube amp assembly bench

Although all my parts and most of my tools are still in boxes, my household move is complete! It will still be a while before I’m spending free time in the workshop, but it’s never too early to daydream a bit about how I’ll set things up.

I have a blank slate of dry, mostly bare, concrete and studs/joists to work with. Aside from a sump in the corner, I’ll be able to utilize all 15ft x 10ft for my own storage and working space. Coming from space that was split between a garage and a shared basement, the 150 sqft is palatial. That said, my goal is to use it as efficiently as possible.

Amp Work Zones

Taking a cue from kitchen design, I’m planning the room layout by functional zones. Specifically, building tube amps involves three key processes: chassis work, electronics work, and parts/materials storage. Each process is a Russian nesting doll of other steps and tasks, of course, but these three general areas represent unique workflow challenges that are more or less shared by the sub-tasks that make them up.

Chassis Work

  • Messy in terms of generating dust, shavings, waste
  • Work is usually done standing
  • Physically large, sharp, and/or spinning equipment
  • Space has high potential to be used in non-hobby activities

The woodworking-focused area would benefit from mobility. Rooms adjacent to the workshop can be used as ad-hoc work space for projects that don’t fit in the smaller shop. Keeping tools and workbenches on casters will also let me rearrange as the work or materials require. Additionally, being able to moves benches helps with dust cleanup.

The old chassis work space

This will probably involve a 8ft+ bench for assembly with an integrated miter saw bay. Space under the bench will house a shop vac (dust control) and extra power tools and tool boxes. My drill press will live on a separate cart, as will my router table, for maximal flexibility.

I need to figure out a good way to deal with casters and the unevenness of basement floors.

Electronics Work

  • Benefits from flexibility in lighting and seated/standing work
  • Equipment is stationary and shelf size, but numerous
  • Results in many small parts used simultaneously

I’m planning to reuse the t-track chassis cradle I had in my last workbench. This was one of the best ‘tricks’ I picked up for building and working on projects. The rest of the space above the workbench will also be very valuable to keep power supplies, variacs, and equipment close at hand while working on a project. The worktop itself should have some kind of padding.

A project on the “easel”

This will likely also be about an 8ft long bench. I do not see a lot of benefit in making this a mobile bench due to its specialized nature and the manageable size of projects. Owning a CNC is one of my long-term hobby goals, so the extra long bench may come in handy (though it kind of breaks the ‘zone’ philosophy).

I’d like to come up with some kind of solution for keeping component parts organized while they’re out and being used for a build. This could be as simple as having tackle-box organizers on hand.

Parts and Materials Storage

  • Materials typically include long lumber and extrusions and plate under 18″ square
  • Component parts are numerous, but physically small and commodities
  • Tubes and transformers vary widely and some are fragile/valuable

The space has shelving built between studs that will help with storage for some things (mostly paint cans at the moment). Storing long materials high on the wall in a rack makes sense. Aluminum plate could be stored on edge without worrying about it deforming. The numerous component parts, tubes, and transformers, present an interesting challenge though.

In the past I’ve kept tubes in Rubbermaid drawers, but I’ve found that I’ll often forget about what I actually have on hand. Curing that will probably require some way of organizing tubes in a single layer, preferably vertically to conserve floor space. I have had my eye on french cleat style storage walls and am thinking about ways to adapt this to small parts and tubes. The modular nature of it has some potential benefits.

First things first

Before I start cutting too much wood for benches and storage, I plan to give the whole room a good scrubbing and a fresh coat of paint. Other rooms in the new basement have a coat of epoxy that I find to be very appealing, so I will likely treat the floor to make it easier to clean and generally more attractive. I’ll also switch out the bare bulb light fixtures to shop lights and have some additional outlets put in on their own circuit.

Yes, there’s lots to do in the new workshop! I plan on building a simple tube project or two in the meantime. Refreshing my memory of what it’s like to start out without a bunch of tools is probably a great exercise in and of itself!

Letters to WTF: how do you test a build?

Q: Hi, I’m working on a schematic from your website. How do you usually test your circuit, as you go, or once everything is wired?

This is a great question. The short answer is that it depends. On a simple build with just one or two stages and passive loads and power supply filters, I will probably finish all my wiring and then power up and test. On a complex build with things like active loads, multiple bias voltages, or regulated power supplies, I will test as I build. In both cases, my general testing process is fairly similar.

  1. Connect the project to a variac or light bulb current limiter (if available).
  2. With only rectifiers installed (no other tubes), power on and measure B+ voltages. These will be higher than the voltage levels with the rest of the tubes installed, but should be in the ballpark.
    • 2a If using any circuits on PCBs, I will test before installing in a chassis if my external power supply and loads allow it.
  3. Install preamp tubes and measure bias points to be sure they’re in the right ballpark. If fixed or directly biased output stages, measure bias levels. The B+ is still a little high at this point.
  4. Install output tubes and dummy loads, and measure current draw and bias point. The B+ should now be at roughly the calculated level. Adjust bias if needed.
  5. Connect to cheap speakers and debug hum/noise. Let the project run for extended periods of time and generally abuse it a bit.
  6. Hook-up to the main system and crank it!

At each step, any trim pot adjustment appropriate to the stage would be adjusted as needed. Typically I will have one digital multi-meter (DMM) on the B+ at all times and additional meters to measure individual tube bias. I use alligator clips and connect/disconnect meters with projects powered down. Don’t poke around live amps if you can help it!

The Quality Amplifier

Having found an irresistible deal on a pair of Hammond 1620A output transformers ($35 each), I have started some preliminary research on suitable amplifiers to build around them. These transformers have a 6.6k primary and are rated for 20W. This is around the power and impedance used in classic amplifier topologies like the Mullard 5-20 or the Williamson Amplifier. Both would probably provide blameless performance, but I’ve got this incorrigible itch to do things the hard way.

Dennis Grimwood’s website Optimized Electron Stream has a great collection of articles and reading. In particular, his history of the Williamson Amplifier caught my attention. According to Grimwood, the Williamson amplifier was an evolution of a design published as The Quality Amplifier in Wireless World in 1943 (and updated in 1946).

Note you can find the archive of Wireless World back issues here at American Radio History.

The Quality Amplifier was the work of WT Cocking (who was also a prolific writer about valve electronics). In contrast to other contemporary designs using interstage transformers, Cocking exclusively used RC coupling between stages and a concertina phase splitter at the input. Much time is spent in his Wireless World articles detailing the care and feeding of capacitors, something we take for granted today.

Several potential triode output stage configurations are detailed in the 1946 article:

  • the original push-pull PX-4, producing 4W (or 8W with higher supply voltage)
  • push-pull PX-25, producing 12W
  • push-pull 6V6G triodes, producing 2W

All of the designs recommend MH4 valves in the phase splitter and driver stages, but list the 6C5 as an alternative. The 6C5 is a forefather of the modern 6SN7. None of the variations use global negative feedback. Here’s an example schematic showing the topology:

Apart from the appealing simplicity, I note the coupling caps needed at the input and between the stages. This is one more RC coupling than used in the Williamson, but there’s no global negative feedback to complicate the phase shifting. The placement of the concertina is also interesting here. By splitting phases before the drivers, rather than after such as seen in the Dynaco ST-35, Cocking is getting more gain per phase. Given the limited Mu in the tubes of the day, this was probably necessity.

This brings me back to the Hammond 1620As. I’m not going to be building with PX-4s or PX-25s, but the 6V6G that put out only 2W has had some spec bumps since Cocking’s day. We also have the benefit of transistors to assist us in squeezing out a few more watts and otherwise modernizing parts of the original design. Specifically, I’m eyeing the A2 grid lines provided on the 6V6 datasheets…

The 6V6GT triode-strapped is praised for its tone but lamented for its limited power. The Cocking Quality Amplifier looks like a great template that, with a few modern touches, will minimize the 6V6’s weaknesses and maximize its strengths. The push-pull loadline above looks like about 10W triode, a five-fold increase over the original application!

Removing DIY barriers

It seems to me that there are three fundamental obstacles for beginners in the DIY tube hobby:

  • Layout and connection of component parts for best hum/noise performance
  • Choice of parts for correct and safe ratings/types/etc
  • Chassis fabrication and layout

Complete kits with chassis, parts, PCBs, and the whole ball of wax hit all of the points, but they are a daunting investment in both time and parts. See great examples from Bottlehead or Elekit. In a baby-steps approach, I’ve begun experimenting with putting entire circuits on a PCB design (image shows the El Estudiante). This addresses the first point.

I have ideas on ways to tackle the other challenges that minimize capital requirements and keep the hypothetical business idea agile and scalable (brushin off the old business and supply chain lingo). It might even be enough to turn into a respectable side-hustle. Hopefully I’ll be posting more on what I’m calling “quarter kits” in the near future.

Designing for DIY

At a recent audio swap meet, I had the chance to meet Matt from Toolshed Amps (check out his great looking work here!). We talked quite a bit about tubes and audio design and our different approaches to the same goal (quality sound). It was interesting and relevant enough for me to want to share some thoughts here on the blog as well.

On the surface, the differences between what Matt and I create seem obvious. Matt favors classic triodes like 2A3, 1626, or 45. Supporting components include tube rectifiers, big can caps, and Magnequest (!) iron. His amplifiers are housed in meticulously handmade chassis with intricate etching. In short, Toolshed Amps lives up to its name and the cottage industry tradition of passionate small-batch craftsmanship. I love it.

WTF Amps is a DIY-focused project first and so I try to design with other builders (not just end-users) in mind. In addition to quality audio, this creates some hobby-specific goals that guide many of my design decisions. At times there is even conflict between these goals:

  • Parts availability and flexibility
  • Novel and exploratory circuits
  • Simplicity and intelligibility

I like looking for NOS tube hidden treasures and am always hunting for a deal on second-hand transformers. When I publish a design to be replicated by others though, I have to be cognizant of the availability of the parts I specify and whether alternatives exist. You’ll find more Hammond/Edcor iron in my designs than Tango/Magnequest not just because of costs, but because they’re widely available. Similarly, although I love the 5965 tube, I’ll probably specify a 12AT7 because they’re in current production. Where I favor easy-to-find parts, I still design (and write) for flexibility in upgrades or tube substitution.

I believe that the DIY tube hobby (like most hobbies) is a journey. As we progress in the hobby and our repertoire of concepts and circuits grows, the uncommon and novel designs are what keep us building and learning. Building leads to experience and self-evaluation, which leads to conceptualization and experimentation (side note: andragogy is the method and practice of adult learning). I should note that playing with new circuits and approaches are as much for myself as they are for the readers!

The last guiding principle (simplicity and intelligibility) is often at odds with the need to explore new things. If I publish a DIY design, I would like to be able to explain it in a project write-up as well. Some of this is accomplished when I’m researching topologies but complex projects (even if the component parts are simple) are a daunting task. The urge to push the design envelope is always there, but I’ve learned to take baby steps and rely on conceptual stepping-off points for published projects. This is good general advice for the hobby as well. Don’t rush it; build what you know and iterate.

So in summary, do I want to build an A2 DHT amplifier with Tango iron, 274B rectifiers, regulated everythings, TVC attenuator, and a rosewood enclosure? You bet your butt. Do I respect guys like Matt who do (and do it well)? Darn straight. But this kind of all-out end-game amplifier wouldn’t quite fit with my DIY-friendly design goals. On the other hand, WTF Amps will try to get you as close as possible to building one of these yourself with available parts and easy-to-understand write-ups. The last mile is just up to you.

Workshop toys and a different style chassis

I recently spent some time in the garage workshop on a non-tube project I’ve been meaning to do for quite some time. Up to now, my chassis have all been a mitered box with a lip to inset the top and bottom plates. I cut the lip with a straight edge and a hand-held router. This works fine and produces good results, but setting it up for the cuts was a chore. The better solution for this kind of rabbet with a fixed width/depth is a router table, so that’s what I built!

My new table includes a white-board top, inset aluminum plate for the router, and t-track to adjust the fence distance. The fence has adjustable stops so I can get close to zero clearance and add shims to plane edges or remove small amounts of width from boards. This will all make cutting the same chassis I’ve been doing much quicker. It also opens up some new chassis possibilities:

Here’s a different style chassis that I’ll use in the next preamp build. The sides are still wood and include a lip to inset the top and bottom, but the front plate is 1/4″ aluminum and the back is an aluminum c channel. This will make drilling for jacks and controls much easier than using a 3/4″ wood panel. Making the same enclosure with decent precision would have been much more difficult without the new router table.

More updates to come as I begin this preamp project!

A Mighty Cacahuate caught in the wild

Student “D” sent me some pictures of his Mighty Cacahuate project with a twist and it’s too unique not to share. D developed a PCB for his build and mounted all the amplifier parts to a top plate as would usually be done. Instead of a boring wooden box, D dropped this into a boombox enclosure for all-in-one listening. I love to see the creative use of a basic schematic I posted here on my little website.

We initially troubleshooted some wiring over email, mostly due to my omission of the details of heater wiring and pin numbers on the original schematic. Once sorted though, D says the amp started playing and sounding great without a hitch.

It was an amazing feeling the first time they powered on.

-D

Careful there, D, that feeling is habit forming!

Update on the mono-blocks: left channel is done and right channel is coming together quickly. Write-up for the project is also underway. Looking forward to playing in stereo!

EL34 tester tubes, also considering springing for KT88s on this project
Night shot. Red glow through the vents is LEDs used as voltage references in CCSs inside.

American Radio History

Today I came across a website project called American Radio History and it reminded me that the internet is an amazing resource for education and sharing/archiving information. American Radio History is an online depository for endangered antique electronics knowledge captured in the form of hobbyist magazines from the early 20th century (back when tubes were the dominant active device).

The site’s FAQ provides a glimpse into how it came about:

There is so much printed material about radio and television that is becoming harder and harder to find. Libraries are discarding (often to the recycle bin) many titles. Other collections are very limited in access so “the rest of us” can’t find information we want.

The site began over a decade ago when I found I was often being asked questions I could answer from my own library. So I went digital for all to see!

Just one person does most of the work. I have off-site backups at several locations under the custody of well-respected radio historians. I also have several hearing-impaired persons who help with the major flatbed and Atiz Bookscan digitizing projects.

Here’s a short list of some of the dozens of relevant magazines for DIY tube reading:

Elektor

Radio TV Experimenter

Audiocraft

It is not difficult to find schematics in our hobby, but it is somewhat more challenging to find schematics accompanied by articles that reveal their inner workings. That is one advantage still held by professional publications like the audio magazines cataloged by American Radio History.

Bench tips: organizing resistors

There is a new post at Hackaday detailing a very cool 3D printed resistor organizer. Resistors are an indispensable component for DIY tube projects and we typically collect a large variety of both resistance values and power ratings. If you’re into boutique parts and Holco/Riken/Caddock flavors, it is even more overwhelming. On the upside, resistors are pretty cheap to buy in bulk; but if you buy in bulk and don’t have a way to organize things, you end up with a tangled mess of kinked-up leads and unmarked parts.

That was me a couple years ago: always buying extras and just tossing them into a parts organizer with too few cubbies and not enough labels. Eventually I got sick of repeatedly buying the same values because I was too lazy to sift through my inventory. Doing a little reading on hobby forums, I came across a really cheap and useful solution: trading card binder pages.

Trading card pouches are just the right length to store resistors without having to bend up the legs. This is best for 1W and smaller sizes, so I still have some storage for power resistors. But I also don’t buy power resistors in bulk very often (because I’m cheap), so there’s less to store. Go for either tape and reel (see above) or keep your loose resistors in baggies:

The parts baggies you get from Mouser/Digikey are already labeled, so you just need to fold it with the value facing up and slide it into the card slot. Easy peasy. Best of all, the trading card pages and the binders themselves are easy to find. The only downside to this resistor storage solution that I’ve found so far is the high likelihood of dumping everything on the floor if you pick the binder up upside-down (true story, twice). I’m currently on the lookout for a Hello Kitty trapper keeper with a zipper to solve this. For science.

Bench update: the push pull mono-block project made its first music through a speaker yesterday. I’m looking forward to cranking out the second amp and posting the project for others.