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!

An Amplifier Primer: Technical Terms for Beginners (Headphonesty article)

Early tube computer modules with some of the female mathematicians and physicists who worked on the projects

I just finished an article for Headphonesty on headphone amplifier basic terms. It focuses on building block concepts and aims to make technical marketing copy more comprehensible by users without an electronics background.

Here are the topics covered:

  • Single-ended and push-pull
  • Classes of amplification (A, B, AB)
  • Output coupling methods (direct, capacitor, transformer)
  • Amplifying devices (tubes, transistors, ICs)
  • Negative feedback

This article is probably a good introduction to articles on WTF Amps like the Output Stages page or the various headphone amplifier projects.

Read An Amplifier Primer on Headphonesty here!

Modern Quality Amplifier Conceptual Outline

May is a very busy month for me personally so I have only a short update this week. My last post was about the Quality Amplifier, a forerunner to the well-known Williamson amp. I proposed a modernized 6V6 A2 version based on the same topology that could probably crack double digit power.

In the conceptual outline above we have a MOSFET rather than a tube as a concertina splitter to save heater power. The unity gain MOSFET splitter feeds a tube differential pair to generate enough voltage gain to drive the outputs. MOSFET grid drivers with a CCS load help the differential stage cope with the drastic impedance changes at the output tube grids in A2 operation. Outputs are wired as triodes, of course.

The A2 drivers will go on a PCB. I also have one designed for the MOSFET splitter, though that’s simple enough to wire any way you might want. We’d need just three tubes per channel: a pair of 6V6s and a dual triode driver. My driver pick at the moment is probably the 5965 (or a pin-compatible 12AT7 if it needs to be new production).

I’m also happy to report that progress is being made on the preamp project. Hopefully I’ll have pictures to start sharing in the very near future!

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.

Guest editor at Headphonesty

Headphonesty is a digital magazine dedicated to high-fidelity headphone audio culture. Outside of designing and building a few heapdhone amps, I’m personally a regular high fidelity headphone user, especially now that I have a toddler in the house. I recently connected with the chief editor at Headphonesty on Reddit and agreed to lend a hand with an article on headphone and amplifier impedance.

Unlike speakers, which are really rated at 4 or 8 ohms 99% of the time, headphones have a wide spectrum of impedance ratings. From 16 ohm in-ear-monitors to 100k+ ohm electrostatic headphones, matching sources and loads is a confusing aspect to the hobby that’s often done through trial and error or hearsay. This article written by Trav Wilson and edited by yours truly seeks to explain some of the concepts in an easy-to-read headphone-centric way.

Read Headphone Impedance Demystified on Headphonesty here!

Line stage + phono build underway

In a recent post I set a goal for myself of creating a couple of preamp designs that included both line stage and phono preamp circuits. The first of these builds is now underway! Because of the number of input and output jacks as well as switching and volume controls, I’m using a different style chassis than the all-wood apron approach in most of my builds. The face plate and rear of this enclosure are aluminum with wood (walnut here) used as accent panels on the sides.

I’m emphasizing the sleeker look by mounting all of the transformers inside the chassis. The power supply is mounted to a section of aluminum c channel that also serves to section off all of the AC power from the rest of the chassis (which will carry sensitive signal circuits). So far so good. We’ll know if the approach to shielding and layout is effective once it’s powered up and playing. Placement for the signal portion will be finalized after I’ve mounted the front panel controls into the 3/8″ aluminum flat. I expect that to be a bit of an adventure…

The generic circuit for this build is below (final values will be published once it’s tested). Tracing the path of a phono signal: the resistor loaded input stage feeds the RIAA correction filter which feeds a gyrator loaded output stage. I’m using a gyrator as a flexible load to allow for tube swapping as well as a low output impedance device to effectively drive the follow control that follows after the selector switch. The volume control feeds a transformer loaded 6H30.

The Edcor GXSE 15k:600 output transformers are an experiment here. Reading through others’ experiences and measurements, I think the 6H30 is going to be a suitable driver with good bandwidth if it’s given enough current. I expect to do some experimenting with loading the secondary.

All in all, this will be all 9-pin current production tubes and parts. If the execution works out as well in real life as it does on paper, it will be a great, relatively-affordable preamp build.

A cheap sleeper tube thought experiment

One day I was combing through tube characteristics using the parametric search function in the Tube Data Sheet Locator desktop app and I came across an interesting 7-pin triode. As luck would have it, I happened across a couple at a swap meet about a week later when the tube type was still fresh in my mind. I bought them, thinking they might be interesting to experiment with.

The 6AF4 is a 7-pin indirectly heated triode. I found the 6AF4 interesting because of its fairly low amplification factor (Mu of 15), decent transconductance, and high perveance. These characteristics suggest that a really simple and fairly low voltage preamp may be within reach. Check out the datasheet here (link to PDF).


I’d power this with a pair of 48V SMPS in series and wire the two 6AF4 heaters in series to be powered by a 12V SMPS. This would bring PSU costs and size down significantly. Voltage gain here would be about 10 and output impedance should be a little under 2k ohms.

Because resistor loading will still provide a lot of gain, even with a low Mu of 15, this version uses a matching transformer to step down gain and output impedance. Voltage gain here would be 2 or 3 and output impedance should be a couple hundred ohms. Edcor makes affordable candidates for output transformers in this application (WSM series). I’d also look for second hand matchers in the 2:1 to 5:1 range.

As far as a loadline and bias point, the blue blob area looks pretty good to me. Note how low the supply voltages are here. These low voltages make parts both cheaper and smaller. While I like the lower output impedance and fanciness of parafeed, the resistor load would probably sound a little tubier. It would also be simpler, cheaper, and more compact.

Whelp, I seem to have talked myself into it…

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.