Tag: dht

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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.

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Thomas Mayer’s new site

I’ve followed Thomas Mayer’s hobby website for a long time. He uses a tantalizing mix of high quality transformers and DHT tubes to build some beautiful audio devices. The tube of the month series is also a must-read review of odd-ball tubes and applications.

Now we can all see how much Mayer charges for his impressive tube builds. It’s about what one might expect based on the craftsmanship he clearly puts in and what the audiophile market supports in other products.

Check out vinylsavor.com here. If nothing else, browse the galleries and drool over the very Scandinavian glass, wood and metal work.

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DIY DHT filament strategies

Having a small stash of #26 tubes and always being curious about it as a preamp tube, I’ve embarked on some preliminary research of successful implementations. There’s a huge thread on diyaudio.com, but some choice references are Ale Moglia’s iterations and Kevin Kennedy’s classic implementation.

In general, the prototypical 26 preamp is a fairly simple single tube grounded cathode gain stage. Perhaps this topology simplicity is why there is so much experimentation in the support circuits. Gyrators, current sources, and line output transformers all make an appearance as the anode loading strategy. The B+ supply is similarly diverse: SS regulators, tube regulators, VR tubes, etc. It seems the popular consensus is for fixed bias: using the filament current drop across a resistor to set the cathode current. But there are fixed bias and traditional cathode bias implementations as well.

All of the above is fairly comfortable stuff coming from the general tube world of 9 pins and octals. The filament (AKA heater) supply, on the other hand, is something new for those used to indirectly heated tubes. In indirectly heated tubes, the cathode is a sleeve surrounding the filament heating it; this mechanical separation helps prevent heater hum (50hz or 60hz AC) from entering through the cathode. In a directly heated tube, on the other hand, the filament and the cathode are one and the same.

Depending on the circuit, AC filament power with balancing resistors and/or a ‘humdinger’ pot may be enough for an acceptable hum level. With the higher Mu (8-9) of a #26 and the need to keep front end noise/hum to a minimum (because it will only be amplified by everything following it), a DC heater solution is in order. We are looking at a requirement of about 1A at 1.5V for a #26 tube. The general approaches I have found are:

A low voltage SMPS and a dropping resistor requires no explanation if you know Ohm’s Law and can find something quiet with the right ratings (here’s a good read on this topic). Voltage regulation and current sources/sinks have been covered in principle a few times in projects and general information pages as well (see links above). Mixed strategies are what have piqued my interest the most.

Kevin Kennedy’s article suggests a 7805 followed by a LDO CCS to supply #26 filaments. From what I can gather, this is the principle also behind the Ronan Regulator (which I see mentioned frequently but I can’t seem to find the ‘official’ schematic). In these strategies the voltage regulator makes a first pass at cleaning up the raw DC and absorbs some power dissipation. The constant current source follows and sets the filament current to a fixed value (in turn setting filament voltage as per Ohm’s Law). Including a CCS to limit current has a protective side-effect as well: cold filaments are otherwise eager to soak up a lot of current, potentially stressing the power supply and filament/cathode itself.

Rod Coleman also has a very interesting approach to DHT filament regulation. You can find boards/kits for sale here (no commercial interest, just admiration for the design).

coleman regulator

This circuit feeds the filament from the ‘positive’ end with a gyrator, also known as a cap multiplier in this configuration. The transistor Darlington pair sees a low-passed capacitor at its base and works to amplify this smoothed signal at its low impedance emitter (effectively making the cap seem much bigger than it really is). This doesn’t regulate voltage because the gyrator doesn’t have a fixed reference, but it does reduce ripple drastically.

The ‘negative’ end of the filament is connected to a constant current sink. This is a ring-of-two CCS design which will have a lower operating voltage requirement than a cascode CCS or many ICs. Because our current is relatively high, low dropout voltage is a benefit in reducing overall power dissipation.

The filament is fed a low ripple voltage with a CCS setting the current. It’s simple, but reports seem universally positive for Coleman’s regulator approach. Whether filament bias or cathode bias, the filament supply should be left floating (it finds ground through the bias resistor or grounded cathode).

There’s little reason to reinvent the wheel here as far as I can tell. Although I’m still casually reading, I’ll more than likely try one or more of the above approaches to powering the filaments in my upcoming #26 project. More to come on this project as parts arrive and the ideas ferment.

P.S. here’s a FET version of the same gyrator-CCS one-two punch:

FET filament reg

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Big PP problems

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Hoffman’s Iron Law impacts all systems, regardless of the type of amplifier. It states that speaker designers may only optimize for two of three performance goals: efficiency, size, and frequency extension. Modern speaker design goals trend towards slim and minimally-intrusive boxes. Because few are willing to give up low frequency ability, this aesthetic trend has resulted in lower-efficiency speakers, requiring ever more powerful amplifiers.

When you have a set of bookshelf speakers or less-efficient towers, a single-ended triode amplifier may not cut it for power. Larger push pull (and parallel push pull) amplifiers are capable of using lower turns ratio output transformers and delivering more power to a load. That could be the difference between realistic dynamics and a more compressed musical presentation. Of course, there are some things to overcome when you upsize your tube amplifier.

  • The power supply – If building an amplifier with (parallel) push pull power tubes, you’re going to need a lot of heater current. You’ll also need a lot of high voltage current. This means solid state rectification is the way to go. Using a bridge rectifier (four diodes) rather than a full-wave (two diodes) also saves some efficiency in the transformer.
  • Size and weight – More current demands from the transformer(s) directly translates to a larger size and weight. Again, bridge rectifiers will help reduce the power transformer size slightly. A switching buck converter for heaters is also something worth looking into for efficiency’s sake. Building as monoblocks is a good solution, but you’ll probably spend twice as much on chassis, power supplies, etc.
  • Current sharing – To keep standing DC currents from saturating the output transformer core, we want all our output tubes to share current equally (or at least balance per phase in each channel). Bias servos, Blumlein garter bias, individual fixed bias, and individual cathode resistors all have their advantages and disadvantages. Careful consideration here is key.
  • Driving Miller capacitance – With a bunch of parallel output tubes, the input and driver stages will need some grunt to keep Miller Effect from rolling off high frequencies. This is especially true if you’re driving triodes in the output stage. A follower of some type may be needed to ensure a low enough source impedance.

If you haven’t already gathered, I’m a bit preoccupied with how I’ll utilize the big old chassis I picked up recently. Clearly something large is in store. The present question is octals or DHTs and two or four output tubes per side. The chassis originally held some monstrous iron, so there’s space for just about anything.

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