One little project comes to fruition

Early this year, I wrote a post about simple RIAA correction with opamps. Although it doesn’t involve tubes (yet), I recently completed a PCB-based build based on this post. This was both to test the calculations/theory as well as good practice in PCB design.

This ultra-simple phono preamp runs on just a pair of 9V batteries for power and utilizes a mix of feedback and passive EQ for RIAA correction. The batteries should last about 24 hours (playing time), but a bipolar AC-derived supply could be substituted without trouble. Gain is easy to adjust with just a couple of resistors (set at 40db in my build). The bill of materials runs about $25 with 5532 opamps and 5% tolerance WIMAs.

I’m planning on building a couple of these with coworkers and basing build instructions and any revisions on the experience. I do have some extra boards from this first run. Shoot me an email if interested!

New Page: Phase Splitters

It’s no secret that I’ve been working on a larger push pull project, but it may still be a surprise because progress has been so slow. Don’t get me wrong though, dad stuff is the best.


Writing time is sometimes easier to come across than bench time, so there’s a new overview of phase splitters page for your enjoyment. Happy holiday!

Edcor has new secondary options

Finding output transformers with turns ratios suitable for headphones used to be an exercise in futility (and endless eBay refreshing). As the market for high-end headphone amplification has grown over the past few years, it seems like Edcor has taken notice. I’m really happy to see some new options for high impedance headphone transformers on the Edcor website. Relative to vintage UTC or custom winding, these look like very affordable options:

edcor heapdhone OPT options.png

In the past I’ve resorted to matching transformers in a parafeed arrangement or speaker transformers with low impedance headphones. I look forward to trying some of these transformers in future builds!

Direct coupled stages

If you spend enough time haunting DIY tube amp websites and books, you will inevitably come across the theme of direct-coupled tube circuits. N-type and p-type transistor sandwiches make direct coupled circuits almost trivial. Tubes, which are “n-type” only, are not quite so simple to marry anode to grid. And yet the siren sings, drawing in the adventurous tube spirits.

“Why should we want to direct couple in the first place?” you ask, your socratic gland tingling. Many solid state amplifiers take advantage of the direct coupling to increase the levels of negative feedback. With tubes, we’re often more interested in maximizing the inherent linearity of triodes in open loop Class A amplification (but for a good counter example, see Jones’s Crystal Palace in Valve Amplifiers 4th ed).

Ostensibly, eliminating a coupling capacitor or transformer leaves less in the signal path between input and output, making whatever you are building more transparent (if you consider caps to be a significant source of coloration). Eliminating a coupling capacitor also removes a potential source for blocking distortion (if you are prone to driving amps to clipping, though a cap-bypassed cathode resistor can still cause you problems). In my opinion, the most compelling reason to direct couple is that it makes A2 (positive grid bias) operation a possibility.

Following are some (mostly untested) scratch-pad ideas and notes for “simple” direct-coupled SET amplifiers.

Fig 1: Simply using an abnormally large cathode resistor under the output tube raises its cathode above the anode voltage of the driving stage. This dissipates a lot of extra power in the output section and doesn’t really contribute anything to A2 operation. Still, a fun party trick.

resistor load dc

Fig 2: Using a resistor divider to lower the dc voltage seen by the output tube’s grid. This reduces the gain of the first stage and probably still requires you raise the cathode of the output stage (see Jones for good reading on this).

level shift dc

Fig 3: The Free Lunch style of choke loading the driving stage is as nifty as it is temperamental (in my experience). You are still dissipating power in the cathode of the output tube. See also Loftin-White variations discussed at TubeCAD.

choke loaded dc

Fig 4: Currently simmering on my back burner, a MOSFET gyrator sets a reliable voltage on the grid of the output tube and its low output impedance enables A2 operation. Rather than raising the cathode by dissipating power in a bias resistor, the cathode is raised by a separate power supply (must be rock solid). Additional stacked supplies provide B+ for the output tube and driving stage.

mosfet dc

It should be pointed out that direct coupling will almost always require some extra calculating, measuring, and adjusting of whatever you build (you get a glimpse of this with the El Estudiante cathode resistor trial and error). You’re also likely to pigeon hole a direct coupled circuit to very specific tubes, not to mention bias points (which must be maintained). But despite these warnings, once you’ve heard the legend of the circuit without caps, it may already be too late.




“Interview” with Bottlehead’s Doc B

As some readers may know, Reddit is my social media haunt of choice for casually chatting tubes. Around two years ago, I created a forum/board/subreddit called /r/diytubes which has grown to almost 6,000 users. Last year we did an AMA (Ask Me Anything) interview with Doc B from Bottlehead where the community asked questions and Doc patiently responded with his witticisms, opinions, and technical insight. If you haven’t already, you should read the whole interview here.

Some choice excerpts…

Q: What’s your daily setup?

A: A good speaker setup in a well treated room really floats my boat. My daily setup changes often, as it is in our listening room at Bottleheadquarters. Here’s a pic of what it looks like today. It’s mostly Bottlehead gear (our new speaker kit prototypes, Kauji amp, BeePre preamp, Eros phono preamp, Tube Repro tape preamp, Bottlehead DAC (NLA), Mac mini/Audionirvana, Empire 208 modded by Analog Engineering, SME309, Haniwa HTC-R01 cart, MQ/Peerless stepup transformer, Nagra T audio tape deck. All Bottlehead cables. The amps on the floor are prototypes that are being run-in for future evaluation. In my office I do use headphones – AKG K1000s with our Neothoriator headphone amp

Q: What would you recommend as a basic to intermediate set of test and measurement tools for building(testing/measuring,etc) tube circuits?

A: In terms of basics, I would suggest getting a decent DMM first. A Fluke is really nice but not necessary. Inexpensive DMMs are surprisingly good these days. Just make sure it is rated for the kind of voltage your tube gear may put out. 500VDC max rating is usually OK, but may be marginal for higher powered gear. 1000VDC is a safe bet…

…If you want to spend on something fancy, put your money into a good solder station. I have tended towards obscure and expensive irons like Hexacons and Ersas over the years. Unecessary to spend that kind of money. A good moderately priced station like a Hakko will make soldering so much easier that you will need test gear to sort out your messups far less often. Good wire strippers are another must have. I like the Ideal type, but whatever works well for you is the thing to use. Ditto good side cutters, pliers and a solder sucker.

I also get asked about tube testers. If you plan to buy and sell a ton of tubes you might be able to justify a TV-7 or a Hickock 539. We use a TV-10 I bought 25 years ago for $50, as we go through a dumpload of tubes here. But all those testers do is exactly what the device you plug the tube into does when you measure it with your DMM. You actually get a more accurate evaluation of a tube’s performance by putting into the piece of gear you want to use it in and seeing if it pulls the right voltages. Then give it a listen. If it measures right and sounds right, it’s a good tube.

Q: Where do you see DIY audio going in the next several years? Are the best days ahead or behind?

A: I’m all for new fangled technology if it sounds better. In terms of construction for the newb, I don’t think it’s something worthwhile to get into at first. Point to point wiring is just plain easier to understand and implement.

…DIY audio is like everything else. It will continue to evolve and the best days are yet to come. I caution people not to get caught in the whole “Golden Age” nostalgia thing. Sure, there were some very nice pieces of gear made back then, and Heathkit, Dynaco, Eico, et al. were a huge influence upon me to get into this stuff. They set a standard to be broken by newer designs…

…One thing that thrills me is the number of millenials getting into hi fi through headphones and also into hi res media through vinyl. That is exactly what needs to happen to get a new generation inspired to keep improving the breed.

In conjunction with /r/diysound (where I also help out), there will also be another round of AMAs this summer. Hopefully some will be tube-centric again this year. If you have ideas for companies or designers to reach out to, I’d love to hear them.

Finally, Doc B was an awesome sport with this interview and really shows his colors with regards to the DIY hobby. If you’re looking for a place to start with DIY tubes, consider Bottlehead’s kits.

Note: I have no commercial interest in Bottlehead and received nothing in return for this post. I just like the smell of what Doc is cooking.

Hackaday published a great series on transistors


Related image

Hackaday, a great blog focused on all things DIY, has been doing a short series on transistor configurations. In much the same way that I try to focus on basic tube know-how for beginners, Hackaday decided to do something about the “relative paucity of education with respect to the fundamentals of electronic circuitry with discrete semiconductors.” Needless to say, I’m head over heels.

If you like tinkering with electronics, especially audio circuits, you must read the three part series:

The Common Emitter – see analogous tube circuit the Grounded Cathode Amplifier

The Common Base – see analogous tube circuit the Grounded Grid Amplifier (touched on in the Cascode page)

The Emitter Follower – see analogous tube circuit the Cathode Follower

As you’ve hopefully found from reading some of the projects on WTF Amps, it pays to have transistor theory under your belt even if you like building with tubes. They’re ubiquitous and useful little guys for all kinds of helpful circuits.

Another batch of boards

Another small batch of prototype boards for projects has arrived. I’ll test these as soon as I find the time to stuff and test the last batch of experiments…

dc heaters
Simple 6.3V to 12V regulated heater board (LM7812 based), good for phono builds
mini booster
A miniature DC booster board with on-board USB charger for battery power B+
opamp phono
A simple opamp phono preamp powered by a couple of 9V batteries just because

Radial-updated Dynaco ST-70 is now shipping

According to AudioXpress, the redesigned Dyanco ST70 is finally shipping. You can read AudioXpress’s article here. You can also read the thoughts of the lead engineer on the Dynaco website here.

The ST70 is an iconic amplifier for very good reason. Radial Engineering now owns the Dynaco name (Hafler as well) and is reviving some of the classic pieces of gear from these brands. The new ST70 ditches the old pentode front-end in favor of a grounded cathode feeding a cathodyne, feeding a grounded cathode driver. Classic Williamson:


According to Dan Fraser (lead engineer, Dynaco)

During the listening phase of testing process, we found the sound quality of the pentode-triode design to have the lowest sound quality. Still good – but not as good as the other two. When comparing between the triple triode [12AX7 feeding 12AU7 LTP] and the quad triode [12AU7 Williamson] the mid and high end seemed indistinguishable. Both were excellent. However the quad triode with the Cathodyne phase splitter was felt to have a punchier low end.

With the high level of negative feedback, more stages proved better sound. It was also better behaved with regards to balance, according to Fraser. The long-tail-pair version had trouble maintaining balance (though it isn’t clear whether constant current sinks or negative voltage rails were used to maximize the tail).

Fascinating stuff, doubly so because I’ve been working on a large push-pull project recently. By “working on” I really mean collecting parts, obsessing over minor details, and waiting for free time to start cutting wood and punching holes in aluminum plate. I’m planning an Allen Wright-inspired designed with only two stages. Rather than the all-tube cascode Wright employed (successfully), it will use a shunt cascode hybrid for maximum nerd. Hoping for material updates to this project soon!

The Nuvistor and Bob Katz’s Audio Blender (via Inner Fidelity)

Bob Katz has been writing a series of articles over at InnerFidelity for several years and they’ve recently taken a turn down a more experimental path. His most recent article details a device that mixes a transparent solid state signal and a Nuvistor signal biased to provide a distortion spectrum with just a small percentage of second harmonic. Check out his write up here!


A Nuvistor is a small metal and ceramic tube released by RCA just as transistors began supplanting vacuum tube technology in most electronics. They are a true vacuum tube with familiar triode operation and characteristics and an indirectly heated cathode. The most common Nuvistor in consumer electronics was the 6CW4 (high Mu) though there are several triode flavors and even a couple of tetrodes.

nuvistor cutaway.png

Because they were originally intended for radio and TV usage, Nuvistors enjoy very good bandwidth, low noise, and high gain (high Mu variants).  The metal envelope is integrated with the basing and the tube plugs into what RCA dubbed the Twelvar base. You can probably guess how many pins that had. With the Nuvistor, RCA also introduced the RCA Dark Heater, a lower temperature filament that guaranteed higher stability and less AC leakage. Despite this innovation, most Nuvistor heaters require around 1W to light (e.g. 150mA @ 6.3V).

The 8056 used in Bob Katz’s project has the following characteristics:

8056 characteristics.png

And the following very respectable plate curves:

8056 plate curves.png

With a modest Mu, low plate resistance, and very low B+, it’s no wonder Bob decided to marry this interesting tube to a solid state partner for his Blender. The 8056 heater requires 6.3V at 135mA. At this voltage and heater requirement, it’s close to being practical for modern portable devices. In their heyday Nuvistors were used in battery-powered and efficiency-critical applications like the US Space Program and military radios and communications equipment.

Would I ever build something with Nuvistors? It’s tough to say.  I’ve been on a casual hunt for tubes that might be suitable in a portable battery-powered application. Other candidates are the Korg Nutube or the sub-mini 6088. Like all things in this hobby, there are trade-offs. The Nuvistor 8056 heaters are hungry relative to these other options, but the other characteristics are very attractive. In all likelihood, I’ll try them all eventually. This is why I DIY.

Top 10 tips for PCBs on EasyEDA

I’m a believer in point-to-point construction, but not because I think it necessarily sounds any better. Point-to-point is simply the quickest and easiest way to try a new circuit. With tubes, high voltage caps, and through hole resistors, building without boards is straightforward (once you have some understanding of properly grounding circuits). When incorporating TO92, TO220, heatsinks, etc, a PCB starts to look much more appealing.

If you’ve been following my adventures, you’ve seen that I’ve been experimenting with PCBs recently. Most of my schematics are drawn in EasyEDA. EasyEDA is part of the JLCPCB family, where you can quickly import and order boards (you can also export Gerber files for purchasing elsewhere).  Based on my experience so far, and the advice of a cool dude name Matt, here are some thoughts and tips for working with EasyEDA/JLCPCB:

  1. Take advantage of the grid/snap spacing for layouts: I set my grid to 125mil (1/8″), the snap to 31.25mil (1/32″), and the alt snap to 12.5mil.eda grid
  2. Create m3/4-40 mounting holes: I make my holes 125mil (1/8″)
  3. Make all connections on one side of the board (where possible)
  4. Use the reverse side as a ground planephono.png
  5. Don’t be shy about creating your own parts (this is the PCB Lib function)!
  6. Don’t use standard TO92 packages; use TO92 ammo package footprintsto92
  7. Enlarge holes for wire-to-board connections and make the pads generous where you can
  8. My preference is to keep tube sockets off boards to prevent footprint issues, board flexing, and keep heater wiring flexible
  9. Avoid parallel tracks for anything carrying AC (signal or power): see Doug Self for good reading on coupling
  10. Add multiple hole connections for film caps (options for multiple radial and axial types)film cap

April/May has been a whirlwind in the WTF Amps household, but we should be returning to actual building soon. I’ve got about 30lbs of transformers and aluminum plates impatiently waiting for some actual hobby time to shake out of my schedule. Big boy mono blocks.