Schitt is getting into DIY, except they’re definitely not

Pic from Schitt product page

If you’ve missed the recent hubbub, Schitt is launching a line of drink coasters. It just so happens that these coasters double as unsupported DIY projects with scarce documentation.

The schematic shows a 6418 sub-miniature triode direct connected to an AB push-pull transistor output stage (cap coupled). Power is a blistering 30V to the tube and 15V to the output stage, cleverly derived with a bridge doubler and regulated with LM317s.

Schitt is hedging the product with extra coy advertising (AKA the Schitt Shtick) and reinforcing in several places that they are not a DIY company. Hence the product is a coaster, not a miniature hybrid amplifier. It will come at no surprise to Schitt when DIY documentation is created by early adopter hobbyist communities, I’m sure.

Some of the quoted specs:

  • Frequency Response: not terrible, but not exciting (like 10-100K, -1dB or so)
  • Power Output: much less than anything else we make (like, less than Fulla 2, maybe 400mW into 32 ohms, all in, 10% THD or so)
  • THD: about 0.5% at 1V RMS (6418 tubes) or about 1.5% at 1V RMS (6088 tubes)
  • IMD: didn’t bother measuring, this amp ain’t about measurement
  • Output Impedance: about 8 ohms (yes, 8, not 0.8, not 0.08), in case you didn’t get the memo, this ain’t a high-performance amp

What is it about Schitt’s non-committal and self-effacing copy that gets so many people so excited?  Why did I just buy a small lot of 6418s? Why are coasters already on the way to me?

Schitt, you sly dogs.

Letters to WTF: is tube linearity just a matter of lower gain and higher operating voltage? Or it is inherited with the medium – vacuum vs semiconductor?

Generalizations are difficult to make, but you can look at the theoretical mechanisms of operation for some interesting nuggets.

In a tube, current is transferred between anode and cathode by the space charge in a vacuum. Child’s Law states that current in a vacuum is directly proportional to anode voltage (to three halves power) and inversely proportional to the distance between electrodes (squared). The speed of electrons depends solely on the applied voltage.

In semiconductors constructed of doped sandwiches of semiconductor material, Child’s Law doesn’t apply. Here we use the Mott-Gurney law. This states that the current density in a semiconductor is directly proportional to anode voltage (squared) and inversely proportional to the thickness of said material (cubed). The speed of the electrons depends on both the electron mobility of the semiconductor (assumed to be constant) and the applied voltage.

There are extra constants for calculations in either case, but notice the similarities. In both cases, we can assume distance between electrodes (whether separated by a semiconductor or a vacuum) doesn’t change. The difference here is the power for the voltage term. The generalization is that current density through a vacuum is less affected by changes in anode voltage than is current through a semiconductor.

Is this the theoretical mechanism to explain the heuristic that vacuum tubes are more linear than transistors? Maybe…at the very least it’s an interesting observation. In reality, geometry, application, and other factors matter, aside from just materials. Some tubes are more linear than others just as some transistors are more linear than others. There are bad ways to bias and operate tubes just as there are good ways to bias and operate transistors.

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.