The TubeLab UNSET is coming?

One of my first tube projects was a TubeLab SSE. In fact, I still use this amp as a reference whenever I build something new.

George, of TubeLab, is notorious for torturing tubes and generally just blowing crap up in his experiments. He is also a fervent supporter of DIY and frequent poster on diyaudio.com.

A couple of my recent posts have looked at ways of applying local feedback to pentodes in order to force them into more triode-like behavior. It’s funny how experiments and research in the DIY hobby community converge. Here’s yet one more example.

Yesterday George posted some exciting but cryptic experiences with a new design here. The challenge, in his own words:

[With triode strapping] the pentode takes on triode like qualities with the associated triode disadvantages, most notably the inability to pull its plate down near the cathode voltage thus limiting the available power output. Another issue that needs to be overcome is the screen grid voltage limitations of most TV sweep tubes. Wire them as a triode, and most will eventually blow up when left alone idling which is worse case for a class A amp (maximum dissipation). 

This is a great summary of the limits of single-ended triode amplification. Power is limited to single-digits by tube perveance, voltage maximums, and the ability to dissipate heat. Pentodes, able to swing outputs much closer to zero and operated with fixed screen voltages, go a long way towards solving the conundrum. But the trade-off is linearity and output impedance (which is why SETs are popular in the first place).

George goes on to tease his new design:

I arrived at a new topology that I can’t find anywhere in recorded vacuum tube history…..yes, there are several close similarities, but this is truly unique…I called this topology the Composite Electron Device for lack of a better name, since it is a composite of a vacuum tube pentode, a mosfet, and a hand full of discrete parts to create triode like curves. 

We don’t get any schematics (yet), but he finally gets to the measurements pudding:

THD was 0.197% at 100 mW, rising to 0.235% at 1 watt. 5 watts brings 0.662%, 10 watts 1.61%, with 2.48% at 15 watts and 4.04% at 20 watts clipping sets in at 20.8 watts where the THD hits 5%. 

So what is it, this single-ended not a tridoe? We know there’s some local feedback going on, but George says this is actually something new under the sun. That doesn’t happen everyday in tube land, so I’m following this one with a lot of interest.

By the way, if you want to build a traditional SET and prefer a PCB, take a look at George’s TSEII or original SSE designs!

Stacked SMPS PSU for tubes

I used a 48V switch mode power supply in the El Estudiante headphone amp and am pleasantly surprised with the quiet background and relative simplicity. When it comes to higher voltages though, you will not find many AC/DC switch mode power supplies at vendors like Mouser or Digikey. While a beefy low voltage DC supply could feed a DC/DC booster (see Millet’s 10W booster project and various eBay listings), I’d like to find something that is more easily repeated by others with parts from major PSU manufacturers.

I came across the idea of stacking low voltage SMPS supplies in a couple places and the idea intrigued me as a scalable and affordable approach to creating B+ (see here and here). The Meanwell EPS-15 48 are regulated and isolated AC/DC supplies that sell for about $8. Stacking six of these would supply 300mA at about 300V.

This schematic shows the general outline of what I’d like to try with a single-ended amplifier. The V1 supplies produce the anode to cathode voltage for the output tube. These are referenced about 100V above ground by the V2 supplies. The input tube’s B+ is a combination of the V2 and V3 supplies. All in all, this would call for seven 48V supplies, plus another low voltage supply for heaters. The final cost would be somewhat less than a traditional transformer and CLC filter, but there are more interesting reasons to try this.

All the supply gymnastics make it very easy to direct couple the two stages. In this example, Q1 is a gyrator load and Q2 sets the reference voltage. This would also allow us to drive the output tube into A2 operation. My choice for output tube here would be a 6V6: a really sweet sounding triode that otherwise doesn’t produce much in A1 operation. Power would still be low (around 2W), but that would be plenty for headphones or enough for high-efficiency speaker systems.

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.