Brief Hiatus

It’s been a couple weeks since my last post because personal and professional life has been extremely eventful. I’m happy to say it’s all good things, but it will be a little while until I’m caught back up with hobby time. There’s a very appealing light at the end of this tunnel: more room!

In the coming year, I’ll be setting up a new workshop space from a blank canvas. At this point, I’m split between a crowded basement for assembly and a cold garage for woodworking. I’m very excited to setup something purpose-built for the hobby at hand. My own little studio!

Along with the work space, I’ll be able to setup a proper listening room without constraints on speaker size or placement. I’m very mindful of WAF and toddler-proofing now, so I’m looking forward to a little more freedom in the messes I make and experiments I try.

There’s plenty of solder left for me to sniff, so don’t think the recent radio silence is anything but a short break while I deal with real life stuff!

Amplifier design I admire

I’m working on a headphonesty.com article about pretty tube headphone amps and I thought I’d share some of my favorite visual designs here as well. This is a small collection that inspires me. I’ve spent a couple weeks blowing up MOSFETs in the current project so the inspiration is much needed!

Specimen Products 300B Monoblock
Toolshed Amps Transcendance Integrated Amp
Decware Zen TORII Stereo Push Pull
Eddie Current Balancing Act Headphone Amp
Thomas Mayer 45/2A3 Stereo Amplifier

The upcoming article will focus on headphone amps but there are so many beautiful speaker amp designers that I felt compelled to share the above as well.

A (gorgeous) Papa Rusa in the wild!

I just received some pictures from builder MG of his Papa Rusa amplifier. He’s used some beautifully figured wood to build a striking enclosure for his Papa Rusa 6S45Pi parafeed amplifier. It has a cool instrumentation vibe to it with the subtly angled front panel and exposed transformer and heatsinks.

Great build, MG!

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!

Bad Hombre tube headphone amplifier in the wild

I recently received some pics from W of a Bad Hombre Mk2 build (12AU7 + ECC99). This one uses Lundahl LL1676 input transformers and LL2765 output transformers, and features a regulated B+ and a loaded front panel of controls. Very nice!

The LL2765 is a fine transformer with ECC99 output tubes in this circuit. It has a 5k primary and multiple output taps for 32, 150, 600 ohm headphones. The 5k primary will result in more power than the 8k in the original design, but it can also be used to reflect a higher impedance depending on what taps and headphone load one chooses (e.g. 300 ohm headphones on the 150 ohm tap for 10k load).

On a side note, I see Lundahl also now has a LL2774 3k primary headphone output transformer (16, 64, 300 ohm secondary taps). This is a very similar turns ratio but it looks like the LL2774 is available with a bigger gap in single-ended configuration for higher current output tubes. Great to see new options for headphone output transformers!

Multiple B+ voltages from a single transformer winding

For the current project (a line stage with added phono) I needed more than one B+ value. The difference between the two voltages I wanted and current drawn was too large for a single high voltage rail and a filter or regulator to drop the lower rail to the correct value. So I looked for ways to add a voltage doubler to a standard bridge rectifier. Turns out, there’s more than one way to skin a cat:

The “Millet Doubler” is detailed here and uses a single center-tapped winding. The entire secondary is rectified via a bridge rectifier, rather than the usual approach of grounding the CT and using a full-wave rectifier. The center tap voltage is then rectified and feeds the lower half of a stack of capacitors.

The “TubeLab Doubler” is something I found posted on diyaudio.com; it is also discussed in depth here. This one uses a single winding without a center-tap. The doubled voltage rail is somewhat lower than what you’d get in the Millet Doubler, but still potentially useful especially with inexpensive isolation transformers.

I can only find a schematic of the “TubeCAD Doubler” (no discussion), but if you’re familiar with TubeCAD’s blog, it doesn’t look too unfamiliar. See a good article on multiple power supply voltages here. This one looks a little bit like a combination of the other two variations.

In the end I went with the second version because it allows me to use an isolation transformer (and because I found it before seeing the TubeCAD one). Of course a couple of wiring oopsies are being worked out before I can report back on the power supply or preamp it is intended to feed…

More local feedback

I’ve been on a bit of a feedback kick lately, researching both for projects and to add a page to the website about the topic. Local feedback, in particular, has piqued my curiosity (which is usually fixated on triodes and open loop circuits).

A tube’s grid is an inverting input (with output taken at the anode). This makes it a natural point to loop feedback from the anode. TubeCAD covers exactly this topic in-depth in Partial Feedback. But, my favorite variation on this type of feedback uses a P-channel FET in the configuration shown above. The FET does several nifty things here:

  • Provides low impedance fixed bias to the tube grid via the source, allowing for A2
  • Provides high impedance input for the preceding stage
  • Defines the feedback/gain in the circuit with source resistors R2 and R3

DiyAudio alias SpreadSpectrum provides details on a push-pull build using this circuit on his blog here.

E-Linear: an interesting kind of feedback

Oops, I bought another pair of output transformers. This iron is 60W rated push-pull with a 6k6 primary, so the natural pairing would be 6L6GC.

I’ve used the 6L6GC in the past (see the Luciernaga). Because it is common in guitar amplifiers, it’s an easy to find tube both new and vintage. While it’s no power-house in triode mode, the 6L6GC is quite capable in pentode or ultralinear.

Operating the output tubes as pentodes means higher output impedance and distortion. The Quality Amplifier from a couple weeks ago avoids the issue entirely by operating the outputs as triodes. The other approach, which is actually more common today, is to sacrifice some of the circuit’s gain to bring output impedance and distortion back down. That’s called (negative) feedback.

The question is how do we want to apply it?

This post won’t get into the nitty gritty of feedback (that deserves its own page on the website), but there are generally two prevailing approaches. Global feedback is what we see most often; this takes a signal from the output of the amp and wraps it all the way back to the input. The Williamson amplifier is the quintessential example of this.

The second approach to feedback is to use local loops. These affect a circuit just like global negative feedback, but are isolated to just a stage or two. Local feedback, because it involves fewer stages and phase shift, is more stable than global negative feedback. That means they’re generally simpler to employ.

The circuit above is a variation of what seems to be unofficially referred to as an E-Linear stage. Feedback from the output transformer primary is applied via the ultralinear taps directly to the load resistor for the input stage. This local feedback drastically reduces the output impedance of the 6L6GC.

The input stage is commonly a pentode because the high plate resistance is a benefit here to applying feedback. In this case, the input stage is a hybrid cascode, which still has a high “plate resistance” owning to the MOSFET upper device. That also gives us more options for the lower triode tube.

I like the simplicity of this circuit quite a lot. In Class AB, it looks like a good 25W should also be available with a pretty modest B+ (or a little less in Class A). Seeing as how I’ve got the iron on hand, I hope to give this one a test at some point!

Nelson Pass has a tube design available

It’s no secret that I admire Nelson Pass both for his design skills and for what he gives to the DIY audio hobby. Unfortunately, us vacuum tube enthusiasts are mostly left out in the cold when Pass flexes his design muscles. That is until Burning Amp 2017 when he presented a pre-amp using the Korg NuTube.

Ok, so it isn’t the first “tube” that comes to mind when we think thermionic emission, but hey, it’s got a vacuum at least!

The Korg NuTube is a twin “triode” made by adapting vacuum florescent display technology to audio applications. Just like a DHT, it has an anode, a grid, and a directly heated cathode. The principals of operation (i.e. emission from cathode to anode modulated by a grid voltage within a vacuum envelope) are essentially identical to the little glass bottles we all know and love.

However, the low-voltage miniaturized technology requires certain compromises. With a max dissipation of only 1.7mW, the NuTube is limited in the maximum anode voltage and this forces positive grid operation, requiring a buffer to drive the inevitable low impedance. Like wise, the high plate impedance also necessitates a buffer on the output for most applications.

This is exactly what we see in Pass’s design, using his signature CCS-loaded JFET follower style buffer stage at the input and output. The result is a very compact and relatively low-voltage tube preamp with around 16dB of gain (as designed). Best of all, you can pick up a board and parts at the DIY Audio Store here!

I had the opportunity to hear this preamp not long ago in a nice system (Magnepan speakers, tube and solid state amplifiers) and compared it with an Alex Cavalli designed tube buffer and a MOSFET-based preamp. The Pass preamp sounded fantastic in this good company.