Having recently moved and had a second kid, tubes should be the last thing on my mind, right? Maybe. Then again,personal hobbies should always have a place in day-to-day life, and especially when that life gets a little chaotic. Such hobbies are about exploration and continuously challenging oneself. And yes, sometimes a little bit of dorky obsession.
Hence, with all my tools in boxes and precious little spare time, I have a great opportunity to revisit quick, cheap, and cheerful tube design (at least this is what I tell myself).
Last spring I posted about a sleeper tube that looked to me like it had potential in a simple linestage. The single triode has modest B+ requirements, low Mu, and respectable transconductance, exactly what we might look for in a bare-bones capacitor coupled preamp. To boot, the tube is called 6AF4. AF. How could I resist?
Here are the plate curves, highlighted where it looks pretty good to me:
It looks like we could work with as low as a 100Vdc supply here, but plate resistance appears lower and more consistent if we have 150Vdc on tap. With 150V B+, a 7.5k loadline passes right through the middle of my target zone of operation, intersecting the y axis at 150V/7500 ohms = 20mA. A 220 ohm cathode resistor looks like a good place to start and should allow a dozen or so milliamps through the triode. The result, fleshed out, looks something like this:
Back in my original post on the topic, I mused about 48V switching supplies wired in series to generate a B+. That’s a monkey I still haven’t gotten off my back, so I drew up a little PCB based on the XP Power VCE05 module. This is a $12 encapsulated AC/DC converter that puts out 100mA+ at 48V. Three in series gives ~150V in a very compact footprint and at a cost comparable to a small Hammond EI transformer.
Although a pair of 6AF4s will only need 25mA or so, a pair of these boards would (hypothetically) be capable of powering a small power amplifier (300V @ 100mA). It’s worthy of some exploration.
Although all my parts and most of my tools are still in boxes, my household move is complete! It will still be a while before I’m spending free time in the workshop, but it’s never too early to daydream a bit about how I’ll set things up.
I have a blank slate of dry, mostly bare, concrete and studs/joists to work with. Aside from a sump in the corner, I’ll be able to utilize all 15ft x 10ft for my own storage and working space. Coming from space that was split between a garage and a shared basement, the 150 sqft is palatial. That said, my goal is to use it as efficiently as possible.
Amp Work Zones
Taking a cue from kitchen design, I’m planning the room layout by functional zones. Specifically, building tube amps involves three key processes: chassis work, electronics work, and parts/materials storage. Each process is a Russian nesting doll of other steps and tasks, of course, but these three general areas represent unique workflow challenges that are more or less shared by the sub-tasks that make them up.
Messy in terms of generating dust, shavings, waste
Space has high potential to be used in non-hobby activities
The woodworking-focused area would benefit from mobility. Rooms adjacent to the workshop can be used as ad-hoc work space for projects that don’t fit in the smaller shop. Keeping tools and workbenches on casters will also let me rearrange as the work or materials require. Additionally, being able to moves benches helps with dust cleanup.
This will probably involve a 8ft+ bench for assembly with an integrated miter saw bay. Space under the bench will house a shop vac (dust control) and extra power tools and tool boxes. My drill press will live on a separate cart, as will my router table, for maximal flexibility.
I need to figure out a good way to deal with casters and the unevenness of basement floors.
Benefits from flexibility in lighting and seated/standing work
Equipment is stationary and shelf size, but numerous
Results in many small parts used simultaneously
I’m planning to reuse the t-track chassis cradle I had in my last workbench. This was one of the best ‘tricks’ I picked up for building and working on projects. The rest of the space above the workbench will also be very valuable to keep power supplies, variacs, and equipment close at hand while working on a project. The worktop itself should have some kind of padding.
This will likely also be about an 8ft long bench. I do not see a lot of benefit in making this a mobile bench due to its specialized nature and the manageable size of projects. Owning a CNC is one of my long-term hobby goals, so the extra long bench may come in handy (though it kind of breaks the ‘zone’ philosophy).
I’d like to come up with some kind of solution for keeping component parts organized while they’re out and being used for a build. This could be as simple as having tackle-box organizers on hand.
Parts and Materials Storage
Materials typically include long lumber and extrusions and plate under 18″ square
Component parts are numerous, but physically small and commodities
Tubes and transformers vary widely and some are fragile/valuable
The space has shelving built between studs that will help with storage for some things (mostly paint cans at the moment). Storing long materials high on the wall in a rack makes sense. Aluminum plate could be stored on edge without worrying about it deforming. The numerous component parts, tubes, and transformers, present an interesting challenge though.
In the past I’ve kept tubes in Rubbermaid drawers, but I’ve found that I’ll often forget about what I actually have on hand. Curing that will probably require some way of organizing tubes in a single layer, preferably vertically to conserve floor space. I have had my eye on french cleat style storage walls and am thinking about ways to adapt this to small parts and tubes. The modular nature of it has some potential benefits.
First things first
Before I start cutting too much wood for benches and storage, I plan to give the whole room a good scrubbing and a fresh coat of paint. Other rooms in the new basement have a coat of epoxy that I find to be very appealing, so I will likely treat the floor to make it easier to clean and generally more attractive. I’ll also switch out the bare bulb light fixtures to shop lights and have some additional outlets put in on their own circuit.
Yes, there’s lots to do in the new workshop! I plan on building a simple tube project or two in the meantime. Refreshing my memory of what it’s like to start out without a bunch of tools is probably a great exercise in and of itself!
As I mentioned in a couple recent posts, my workshop is in boxes while moving. Good news is that those boxes are finally in the new space and just need the shelving built to unload them.
In the meantime, here are some pics of a Muchedumbre build by someone else! As you can see, PB has a very tidy build on a compact footprint. He reported no issues with the simple circuit in our email exchange and says it sounds fantastic! I love the way he’s matched it with the speakers and other DIY components. Very attractive.
I hope to be back posting about my own adventures again soon. First step is to measure the new space and start laying out the shelving and workbenches for the shop…
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!
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!
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.
Here’s another stunning build by another DIY tube hobbyist, this time of the Sofrito Preamp. Beyond the great chassis work and wiring, Joshvito has an awesome write-up on his build in his Imgur gallery here. He has a lot more progress pictures than my page, so this is going to be a great reference for others!
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.
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!
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!
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…