Here’s a sneak peak of the long-term push-pull project. The second monoblock is one cathode bypass capacitor away from being ready for playback. A bad tester tube took out the cap on one side during testing with a bang, but I’ll have replacements soon. Until then here’s the intro of the project write up.
The monkey on your back
Everything should be made as simple as possible, but not simpler.-Einstein
There comes a time in every DIY builder’s life where he or she gets the urge to stretch beyond single-digit output power and single-ended amplification. There is no shortage of worthwhile projects to choose from: variations on Williamson, Mullard, or Dynaco push-pull topologies are easy to find discussed in forums and tweaked to compensate for modern parts. You can even find kits for something like the Dynaco ST-70.
When the double-digit power bug bit me I could not bring myself to abandon my usual no-feedback, triode output, class A comfort zone. This is the simplest (but not the only) path to good sound and my speakers are efficient enough. I’m also too lazy to do feedback math but that doesn’t mean open-loop, class A triode designs aren’t an engaging challenge. This build faced the following complications (which are common to many push-pull amplifiers):
- Class A requires healthy current in the output stage: this needs to be balanced in the output transformer to preserve inductance
- Two cascaded grounded cathode input stages is too much gain, but one stage is generally not enough
- The input stage must have low enough output impedance to drive the triode output tubes
For the most part, my solutions to the challenges strive for simplicity. As is often the case in tubes and life, simplicity in some areas is traded for complexity elsewhere. This push-pull amp has only two stages, the outputs are cathode biased, and it requires only three tubes per channel. To make this seeming simplicity possible, I used solid state helper circuits on PCBs. While these helper circuits are not technically complex, they drive up the parts count and require some measurement and adjustment.
Here is the conceptual topology for Los Monos:
Pictured is a two-stage triode output push-pull amplifier. The output stage is garter biased and the voltage gain and phase splitter stages are combined in a folded cascode long tail pair. This is all described below with a full schematic (showing lots more parts).
More of this write-up is on the way as soon as I’ve got both channels playing and glamour shots are taken!
Student “D” sent me some pictures of his Mighty Cacahuate project with a twist and it’s too unique not to share. D developed a PCB for his build and mounted all the amplifier parts to a top plate as would usually be done. Instead of a boring wooden box, D dropped this into a boombox enclosure for all-in-one listening. I love to see the creative use of a basic schematic I posted here on my little website.
We initially troubleshooted some wiring over email, mostly due to my omission of the details of heater wiring and pin numbers on the original schematic. Once sorted though, D says the amp started playing and sounding great without a hitch.
It was an amazing feeling the first time they powered on.-D
Careful there, D, that feeling is habit forming!
Update on the mono-blocks: left channel is done and right channel is coming together quickly. Write-up for the project is also underway. Looking forward to playing in stereo!
I can’t for the life of me remember a louder year than the one that ends next Tuesday. Personally, professionally, politically, it was a hectic twelve months. In retrospect, concrete and measurable goals (AKA resolutions) are probably what kept me alive and sane amidst the chaos. I’m no self-help guru or productivity genius, but I feel like I have survived a sink-or-swim situation over the past year, so I’ll share some thoughts on living with a hobby in the real world. First, I’ll pat myself on the back for my accomplishments:
Finish master’s degree:
- Done! The last year and a half were a challenge (my daughter was born spring of 2017), but I somehow found the energy, motivation, and focus needed to finish my MBA. I hope that being done with school will now free up the mental space for other parts of life (family, friends, projects).
Post to blog every week:
- 99% successful! I missed one or two weeks and not every post was laser-focused on tubes, but I’m happy that the blog and website survived a hectic year for me.
- Exercise is an important ingredient for mental focus for me personally. I also have my exercise equipment in the same room as my electronics workbench and the multi-tasking kept audio projects moving forward (albeit slowly). My explicit goal for this year was to get into the 1,000lb club (weightlifting) and I did it with some room to spare.
I’m a big fan of quadrant prioritization to keep daily life in perspective (this idea is credited to Eisenhower, I believe). The gist is that we should rank our goals as high/low importance and high/low urgency and pursue them accordingly. I draw one of these diagrams at least once a week:
If we spend our time in quadrants I and II, we’ll get the important stuff done. The hard part is understanding your goals well enough to see the difference between urgency and importance. There is plenty of subjectivity to this importance/urgency classification and not everyone would come to the same conclusions.
For me, tube projects are a solid quadrant II item and that isn’t going to change this year. However, being done with school now means that high urgency and high importance things are screaming a little less loudly. That lets me spend a little more time on long-term goals.
This is all just a long way of saying that I intend to build more this coming year. On the short list are:
- EL34 push pull mono-blocks (see pic for one very near completion)
- Transformer-coupled line stage (to pair with above, probably DHT)
And on the list of schematics-in-waiting are:
- EL84 push pull stereo amplifier
- Line stage with integrated phono
- SMPS-powered 6V6 SET headphone amp
- Battery-powered experiments
On top of the workbench and test equipment projects that builds usually create, I’ll be happy if I can finish four of the above in 2019. I’d really like to return the website to its roots as a practical resource for projects and reading for beginners in the tube audio hobby. As I told someone else recently, my goal with this is creating the resources that didn’t exist when I started in the hobby. While this may not be urgent, it is important!