I am going to describe my first tube amplifier project, a KT77 tube amplifier with hybrid fixed and cathode bias.

What to name my first tube amplifier project? After some deliberation I went for: “Project 1”

Project 1 is an integrated 20W push-pull tube amplifier, based on Menno van der Veen‘s UL40-S2.

I implemented some tweaks and design changes here and there. For example, I tuned some of the component values to better suit the KT77 tubes and to my taste, I redesigned the physical layout completely, it features a tone control and a μController to monitor and control various aspects.

I will introduce the design in a series of posts. I will describe my experiments and results. And to make it more interesting perhaps, I will share some of schematics and photographs.

I will give you a quick preview below.

Based on the UL40-S2

The UL40-S2 tube amplifier kit is since 2018 no longer for sale, but the construction manual is still available through Menno’s website. I considered that if I can no longer buy it, I should build it myself. So I did… and there my journey started. That involved sourcing (for me often uncommon) components, designing the PCBs and getting them milled, soldering, testing, adjusting and enjoying the whole effort.

I am really impressed by the amount of research that Menno invested into the design. He tested and evaluated many configurations and published quite some improvements over time. In addition  Menno assessed quite a list of modern tubes and scored them on quality within the UL40-S2. Furthermore, I like the toroidal output transformers (OPT) that Menno designed and sells through his website.

I selected the Gold-Lion Genalex  KT77 tubes that work very well in the UL40-S2. Furthermore, I use a hybrid cathode/fixed bias with balancing of the quiescent currents between both tubes and I included an electronic choke filtering B+.

After some A/B listening tests, I decided to use the configuration that the Menno describes as “Super Triode“.

Finally, I implemented a tone control. You might wonder why I did that, I’ll get to that later.

Further inspiration to build my tube amplifier

John Broskie inspired me to implemented a quiescent current balancing. Furthermore, I got inspired by Max Robinson and valveradio.net and quite a few others.

I’ll discuss that all in later posts. Stay tuned and look out for posts in the Project 1 category.