The Randall IV: Building a Tube Amp

Randall IV EL84 tube stereo power amplifier

There are many reasons that San Francisco is a special city. Here's one more: the SF Parks and Recreation Department offers a "build your own tube amp" class!

The class is one of a series taught over the past 10+ years, with previous sessions that took on other tube-based electronics. Apparently an outgrowth of a "tube audio club" that used to meet at the Randall Museum of Science, Nature, and the Arts, the class is taught by Ed Yang, a retired engineer and long-time tube audio hobbyist. Nine students (including yours truly) were in the class, which ran on ten consecutive Saturday afternoons.

The Randall IV stereo amp is the product of 4 years of design and tweaking by Mr. Yang. It uses four EL84 power tubes, running in push-pull mode, good for ~12 watts per channel. A pair of twin-triode 6SN7 tubes serve as the input stage (one acting as the phase inverter, the other providing voltage gain). A single 5AR4 tube provides AC-to-DC voltage rectification. Transformers were sourced from EdCor.

In addition to providing the design, all the parts, and supervision on our builds as they progressed, Mr. Yang also provided lectures on the basics of tube amplification, covering voltage rectification, feedback, the various amplifier classes (A, AB, B, D), and related topics.

Before we all completed our amps, Mr. Yang hosted a "tube roll" event, where we could hear what various different tube brands sounded like in the completed amp. We all readily heard differences. The consensus that emerged was for JJ tubes, made in Slovakia, as the best cost-for-performance option for the complete tube set. But the nod for best-at-any-price went to Psvane EL84s and the Cossor 6SN7 (only room for one of those, given the tube layout, with preference for it as the voltage gain tube). Various 1950s and 1960s vintage American tubes also made a strong showing.

How much did it cost? Tuition was $310, with parts adding another $375. The cost for the tube complement depended on what you wanted, but after some critical listening, I ended up choosing Psvane EL84s ($170 for a matched quad), JJ 5AR4 ($18), Cossor 6SN7 ($59), and a vintage Raytheon 6SN7 ($15). Total out-of-pocket cost: $950. (I also bought various extra tubes to try out that I didn't end up using, which added about another $150.) In addition, I probably spent ~30 hours building the amp. But it sounds surprisingly good, and the pride of building it myself is priceless. While ~$1,000 is a lot of money for an amplifier, given the parts quality and the careful choices made by the designer, I'd say it's a bargain for the ultimate product.

For now, the amp goes into my dining room system, driving a pair of Falcon LS3/5a speakers (a 15 ohm British BBC-spec'd location monitor classic). 

Ed's tube roll set-up!

Nearly complete build