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Build an EL84 Tube Amplifier Class – San Francisco, by Ed Yang

Ed Yang | Published on 11/25/2019

Ed Yang will be teaching a build your own tube amplifier class at the Randall Museum in San Francisco.

By popular demand I will be teaching a second session of the Randall IV amp for the tube audio DIY class at San Francisco’s Randall Museum. Beware that this is not your run of the mill EL84 stereo amplifier. The goals of the amp is sound as close to a live performance as possible, keep the cost down, minimize component count so that it will be easier to build, must look pleasing both inside and out.

The topology of The Randall IV employs 6SN7 tubes for the input/voltage gain stage and for the cathodyne phase inverter. The power stage uses EL84 tubes in a push pull pentode configuration. This allows for a simple design that has a minimal parts count and thus easier to build. The 6SN7 tube was chosen because they are still in production; there are plenty of serviceable used tubes and NOS on the market. The 6SN7 is known to have a smooth midrange and low distortion. The EL84 tube was chosen because it is still in production and inexpensive. They have a reputation for solid bass and sound ok. For me the EL84 has annoying treble and less than stellar midrange. I decided to put the two tube types together to see what happens. Lucky enough the combination worked, the amp have excellent definition and warn timber.

Voicing of the amp was done by first by implementing the topology using the intrinsic character of the tubes. To tailor the frequency response and control distortion a simple filter network is used for the global negative feedback system. To further refine the sound select resistor, capacitors and transformers were sourced. I’m a firm believer that “boutique components” are mainly hype and not worth the money. Most of the parts are sourced from Mouser, eBay and Heyboer Transformers.

The tone of the amp is based on my experience attending the San Francisco Opera and opera recitals. This form of music is produced without sound reinforcement equipment. With opera I have a true reference for how I want my equipment to sound. No music reproduction system I’ve ever experienced can come close to an opera or recital. This amp is my best effort to capture as much of the tone and presence that acoustic instruments and voices have to offer.

Randall IV has been through several “shootouts” with well-regarded vintage and contemporary EL84 amps. On every occasion the Randall IV was the preferred amp. In closing the Randall IV is not for everyone because of its limited power output – around 14 watts. For a reasonably efficient speaker system, this amp exceptional.

Details for those interested in taking the class and building the Randall IV:

Randall Museum 199 Museum Way, San Francisco, CA 94114.

10 two and a half hour Saturday sessions from 1/11/20 to 3/14/20.

Parts cost (less tube set) is $375.

SF Rec & Park class fee is $370 (museum member) $380 (non member).

Registration and details:

Be warned that building the Randall IV will require concentration, dedication and dexterity. This class is good for students that have intermediate or advanced electronics projects skills. If you are a beginner please contact me before taking the class.

Email me (Ed Yang If you have question about the class or Randall IV amp.