This amp is the quintessential triode amp. It just does its job, period. It is honest and accurate and sweet and will drive a surprising number of commercial speakers to acceptable levels. If you use the best available parts and materials in constructing it (as well as stash away a few tubes for a rainy day), it will pass down to your children and continue to wow 'em in the next millenium. - J.C. Morrison (1993)
A direct coupled driver stage and an all low mu triode array are the specific reasons why this 300B SET design works so well. Good audio output transformers are the key to really outrageous performance. Recommended are the Magnequest FS-030 audio outputs by Michael S. LaFevre. The First Series "030" (FS-030) is specifcally designed for use in sophisticated low-to-medium power single-ended circuits. For best performance you will want to used the best audio output transformers you can get your hands on. It is not necessary to use Western Electric 300B vacuum tubes. Current (new) production 300B tubes like the Electro-Harmonix 300B and the JJ 300B sound wonderful at a fraction of the Western Electric 300B price. Stamou has built this amplifier with Lundahl LL1623 audio output transformers. To see his build, visit the DIY 300B Single-Ended-Triode (SET) Hi-Fi Amplifier project page.
The schematic for the J.C. Morrison 300B Single-Ended Vacuum Tube Amplifier is shown below. Using audio output transformers with a primary impedance of about 3000 to 3500 ohms you can expect about 8W of output with at about 2.5% THD. See
The power supply use a 5U4-GB tube rectifier and a C-L-C filter. There is more information about power supply design with tube rectifiers in the Power Supply Design for Vacuum Tube Amplifiers article.
Stephen (sampleaccurate on the forum) put together a 300B SET amplifier following the above direct-coupled 6SN7 driver / 300B SE amplifier schematic. Photos of his build are shown below and additional information is available on the forum in the First DIY amp FINISHED - 300B SET - works! thread.
6SN7 / 300B Single-Ended Triode (SET) Tube Amplifier
6SN7 / 300B Single-Ended Triode (SET) Tube Amplifier - Rear View
6SN7 / 300B Single-Ended Triode (SET) Tube Amplifier - Underside View
The transformers and choke are all from Hammond. The power tranformer is a Hammond 278X (800V CT @ 230mA + 5V CT @ 3A + 6.3V CT @ 6A). The choke is a Hammond 193J (10H, 200mA, 82 ohms) and the audio output transformers are Hammond 1627SEA. Additional photos and information is available on the forum in the First DIY amp FINISHED - 300B SET - works! thread.
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