Circuit: DIY Compact Tube Amp

Building a tube amp is a dream for many engineers and makers alike. But factors like the cost, large enclosure, transformers, and complicated wiring harnesses can scare even the most ambitious away. Back in 2014, Michiel Ter Burg developed a compact solution that was featured in the pages of Elektor Mag. He proved it is possible to build a small tube amplifier with a few watts of output power per channel using ordinary PCB mount power transformers as the output transformers.
 

Tube Amp Circuit

Refer to the tube amplifier schematic. As Ter Burg explains, “The audio input signal on connector K1 is fed to input transformer TR2. The secondary of this transformer drives the two triode sections of the tubes in opposite phase via resistors R8 and R12.”
  “The anodes of the pentode sections of tubes V1 and V2 are connected to the primary winding of transformer TR1 (an encapsulated PCB-mount power transformer), which is recast as an output transformer here. The output signals from the triodes are fed to the control grids of the pentodes via networks C3/R3 and C5/R16. The two secondary windings of TR1 drive the loudspeaker.” Refer to the article for additional information about the circuitry.

The PCB

Refer to the PCB layout for a single amplifier channel. Two must be assembled for a stereo amplifier. Ter Burg notes: “After you have assembled the amplifier board and the power supply, you can adjust the quiescent current and the balance after letting the tubes warm up for a while. As already mentioned, you can start with a quiescent current of 25 mA per tube (250 mV on K5) and adjust the balance so that the currents through the two tubes are exactly the same. Then you’re ready to start listening to the amplifier."
  "The author obtained the best results with the two output windings connected in parallel and a 4.7-kΩ resistor in the feedback path, as indicated on the schematic. With various speakers (either box enclosure or panel type, and regardless of the impedance) it sounds like what you would expect from a tube amplifier: good detailing at low volume and smooth overdriving at high signal levels. Although the measured bandwidth at full power may be somewhat disappointing, in practical listening situations you don’t miss anything because the bandwidth is fairly large at lower output levels, extending to above 10 kHz.”

More About the Tube Amp

The article associated with this tube amp project appeared in Elektor Jan/Feb 2014. Elektor Members have full access to Elektor’s library, which includes the article.