Craig J Coley
This project is the Coley 215SE Stereo Amplifier, a 15W per channel Class A single ended triode (SET) amplifier using triode connected Tung-Sol KT-150 beam tetrodes. The sound of the amplifier throughout its power range is warm, clear, and transparent, an excellent choice for driving open baffle speakers.
The goal of this design was to produce a zero negative feedback single-ended Class A amplifier with enough power to drive a variety of speakers. The Tung-Sol KT-150 was selected for the output tube because of its 60W plate dissipation and good linearity when wired as a triode. The use of a triode connected tetrode has two distinct advantages, (1) 550V plate voltage compared to 1000V for traditional triodes like the 845, and (2) the indirectly heated cathode requires less mitigation of hum. A Tung-Sol 6SN7GTB was chosen for the preamp tube due to its long reputation as an excellent dual triode preamplifier. A modest amount of cathode resistance was used with each section of the 6SN7GTB to stabilize gain.
A pair of Hammond 1628SEA was chosen for the output transformers, a Hammond 278CX chosen for the power transformer, and a Hammond 193J chosen for power supply filter choke. Hammond magnetics are conservatively designed, widely available and modestly priced.
A Hammond 1441-32BK3CWW 17x10x3 walnut side steel chassis was chosen because it can support the heavy transformers and has good appearance. Greenlee knock-out punches were used for all tube socket holes and transformer wire entries. A 1:1 2D CAD drawing was made for all sides of the enclosure which aided greatly in packaging and pre drilling the small chassis. Printouts were made of the drawings, taped to the sides of the chassis and then used as drill guides for all hole centers.
Once all holes are predrilled in the Hammond chassis, the tube sockets and transformers can be mounted in preparation for wiring. Neoprene grommets should be used for all transformer wire entries due to the high operating voltage. The power supply capacitors are 220 microfarad, 600V aluminum electrolytics and mounted horizontally for safety.
The design of the amplifier relied on experience and vacuum tube models available for LTSpice, a free SPICE simulator. SPICE simulation enabled resistor value optimization and stage gain to be determined for the design power level. Since plate dissipation is not an issue in SPICE, the KT-88 model was used in place of the KT-150 with no notable discrepancies. Not shown in the schematic was a low cost remote volume and power control board that was added as an afterthought. These are available from a wide variety of sources, including eBay and elsewhere.
Tips & Tricks:
The construction of this amplifier is straightforward but should only be attempted by an expert builder who is familiar with the safe handling of high voltage. Most of the parts are available from a wide variety of sources, including Parts Express. Trophy plaque companies can make custom metal plaques to label tube types and controls at the builder’s discretion.
The amplifier worked to expectation the first time it was powered up and required very little tweaking, a testament to accurate vacuum tube SPICE models. The performance is outstanding with warm sound, good bass and delicate highs. The power reserve is also outstanding, with enough punch to deliver good bass even with inefficient speakers. The cost of this amplifier will run around $1100.00 as shown for the parts alone.
About the Designer:
This project was designed and built by Craig J. Coley of Burleson, TX. I work as an electrical designer and am listed as inventor on 8 US patents in the field of electro-optics. I have been an electronics hobbyist since I built my first Heath radio at age 7 and a ham radio operator since age 11. I can be contacted at [email protected].