Designer:
Craig J. Coley

Project Category:
Amplifiers

Project Level:
Advanced

Project Time:
20+ Hours

Project Cost:
$500 – $1,000

Project Description:
This project is called the Coley W210 and is a very small “executive” tube amplifier. It is a two-channel, 10W Williamson circuit with ultra-linear output transformers. Each channel uses one 6SL7 dual triode driving a pair of self biased 6V6 output tubes. The THD of the amplifier has been measure at less than 0.25% at 1W. Frequency response at 1W has been measured to be 20Hz to 28KHz ±1.0dB at 1W. The design includes internal Bluetooth 4.0 and RF remote control. A small key fob remote control operates a motorized Alps volume control and the power relay. The miniature front panel meters are incandescent backlit and indicate audio level with 10W at 0dB.

Design Goals:
The project was an extension of my W225 amplifier and was intended to be a small amplifier for my desk at the office. To reduce the component count, the dual 6SN7 preamplifier and driver tubes of the W225 were replaced with a single high gain 6SL7. While the design is my own, it is certainly inspired by the pioneering work of David Hafler at Dynaco in the 1950s.

Tips & Tricks:
Since the design was so similar to my earlier W225, the same wooden panel prototype was modified and used to prove out the W210. It was also tested with the same Heathkit variable supply and General Radio power meter.
Once the design had been proven robust, a new schematic was created for the stereo version. Even though there is a small risk of cathode stripping, limited size dictated the use of solid state rectifiers. The small chassis also includes the same excellent Bluetooth 4.0 module and power relay board used on the W225. Due to limited space, however, a custom remote volume control board had to be designed and is based on a miniature Adafruit RF receiver and key fob transmitter. A Bluetooth receiver in a tube amplifier is unusual but the board used in the W210 is surprisingly good and it allowed for convenient play from a variety of streaming services like Spotify.
For the mechanical work, a 2D CAD drawing was created for the chassis which aided tremendously in producing a symmetrical and pleasing finish. Prior to drilling, the chassis was covered with vinyl tape to prevent scratches while performing the required mechanical work.

Conclusion:
The amplifier performs very well and is my daily amplifier on my desk at the office. The direct cost of materials is around $450.00 and direct labor for construction was 40 hours. The most difficult aspect of this design is its size; it is very difficult to hand wire and I would highly recommend a larger case.

About the Designer:
This project was designed and constructed 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.

Parts Used:
All transformers were ordered directly from Edcor. Resistors under 5W are carbon film, resistors 5W and above are ceramic wire wound. The design was tested with NOS American 6SL7 and 6V6 tubes as well as Soviet 6N9S and 6P6S tubes; all types performed well. Soviet tubes are installed in the final version and were used for all testing. The chassis is a stock Hammond steel model with walnut sides. 400V poly capacitors were used for coupling and Nichicon aluminum electrolytics were used for the power supply.