Craig J. Coley
$500 – $1,000
This project is the Coley SET125 Mono-Block Amplifier. It is a one channel single ended triode (SET) amplifier using a Soviet era directly heated GM70 triode. It can easily produce a continuous output power in excess of 20W while operating full Class A at low distortion. The sound of the amplifier throughout its power range is warm, clear, and transparent; an excellent choice for driving a bass-compensated open baffle speaker.
The goal of this design was to produce a high power Class A SET amplifier that could easily drive a compensated open baffle speaker to high volume. The Soviet GM70 and Chinese 845 tubes were considered good candidate tubes. The NOS Soviet era GM70 was chosen because of its ability to operate easily at 100W continuously, thereby allowing a Class A output approaching 25W peak. The GM70 with carbon anode can usually be acquired on eBay for around $50.00 from the Ukraine, including shipping. Four GM70 and four 6N8S tubes were ordered so that multiple tubes manufactured at different times could be tested. The ultimate goal is the construction of a pair for stereo operation. It was decided from the beginning to use 6-8dB of negative feedback to compensate for transformer overshoot and speaker load dumping that is common at low frequency. The power supply voltage required for the GM70 is 950V, easily lethal, and construction should only be attempted someone who is familiar with the safe operation of high voltage circuits.
A Hammond 1628SEA was chosen for the output transformer, a Hammond 278X chosen for the power transformer, and Hammond 187F20 chosen for the GM70 filament. For power supply filtering, a 6 Henry Triad Magnetics C14X was chosen due to its low cost and 1500V rating.
A black Hammond 12x8x3 steel chassis was chosen due to the weight of the magnetics and it was believed to represent about the smallest enclosure that could be used for an amplifier of this type. 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 magnetic can be mounted in preparation for wiring. Neoprene grommets should be used for all transformer wire entries due to the high operating voltage. All power resistors greater than 5W are metal and screwed to the chassis to dissipate heat. A 15W voltage divider from the 950V supply is used to power the 6N8S preamplifier stage. This method was chosen to both stabilize the 950V supply as well as provide a rapid bleeder for stored energy when the power is turned off. The power supply capacitors are 220 microfarad, 500V aluminum electrolytics and the capacitor connections are covered with a clear plexiglass cover 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, feedback prediction and stage gain to be determined for the design power level. While both cheaper and more expensive magnetics were available, the availability, modest price and performance of the Hammond products made their selection easy. While somewhat uncommon in an SET amplifier, a modest amount of negative feedback was used that helped to flatten the gain and compensate for magnetic energy stored in the output transformer and speaker system. The measured performance was within 1dB from 20 Hz to 20,000 Hz and could produce greater than 20W continuous power (the limit of my power meter) at 1.25% THD.
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 builders might also desire a larger case for packaging ease and this is recommended wherever practical.
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.
Qty(1) Hammond 1628SEA.
Qty(1) Hammond 278X, P/N 122-240.
Qty(1) Hammond 187F20
Qty(1) Triad Magnetics C14X
Qty(1) Soviet GM70 Triode
Qty(1) Soviet 6N8S Dual Triode (or 6SN7), P/N 072-845