This photo shows the input power shield, constucted from 1.5mm cold galvanized, mild steel. It is utilized to minimize radiated EM fields from the 220VAC power input. It was formed using a bending brake. The edges are sealed to the case structure using conductive copper shielding tape.
The Wakefield Heatsink chosen is a type 483. This is a non-standard part with the same width and depth dimensions as the type 486 and 489, however available as a special order, being 3.25 inches in length, versus, 6 and 9 inches respectively. Using this heat sink, oriented properly as shown, each output device, IRFP240 (X2), IRFP9240 (X2), when dissipating 62 watts, will have a temperature approximately 20'C > ambient, please see graph.
Parts list and schematic diagram of First Watt F5 Class A audio amplifier used with the permission of Nelson Pass.
The basic cabinet is an Aluminum, 3U, 19" rack mount style, which was modified to meet design criteria. With a hinged top cover it is now oriented with the short axis as the front and rear of the amplifier. The FET output semiconductors are connected to the R and L channel PCB using flying leads, 16 AWG stranded wire. The lengths of each 3 wire bundle being identical to minimize any distributed inductance and capacitance differences, any such differences could lead to amplifier instability. The 4.7K thermistors are also mounted outboard and secured to the surface of each of the FET's
This page details some of my amplifier and speaker projects. I also provide a service to repair audio and sound reinforcement equipment.
Teaching guitar technique