The Heathkit SB-220 -- a two 3-500Z amplifier -- made a notable impact in the world of amateur radio when it debuted in 1975. It was the first reasonably priced, intelligently designed, SSB-CW, HF amplifier sold to the amateur radio community.
Regrettably, the amplifier kit is no longer being manufactured. Over the ensuing years, the SB-220, which has some excellent design features, has seen numerous modifications.
This page is a collection of images and links dedicated to those who have made a good product even better through creativity, resourcefulness and technical expertise.
Improving the SB-220 (QST)
Since its introduction in 1975, the venerable Heathkit SB-220 has been continually upgraded and modified by its owners.
A search of the QST archives will reveal numerous excellent technical articles specific to these modifications. For more information, please refer to the following QST articles: Nov. 1990, p27; Dec 1990, p41; Feb 1979, p20; Apr 1979, p27; July 1979, p51; Nov 1979, p57. In addition, CQ magazine Sept 1992, p 46.
The following images show a few suggestions referenced in the above noted articles -- some in both a before and after state.
Upgrade of the R3 (0.82 ohm, 2W) resistor on the diode board Before After Switching relay -- Modified to prevent filament transformer meltdown in the event of a filament to grid short Before After New parasitic suppressors Before After Diodes to save the gear Zener diode replacement circuit
Meter damage can be avoided by placing a pair of diodes across the terminals on each meter. Grounded Grid Using the grounded grid approach
Closeup of the grounded grid. Ever wonder why you can't get the cabinet back on the chassis? The wide part goes to the front of the amp. Move more air over the tubes with an alternate style of fan for more efficient cooling. Note the black paint on the cabinet walls surrounding the tube bay -- this reduces the infrared radiation heat being reflected back into the tubes from the bright aluminum cabinet. Nicely modified chassis.
To avoid transceiver-relay contact pitting and eventual failure, add a 100 - 200 ohm, ½-W resistor.