Heathkit SB-220 Modifications

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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 of the 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
Meter damage can be avoided by placing a pair of diodes across the terminals on each meter.	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.	Zener diode replacement circuit
Switching relay Modified to prevent filament transformer melt down in the event of a filament to grid short	Before
	After
New parasitic suppressors
New parasitic suppressors
To avoid transceiver-relay contact pitting and eventual failure, add a 100 - 200 ohm, ½-W resistor.	Nicely modified chassis
Using the grounded grid approach	Grounded grid close-up
On the right is a home-brew version of the . 'Soft-Key' circuit. This simple addition to the SB-220 allows it to be keyed by most modern transceivers.