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Heathkit SB-230 Linear Amplifier
I purchased my SB-230 in 1976 for $339.95. and used it through 1989. Its been sitting on a shelf, unused, since then. With my limited antenna capabilities, I decided that I needed a little more power for those rare and weak DX stations so I dusted off the SB-230 and prepared it for return to service. Much to my surprise the SB-230's triode output tube, an EIMAC 8873, had passed on to those great electrodes in the sky. "No problem", I thought. The original Heathkit parts list had these tubes replacement cost at $50.00 each. I figured they would be possibly a little more, but not at the sticker-shock discovery of $640.00+, nearly twice the amount of the original purchase of the SB-230 itself! (And people thought Krugerrands were a great investment in the 70's.) The EIMAC 8873 is a compact high-mu triode for use in zero bias Class B or AB amplifiers. Its designed for conduction cooling and is nominally rated for 200 watts of anode dissipation. A beryllium oxide thermal link is utilized in the SB-230 to insulate the anode from the heat sink while allowing for heat conduction from the anode to the heat sink. An electrical equivalent, more modern replacement for the 8873 is the 3CX400A7 manufactured by EIMAC and Svetlana. Unfortunately, the 3CX400A7 is forced air cooled, which requires mechanical modification to the SB-230 for operation. The street price of the 3CX400A7 ranges from $400.00 to $335.00, more within budgetary constraints and potential for extending the life-cycle of the SB-230. This page is an in-progress description of the modifications to the SB-230 to accommodate the 3CX400A7 and bring the SB-230 "back-to-life". I am not a real advocate of gross modification of collectable "boat-anchors", yet financial frugality dictates this approach. Modification Considerations As previously mentioned, the SB-230 is conduction cooled. Modifying the unit for forced air cooling requires removal of the rear-mounted heat sink, and idenfying the proper fan for forced air cooling, along with design of the air plenum for the tube. The 3CX400A7 will handle 400W of anode dissipation, whereas the 8873 only 200W. This parameter allows some margin as long as the original operating voltages and currents of the SB-230 are not exceeded.
SB-230 Top View, RF cage upper right 12/19/05 -- I really haven't had time to continue a technical write-up; however I did sucessfully complete the modifications. There is really no electrical modifications needed (except perhaps to replace aged components - and the blower power). Below are photo's of the finished product. The unit can be restored to the original tube and configuration - and a reminder (after blowing a few grid fuses...) is that the SB-230 only needs about 5-10 watts drive for full power out.
There is also information available at http://www.nd2x.net/N8GPQ.html for another modification using the Russion GI-7BT tube. |
