Heathkit SB-230
With A Conduction Cooled GI-7B
by K4POZ and W4ZT
Purpose of this project:
Ron and I are working to develop a conduction cooled GI-7B retrofit for the beautiful Heathkit SB-230 amplifier. These amplifiers were designed to be beautiful and quiet but the price and availability of the 8873 tube has caused these amplifiers to fall out of grace. It is very common now to see the price of a used 8873 tube to be greater than the price of the entire amplifier. We chose the GI-7B because of the low price and the availability of this popular tube. Our experience with the tube, building sockets for it and retrofitting other amplifiers made this project a natural. We hope that we will be able to show that a reasonably priced retrofit can be done on the SB-230 preserving the quiet and the beauty with no visable external modifications and no fan noise. Our efforts will be presented here with pictures and data. Once we are done, we plan to make available the necessary components at a reasonable price to anyone that would like to update their SB-230.

April 23, 2007
The following data was collected with the GI-7B equipped SB-230 drawing static idle current adjusted to 150 mA with a plate voltage of 2000 Volts. That is a steady state plate dissipation of 300 Watts, far in excess of the 200 Watts rated plate dissipation of the original 8873.

The temperatures were measured at the junction of the anode and the cooler right at the the ceramic seal using an infrared thermometer

In each case the amplifier was unkeyed at the ten minute mark. The cool down data was collected for the conduction cooled anode only and the filament power was left on during the cool down period.

Beryllium Oxide can be a significant health hazard if it is not properly handled.

Be sure to read the precautions in your Heathkit SB-230 manual for handling the Beryllium Oxide block.
If you do not have the manual you can download one here: http://bama.sbc.edu/heath.htm

The Material Safety Data Sheet (MSDS) for Beryllium Oxide (BeO) is available here: