|The socket is extremely simple. It is made from three squares
(70 x 70 mm) of PC board: one double-cladded board is used for the grid and the other two (single-cladded)
are used for the cathode and filament plates.
The square plates have central holes of appropriate diameters for the tube elements (38 mm for the grid
plate, 8 mm for the cathode and filament plates) plus
4 mounting holes (4mm dia.) drilled at the vertices of a 60x60 mm square. Hex
tapped spacers are used for the assembly and keep the correct spacing of the boards.
The figure shows the various components of the socket.
The grid contact is ensured by a finger stock ring. I have used a strip of Cu-Be finger stock of 12 mm height. The strip is inserted inside the hole for the grid and then soldered to the copper on the square plate. To have the correct alignment while soldering you must wrap tightly the finger stock ring on a wooden cylindrical form of the ssame diameter as the tube grid ring (36.3 mm), also inserted inside the grid hole. The reason for using a PC board instead of a copper or brass plate is that soldering is much faster and easier since a copper plate will act as heat sink for solder. However a metal plate would be better for tube cooling.
|The cathode contact is a short piece of brass tube. The tube was machined to fit snugly
around the cathode contact (18.3 mm. diameter), with 8 saw cuts giving the tube some elasticity. The brass tube has been soldered
on the copper side of the second plate. The copper was etched around the plate periphery to avoid a short with the
metal spacers and the screws. The plate with the filament connection is mounted back-to-back with the cathode plate, the copper
foil being on the opposite side of the cathode. A simple contact clip from a fuse holder is used for connection with the
tube filament. The socket allows easy insertion and removal of the tube.
|A paper template is used to mark the holes for the socket on the right side panel of the amplifier.|
|The socket is installed temporarily to check tube clearance. The old 8873 socket was not removed and will be used as a tie strip under the chassis. The plate choke is still in its original place but will be moved to make room for the cooling fan.|
|Another view of the socket. Four large holes have been drilled in the support panel to allow for free circulation of cooling air in the filament and cathode area. A copper braid provides a low-impedance path from the grid to the chassis.|
|Top view of the tube and cooling system. A muffin fan (80x80 mm, 12 VDC) cools the tube anode. Be careful that modern fans with electronic speed control (recognizable by the four-wire connection) can be stopped by the RF field when transmitting! Better to use an old classsic 2-wire fan. Another smaller muffin fan (the type used to cool CPUs) is installed on top of the socket to extract hot air from the cathode and filament area. This second fan is crucial otherwise the temperature of the tube in that region can easily reach more than 100 C even without HV. The plate choke has been moved to accomodate the fan.|
|The 12.6 V filament transformer for the GI-7BT is a surplus Stancor P-8130 with the primary wired to the 117 V tap on the power transformer primary. I used the two brown leads originally going to the dial lamp, which is now replaced with a 6.3 V #47 lamp powered by the filament winding of the main transformer. The 6.3 filament winding is also used to provide 12 VDC via a voltage doubler and a 7812 regulator to power the logic board and the fans.|
|The Logic Board is an interface between the relay jack and the internal relay. The SB-230++ can be controlled by any modern PTT line since it uses CMOS logic. The board also contains the bias zener diode for the tube (27V cathode-grid). To handle the tube current the zener diode drives a PNP 2N6051 darlington transistor mounted in the holes of the original power zener diode (TO-3 case). With this bias the resting current of the tube is about 40 mA. As a tribute to the original SB-230 the 2N6051 is insulated by a (toxic) BeO washer ;-)).|