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IZ5PQT's SB-230++

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My SB-230++ is a GI-7BT retrofit of the original Heathkit SB-230 linear amplifier. Several retrofits with this tube have already been presented on the Web, so there is nothing original in this one. I love the SB-230 and, needing a basic linear, I decided to extend its life with a tube transplant.I decided to document my project on the Web hoping that it will be useful to other fellow hams. I wish to thank K4POZ for his friendly advice during the planning, construction and testing phase of my SB-230++.

Useful links to SB-230 retrofit projects are listed here.

The SB-230++ uses the russian GI-7BT ceramic triode. This tube can be purchased cheaply on Ebay and other Web stores. The GI-7BT has 350 W anode dissipation, so for this application is much more robust than the original 8873 EIMAC triode (200 W). Although K4POZ successfully demonstrated a conduction-cooled GI-7BT I felt safer with forced air cooling. Of course this requires a fan and fans mean noise but I find that the noise can be tolerated.

In the SB-230 the power output is basically limited by the HV supply. The voltage doubler provides about 2200 V (=800 x 1.41 x 2) under no-load conditions. However when fully loaded the HV drops to around 1800 V. This is probably due to a loss of capacitance of the original eletrolytics after 30 years or more, so the advice is to replace them with new ones. This job is the next item in my to-do list, and I will document it on this page once it is done.

I wanted to minimize deviations from the Heathkit design, so I kept the original input circuit and the filament and plate chokes. However I decided for a DC ground of the tube grid, instead of grounding the B- side of the HV supply as in the original design. This helps in suppressing potential parasitics. This requires some small modifications to the plate and grid current metering circuits. In this I follow closely K4POZ's schematic.

Finally, the original SB-230 had a PTT control that is no longer suited to modern solid-state exciters. I therefore inserted a small logic circuit to control a small relay thus avoiding the presence of -120 V on the relay input jack.

Click for larger image 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.

Click for larger image 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.

Click for larger image A paper template is used to mark the holes for the socket on the right side panel of the amplifier.
Click for larger image 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.
Click for larger image 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.
Click for larger image 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.
Click for larger image 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.
Click for larger image 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 ;-)).

This retrofit does not longer exist. It was an interesting exercise. But as soon I got a good 8873 I went back to the original configuration with conduction tuning and no blowers.

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