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 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.

For the GI-7B and GI-7BT data sheets click on the links.

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 it is much more rugged 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. I replaced the six filter capacitors with modern units of 220 uF instead of the original 125 uF, but I had no improvement under full load conditions (0.5 A plate current).

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-W4ZT'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 simple logic circuit to control a small relay thus avoiding the presence of -120 V on the relay input jack.

The SB-230++ can produce a maximum output between 500 and 550 W in CW mode, depending on the band, with 60 W excitation. Slightly more could be obtained as PEP power in SSB operation. Clich here for a sample tuning chart. Notice that I did not test the unit on 80 m since I do not operate on that band.

THE SB-230++ STEP BY STEP

		The socket is extremely simple. It is made from three squares (70 x 70 mm) of PC board: one double-cladded board (a) is used for the grid and the other two (single-cladded) are used for the cathode (b) and filament (c). All 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 to ensure the correct spacing of the boards. The picture 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 11 mm width. The strip is inserted in the hole in board (a) and then soldered to the copper foil. To have the correct alignment while soldering you must wrap tightly the finger stock ring on a wooden dowel of the same diameter as the tube grid ring (36.3 mm), temporarily 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 was the soldered in place on the copper side of the board (b) using the GI-7BT tube as a jig. The copper is etched around the board periphery to avoid a short with the metal spacers and the screws. The filament board (c) is mounted back-to-back with the cathode board, the copper foil being on the opposite side of the cathode. A simple contact clip obtained from a fuse holder is used for connection with the tube filament. The socket allows easy insertion and removal of the tube. The clip support is made from PCB stock soldered in place. Filament supply wires are directly soldered to the copper foils.
		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 I used it as a tie strip under the chassis. In this picture 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 side 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.
		Chassis bottom view. The original wiring of the 8873 socket is discarded but the socket is saved to serve as tie strip. The hole on the left corner was drilled for the filament and cathode connections of the GI-7BT. A new tie strip will be installed using the same screw of the existing one on the left to support the filament and cathode chokes.
		Input circuit layout. The green and yellow wires carry the filament current. The added tie strip supports the bifilar filament choke (RFC3 in the original schematic) and the cathode choke (RFC4). The other choke leads are soldered to the 8873 socket. The two black electrolytics are part of the 12 VDC power supply that uses the 6.3 V filament winding and a voltage doubler circuit. The 12 VDC are used to power the logic board and the two muffin fans.
		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 by a 6.3 V #47 lamp powered by the 6.3 V filament winding of the main transformer. The 6.3 V 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 antenna relay. The SB-230++ can be controlled by any modern PTT line since it uses CMOS logic, 12 VDC and virtually zero current. The A contact of the relay is used to ground the -120 V of the original PTT line thus switching the antenna relay. The other contact switches the GI-7BT cathode line. The board also contains the bias zener diode for the tube (27V cathode-grid bias). 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 ;-)).
		Capacitor Bank The new filter capacitors (220 uF 450 V units) are mounted on a PC board under the chassis. The capacitors protrude above the chassis through the mounting holes of the original capacitors.
		HV Cap Board The filter capacitors board seen from below. Six 3W 100 kOhm equalizing resistors (R3-R8) are soldered directly to the copper foils. A ceramic tie strip supports various components: R9, 1 Ohm plate current shunt (black left); R25, 0.82 Ohm grid current shunt (white left); R11, 680 Ohm and two protection diodes. The black heath shrink tube on the right protects the three 1 MOhm resistors of the HV divider (R12-R14). R15 (100 kOhm) is on the right.
		Old and New Improved technology allows filter capacitors with better performances in a smaller package.

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