Single Tube SB-230 Retrofit Using the GI-7B
by K4POZ, 2008
 

The GI-7B is an ideal retrofit tube for the Heathkit SB-230. The GI-7B can be installed as a conduction cooled retrofit but we don't recommend that retrofit because of the lower cooling efficiency than that of the fan cooled tube with the original cooler attached. Our modification with the conduction cooled anode with the fan on low is better that the original SB-230 cooling but it's still not as good as cooling the stock GI-7B with the fan on high (full 12 VDC). The heatsink on the original SB-230 was marginal for the 8873 tube with its 200 Watt plate dissipation. It is more marginal with the GI-7B and its 350 Watt plate dissipation. It is much better to use the fan and the standard GI-7B anode. Even if you undertake the conduction cooled retrofit we recommend that you be sure to install a small fan to keep some air flowing around the base of the tube.

The previous retrofit that was done with two tubes mounted vertical and the one tube mounted vertical made it apparent that there is no benefit to using two tubes in this amp or mounting the tube vertical. It is for that reason that all our retrofits will be done with one tube only and mounted horizontally.

This is not provided as a kit. There are NO kits available from K4POZ or W4ZT. The pictures below pretty well detail the necessary steps to accomplish this retrofit. If you can not understand what is necessary to do the work, do not undertake this project. We do not mind answering reasonable questions but if you don't understand how this amplifier works, you need to study up on it before you even think about taking on a project like this.

(click on the small pictures to see a larger picture in a new browser window)
1 - In this picture you see where the old grid circuit is removed. A hole is drilled for the connections that go up to the new GI-7 socket.
2 - Here a terminal strip is mounted for the connections up to the new GI-7 socket.
3 - Install the new filament choke, the cathode choke and the input coupling capacitor. Connect cap up to old alc circuit.
4 - Install a terminal strip and voltage doubler components for the 6.3VAC filament to the 12VDC supply.
5 - Completed input circuit: Filament choke, input, cathode choke, 12VDC and input load resistor bank.
6 - Using a template we provide to drill the holes for mounting the new GI-7 socket.
7 - Tube socket is prewired for filament connections and .01uF capacitor across the filament pins. Be sure to note which wire is connected to the cathode side of the filament. Also note the two ground lugs added on the socket grid plate. These lugs are used to add ground straps to the chassis of the SB-230 to complete a low resistance RF current path. The side of the SB-230 is not a good RF conductor back to the chassis so the straps are required.
8 - Tube socket is shown here with the ground straps in place. It is very important to install these grounds.
9 - Move the plate choke to the center wall as shown. Add the glitch resistor before mounting the fan. In this picture the original plate choke has been replaced with one made by K4POZ. In this picture you will see the conduction cooled version of this retrofit. The large solid anode cooler is used in place of the factory cooler. The tube should still have air blown around the base to keep it cooled properly.
10 - Move the plate choke to the center wall as shown. Add the glitch resistor before mounting the fan. In this picture the original plate choke has been replaced with one made by K4POZ. In this picture you will see the conduction cooled version of this retrofit. The large solid anode cooler is used in place of the factory cooler. The tube should still have air blown around the base to keep it cooled properly. The fan shown here is a 12VDC fan. If using the conduction cooled option, use this fan and power it from the low voltage center of the voltage doubler if the noise bothers you. The fan will run at lower speed yet still move enough air to help cool the base of the tube.
11 - Mount the 12VDC fan using double sided tape on the back panel and on the chassis. Add a little hot glue along the bottom edge if you like.
12 - Here you see the W4ZT bias board mounted against the rear panel. A small relay for soft key is mounted against the back panel. A terminal strip is added to the center wall. This terminal strip is used to mount the cut off resistor, bypass capacitors for it and the bias board leads to the T/R relay operate contacts.
13 - Pre-wire the filament transformer and terminal strip. This is a modified transformer from Marlin P Jones. Note that the secondary center tap is NOT used. When powering an indirectly heated cathode tube, the cathode connection should be made through a cathode choke connected to the side of the filament that is connected directly to the cathode.
14 -Mount the filament transformer and terminal strip on the forward wall separating the RF section from the front panel.
15 -This is the filament transformer with a series resistor in the primary. With the MPJ transformer and added turns, the voltage measured at the tube socket with the tube in place should be 12.6VAC. An adjustable resistor can be used here. The typical value is around 25 to 35 Ohms. A 25 Watt resistor should be considered a minimum.
16 - Install the filament transformer on the front side of the wall between the plate tank circuit and the front panel. This is a modified transformer from Marlin P Jones. Install the step start (soft start) circuit (a good example is the SS-100 from Harbach Electronics) or a small 12 Volt relay and step start resistor on the inside right side wall.
17 - Another view of the new filament transformer and the step start.
18 - Filament transformer primary connections are made where the meter lamp 500 Ohm resistors were connected to the terminal strip beneath the meter. The meter lamp is connected to the 6.3VAC from the original transformer.
19 - Note in this picture that the GI-7B tube has the original cooler installed. This is because the GI-7B is cooled more efficiently by the original cooler with a small fan blowing on it than by using a conduction cooler.
20 - Note in this picture that the GI-7B tube has the conduction cooler installed. The 12VDC fan is in place to cool the base of the tube.
21 - Here are meter shunts, multipliers and protection diodes mounted on a terminal strip beneath the chassis.
 
 

 

 

 

 

 

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