YOUNG@MARLBORO.DEC.COM (01/09/86)
I have some comments on upgrading an SB-220 for 1500 Watts output. My first comment is "Don't do it". An SB-220 is good for about 1000 to 1200 watts output, maybe a bit more on 80 meters. It's not likely that anyone would be able to hear the difference if you went to 1500 watts. Almost the entire SB-220 is "flat out" at it's current power. To rebuild it for more power, you would have to look at the following: 1. The Power Supply The transformer in an SB-220 is the acknowledged weak link. Most of the serious failures of this amplifier I know of (and I know people who run *lots* of soup from an SB-220) are due to transformer overheating. The diodes are also marginal, with no capacitor/resistor protection. And it's a voltage doubler. The power supply capacitors may be usable, but with more current drawn from the supply you will have more voltage drop, and more hum. 2. The Output Amplifier If you want to add another 3-500Z, you will have to remove the partition between the amplifier and the power supply. You will also have to redo the cooling, probably adding another fan to cool the new tube. The plate impedance will have changed, requiring replacement of the Pi-network coil, and possibly the fixed shunt capacitors. The coil mounted on the plastic form might not be up to the extra power anyways. While you're at it you might want to check the little open frame T/R relay under the chassis, which might not be up to the extra watts. Note that it runs on DC from the transformer (there is only one transformer, it supplys B+, filament, bias, and relay voltage, and you won't want to run the filaments from a transformer far from the amplifier). The input circuit impedance will be lower with 3 tubes, so you will have to rebuild the input network too. 3. The 4-400 Driver Adding a 4-400 to get enough drive would be a major undertaking. The existing input network would have to be redesigned, and connected to the 4-400. I assume you are running it grounded grid, otherwise you will have to neutralize it, and provide a screen supply. The output impedance of this stage is several thousand ohms, and the input of the finals is under a hundred ohms. You will probably need a Pi-network here, complete with a bandswitch and tune and load controls. Going with an 8877 should eliminate the need for a driver stage, because it has greater power sensitivity (higher gain) than the 3-500Z. But to use it you will have to replace the power supply, rebuild the input and output networks, and completely redesign the cooling, since a ceramic metal tube of that shape will require a blower and a chimney. There won't be a whole lot left of the original SB-220, and of course, you will be QRP until the project is finished. If you really want to be able to run 1500 watts, you would be better off building an amplifier from scratch. That way you won't be barefoot until you finish the amp, and when you are done you can sell the SB-220. I think that would end up costing less than modifying the SB-220, since the parts you remove from it won't be worth as much as the whole unit. Also building from scratch will avoid having to make layout or other compromises based on things Heathkit did, and will allow you more use of the surplus market because the parts won't have to fit in where the old parts were. By the way, if you are seriously considering the 8877, you might want to consider the 3CX1200 instead. It has several advantages, such as lower cost and instant warm-up, and is quite capable of running 1500 watts out. Paul, K1XM --------
jhs@mitre-bedford.ARPA (01/11/86)
To be precise, 1500 watts versus 1200 is slightly less than 1 dB, whereas one "S" unit is 6 dB (except on receivers with inflated S-meters). The stations you work will DEFINITELY not be able to hear the difference, nor see it on their S meters. -John S., W3IKG