Mono Banded, 10 meter only, Heathkit SB-220 unpowered walkthru only. This one all the Ham bands except for 10 meters have been removed on both the Input and Output, and the Band switch has been replaced by a variable Tune cap for the input. Hence, you Tune the Bandswitch for the lowest Input SWR. This amp has the small Harbach relay ‘Soft Key’ PCB and the Harbach HV/Bias PCB. I originally thought the HV/Bias PCB had soft start on it, but looking at the Harbach site just now as I am writing this, that HV PCB updates the HV diodes, Bias, and Metering, but it does not have soft start on that board and this one doesn’t seem to soft start at all. However, this amp uses a voltage doubler for the HV and the original cap bank doesn’t have a ton of capacitance anyway. Hence, though Soft Start is a good thing, with this amp being a voltage doubler and being somewhat underpowered anyway, it doesn’t really need soft start. With 500z amps with more robust power supplies like Henry or Drake, I think soft start is almost a must. SB-220’s are decent KW amps, but try to do more and then you will start having problems. Underpowered transformer, just adequate capacitor bank, just barely adequate Tune and load cap voltage rating. Weak voltage doubler circuit, especially with the underwhelming cap bank. Undersized wiring and no cooling for the tube pins. IMHO for this to be a good powerhouse amp, all this needs to be addressed.
