|The Heathkit SB-200 is an iconic amateur radio linear amplifier first introduced in 1964 with the last unit delivered in 1983 as the SB-201 (minus 10M) – a phenomenal 19 year production run. It was designed to be affordable at $200 in 1964 (rising to $479 in 1983) and to be small and light. Compared to AM amplifiers of the day, which were huge and expensive, it was tiny and provided a significant 1.3 S-units in signal improvement at a very affordable price. Even better, you could build it yourself, with Heathkit standing by to help every step of the way. This was the golden age of electronics kits. Optimized for SSB phone, a relatively new mode in that era, the designer(s) minimized weight, bulk and cost by sizing the power supply, tube dissipation and cooling capability to meet the relatively low transmit duty cycle (about 30%) and average power characteristics of SSB. With 1000 watts input into its twin T160L/572B tubes, it could reliably produce 450 to 650 watts output CW and a bit more on SSB phone, depending on band and state of tune, all on 115 volts. Designed to be driven by the adjustable, tunable output stage of a tube based exciter that could tolerate the variable input impedance and SWR of the SB-200, it was an ideal match for the Heathkit HW-100, HW-101, SB-400 and many other transceivers and transmitters of the era, most of whom used and tolerated control interfaces running at 100 or more volts. If it had any real flaws, they were that the tuned circuits and tubes were marginal on 10 meters, and to a lesser extent, 15 meters, making power output lower on these bands and causing tuning and loading to be VERY touchy on 10 meters. But lots of the tube rigs of that era had the same flaw, and this was expected. A second set of flaws was caused by the very design choices that made the SB-200 so inexpensive and light – it just could not dissipate much average power continuously. RTTY, a 100% duty cycle early digital mode using the now antique Baudot code, stressed the SB-200 to its limit and could easily destroy the tubes and damage other parts in an extended QSO at 500-600 watts out – especially on 10 meters, where the amplifier was at its weakest. Power had to be dialed back to about 300 watts and TX periods kept short. Despite this, the design was and still is elegant, simple, and effective on SSB and is easy to work on and tune due to its intelligent design, layout and minimal component count. Many, many SB-200s were built and hence many still survive to this day, all in various states of modification (good and bad) and repair.|
So why give the SB-200 two stars when this obviously was a 5-star amplifier? “WAS” is the operating word.
Back in the day, a half-century ago, when this amplifier was designed and produced, nearly every amateur was familiar with the dangers, performance characteristics, use and repair of tube equipment. This equipment has no safety circuits other than a circuit breaker or fuse and uses high voltage connections to other equipment for signals and control. That was normal, back in the day. Expectations and skills are different today, as is the age of the equipment – new vs. 40 to 50 years old. Even then, electronics were not generally expected to last more than about 10 years. Who knew you’d be looking at this review 50 years later, while contemplating purchase of a “cherry” SB-200 from the last century, at the local or world-wide-web swap meet - certainly not the SB-200 designers, or its component manufacturers.
Fast-forward to today where the majority of exciters have solid-state control and driver circuits that begin output power fold back nearly instantly with SWRs over 2:1 and expect low-voltage, low-current ALC and keying interfaces. Not a tube to be found. The SB-200 is not natively compatible with modern exciters and will very predictably damage or destroy them if directly connected, unmodified for the modern world. In addition, it is often difficult for a modern exciter to drive the SB-200 to full output due to very fast exciter fold-back in the face the SB-200s variable input impedance, so it’s best to have modest expectations, or an inaccurate wattmeter, or a poorly matched load, which can cause the same optimistic readings. And there is at least one company today that provides very affordable watt-meters that give wildly inflated readings (you know who they are). The average amateur never complains about readings that are too high, so many meter makers believe it’s better to read high than low, hence some reports of 900 watt SB-200 CW power output. That’s just not going to happen, even on its best day.
The next issue is simply age. How many cars, appliances and even people last 50 years without having issues ranging from minor to fatal. None. Zip. Nada. Same with the SB-200. If you are looking at a beautiful all-original SB-200 that has somehow eluded the well- meaning performance modifying amateur, it WILL have issues – like corrosion, dried out capacitors, bad diodes, drifting carbon-comp resistors, sticky meters, flat tubes, RF blasted band-switch. If no one has addressed these issues, it’s up to you to fix, which is great if you love electronics projects, are very familiar with tube gear and its dangers and have plenty of time and soldering skills. By the time you are done, you will have at least doubled your initial investment in the amplifier.
At a minimum, expect the following potential repairs:
1. New power supply board with new capacitors, diodes, resistors and metering resistor string. Harbach makes a drop-in replacement kit that actually improves on the original.
2. New electrolytic capacitors under the RF deck
3. New carbon-comp resistors throughout
4. New TX relay
5. New tubes
6. Input network retuning (on rearmost band switch wafer) – it often shifts on 10 and 15 meters after 40 years, reducing power output to a trickle (you’ve done this before, right?). You will probably need to order a variety of 500 volt, silver-mica caps to do this. They are not cheap.
7. New output bandswitch wafer (call Harbach or ask on the on-line forums – hard to get), often destroyed by poor tuning/loading, bad SWR match or too much sustained power output causing a cascade failure.
Then there are the inevitable “Mods” done to these amplifiers – some necessary, some poor and all pretty much undocumented. Consider that quite a few SB-200s have been converted to 6 meters (where they don’t work terribly well) or changed to use a single (often Russian) pentode power tube in a process that can never be duplicated nor reversed easily, or “converted” to cover the WARC bands at the expense of performance on every other band. It’s not that every mod is bad, in fact some are brilliant. But how do you know? The unknown “golden screwdriver” that made them has often passed on to a better place. And then there are the many safety, compatibility and stability mods discussed on-line, some of which are needed and many of which are dangerous. How do you decide? Hmmm.
Well, in general, the following mods really are needed for compatibility and safety, IMHO – but you need to decide for yourself:
1. Perform a soft-key mod to convert the 120 volt keying input on the SB-200 to a few volts at a few milliamps, letting a modern solid-state radio key the amp without damage. If you can’t “roll your own”, Harbach makes a nifty soft-key kit for the SB-200 that is a real time saver. Just do it. And its backwards compatible with older tube rigs that expect high voltage keying. If you don't do this mod, at least use an external buffer interface relay or box that does the same thing - several manufacturers make them.
2. Swap out the funky RCA RF input jack for something that is positive locking, like an SO-239 or female BNC jack, otherwise the cat and you will have a shocking and possibly smoking experience when Mittens pulls the plug on your next DX QSO. I DO NOT know what the engineers at Heathkit were thinking when that made an (audio-standard) non-locking RCA jack a high-power RF port standard for their equipment. Bad engineer! Bad! Go to bed right now!
3. If you plan to use the ALC input with a modern exciter, look up the Heathkit factory bulletin that recommends installation of an 8.2 volt zener clamping diode to avoid damage to your solid-state rig. And put it in.
Mods that may be useful and at worst are harmless:
1. “soft-start” mod that reduces inrush current (and stops the lights from dimming) when the amp is turned on. Harbach makes a nice kit for this. Not needed, but nice to have.
2. Back to back meter protection diodes. A replacement SB-200 meter is hard to find, and if an HV resistor fails the resulting voltage and current can destroy the meter. This rarely happens, but if it does, you’ll probably wish you’d sprung for the $1 meter protection diodes.
Mods that are likely unnecessary and may be problematic:
1. Replacing the power supply caps with units having twice or more capacitance. Just don’t do it. The resulting inrush current will dramatically increase, possibly popping the circuit breakers every time you turn the amp on, and certainly increasing stress on the amp.
2. Adding a “glitch resistor and fuse” to the HV line. This on-line legend will likely react too slow during a fault to prevent damage, or will cover up a more serious issue, will fail itself during an event, and will add impedance to the critical power supply to the tubes – never a good thing for performance. Your choice, though.
3. Modified anode resistors/inductors and other mods to “improve stability” and guard against VHF self-oscillation. Some of these suggestions and mod kits are harmless while others actually degrade the stability of the SB-200, whose gain falls off so rapidly above HF that its stability is quite good to begin with. Caveat Emptor.
4. Bias scheme modifications, anything that changes the tube type, or the power transformer (dramatically) and anything that adds new bands. These usually result in an un-maintainable amp, or causes unexpected/degraded behavior, or even “It’s dead, Jim” syndrome. Plenty of posts about this on-line.
So… that’s why the SB-200 gets two stars and not five, in the 21st century.
If you’re new to amps and amateur radio, consider purchasing a relatively new tube amp with modern protection circuits, solid-state exciter compatibility and ready support from the manufacturer. Even better – get one with a warranty, because you’ll need to learn how to tune and load a tube amp, and this can be an unforgiving procedure.
If you have the means, consider a no-tune, feather-weight (relatively) low-voltage solid state amp instead. Because this is, after all, the 21st century, and it will likely have a dedicated microprocessor and circuitry dedicated for amplifier/exciter protection – and yours too. It can make operating a lot more fun.
But if you really love projects, and “boat anchors” ( I do), the SB-200 may still be a decent and INFORMED choice.
Lastly, this review is my opinion only, and you need to think carefully through all the options and take responsibility for your own actions and safety.