SB-200 Manual (PDF)
New valves in SB-200
I'm just beginning to appreciate the amplifier, SB-200, put out in kit-form and built by thousands of hams (10s of thousands? how many kits were sold?). The amplifier is a 1kW PEP unit that can reasonably put out 600W for your signal. That's a step up for barefoot rigs, like mine.
There are a only a few expensive parts. Most are inexpensive to replace.
The most expensive part I've found is the transformer. Harbach sells them for $270 each plus shipping (which is considerable given the weight of the item).
The next most expensive component is probably the 572-B tubes. A matched pair will run about $100 on RFParts.
It's difficult to say which is more expensive to replace -- the meter, or the front-panel, etc.. The components themselves (Resistors, capacitors, etc.. ) are probably easy to find. The coils and air-variable caps are probably not so common, so I'm not sure on their availability. Every once in a while they show up on Ebay.
Harbach sells an array of kits to upgrade your SB-200. Fan, soft-start, soft-key, power-supply board system, etc... Beyond those upgrades, the parts to repair your SB-200 could be found from local or catalog-oriented electronic part houses.
Wiring the SK-201 in requires re-arranging some original components already in your SB-200. Namely, the 33 ohm resistor from the Amp Relay jack is moved.
I removed it and re-positioned it rather than just moving one of the leads. It put less stress on the leads of the resistor and makes it look a little cleaner, in my opinion.
Solder carefully, and have some solder-wick handy to remove excess solder on the lugs to help wiggle in the new connections.
After installing the new power module:
Wider view of Power Module
The new fan is under the tubes (new fan and fan-motor modification)
After all the inside-work is done:
And, boy does it sound and run great now. Just a whisper of the fan so I can still hear weak stations w/o having to strain my ears. The HV runs much more steady than before at just under 2.4kV.
Pictures of the Soft Key and Soft Start mod installed (Before the new
fan motor was installed):
High Resolution Picture 3008x2000
Clockwise from lower left corner:
There are two ways to tune the amp.
Some of the old books on amps for SSB used to tell ya to load the amp heavy for better linearity, and over the years of playing with these things I have learned the value in doing that with some of the amps. The SB200 would be a good candidate for this to be done. If you're going to be using it on SSB, you can tune for max output and then increase the loading just a bit more and re-peak the tuning. The output will be down just a bit but in the SSB mode the peaks will be going just a bit higher. You'd need a scope to see that though. And since you have one I'm sure you're going to be playing around and looking at all of that.
- The old fashion way of dipping and loading. Starting with the loading control at minimum, you apply drive to the amp in transmit mode and with the tune control you tune for a dip in the plate current. That dip indicates resonance. Then you increase the loading with the load control which causes the plate current to raise some. You then re-dip the plate current always stopping with the current at the deepest part of the dip. You repeat this process until the bottom of the dip reads the value that it is supposed to be loaded up to. If the manual indicates that it is to be loaded up to 450ma then the dip should bottom at 450ma. And on that amp you will have to drive it pretty close to the 100 watts input to make it load properly.
- The second way is to watch the relative power output meter and tune for max output. And what the manual is trying to say is that you adjust the sensitivity so that the meter stays on scale and is not pegged full scale so you cannot see the peaks. It doesn't matter where, just as long as you can see it peak.
It doesn't matter where the knobs point on the dials. The band marking are an approximate indication and depending on the antenna you are loading into may or may not be right. If the antenna is a reasonable load, they should be pretty close. But always either tune for the peak on the output or the dip on the plate current.
Now I can key the amp safely with the Rig PTT switch and the amp's Soft-Key modification.