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W7BRS SB-200 Page


SB-200 Linear Amplifier in W7BRS station

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.

Getting an SB-200

Getting an SB-200 is fairly easy -- Ebay usually has one for-sale, or you could probably find several at a local ham-fest.

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.

Fixing the SB-200

The manual (see link above) has a full assembly instruction, schematic, parts list, diagrams for as-built units. It's a resource that you will need to debug and fix up your SB-200.

First question to ask

The first questions to check are basically -- does the unit work? Does it put out 500-600W SSB ? You can tell a lot about the amp if you check the High Voltage (HV) meter, with the unit switched to HV, the HV should read about 2.2kV to 2.4kV according to the specification in the manual. If you turn the meter switch to PLATE and then key the amp (which we will talk more about later), then the plate current should be about 90ma. The manual has a list of pre-checks to make. It's worth the time to make those tests before getting too far along. It's easy to miss a broken wire unless you go through and carefully check as many components as you can. The culprit to a problem may simply be a bad resistor, diode or broken wire.

Soft Key for SB-200

The SK-201 Soft-Key upgrade kit sold by Harbach was intalled. It's a small board that mounts on a plastic stand-off under the chassis. You'll need to drill a small hole through the aluminum chassis to mount this kit. The exact instructions for where to drill are in the kit - it's pretty easy.

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.

Soft-Start Upgrade for SB-200

I installed the SS-201 Soft-Start upgrade sold by Harbach. The kit contains everything you need to install EXCEPT the silicon adhesive to glue the board under the chassis. Don't forget to get some quick, high-temp silicon adhesive. Home Depot did not have RTV Silicon in stock so I had to use Loctite Ultra which is rated up to 250deg F, that should be OK on the underside of the chassis. It sets up in a few minutes, but it would be smart to let it cure over 24hrs with a weight over it. The SS-201 kit installed makes the unit have a little ``click'' when it powers on and off. Those are the relays on the SS-201 board. The HV meter shows a nice gradual increase and decrease. You won't hear the difference but the unit is well protected (to paraphrase Harbach).

Power Supply Upgrade for SB-200

There is a power-supply module upgrade for the SB-200. It replaces the power supply board and the components on the orignal. It's a major improvement for the unit overall.

Meter

My SB-200 has the original meter, but another ham Mark VA3MAH updated his SB-200 with a new face plate with a label to go over the replacement. Take a look at the meter on this SB-200, by VA3MAH

Upgrade Photos

Picture of the Power Tank before upgrade:
Click on the photos for the larger format in another window/tab of your browser

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):
1024x768

High Resolution Picture 3008x2000

Clockwise from lower left corner:

  1. Original Power Supply Module
  2. 3 x 3.7M Ohm Resistor Series (inside black insulator sheath)
  3. Terminal block for AC-main (horizontal)
  4. Terminal block Q (vertical)
  5. Fan motor
  6. Slow Start Kit (SS-201) underside visible. The unit is mounted by adhesive between chassis and tops of relay modules on SS Kit.
  7. Above the SS-201 and below the 15W resistor is the Soft Key Kit SK-201
If you do put both SK and SS kits in, allow for room because the SS Kit is dependent on having access to the main bundle of wires running vertical between the SS Kit and Power Module.

572-B Valves

The SB-200 uses a pair of 572-B valves. Mine seem to be made by Centron and are doing OK. I can achieve about 700W SSB on 40 meters with the tubes as-is. I have a spare set of tubes that I may put in to see if these originals are lacking in some way. But, at 750W I cannot really complain at all.

Operating the SB-200

Once you have your SB-200 in working order, now comes the time to to tune it up and operate.

Tuning

I'll cut-and-paste the advice I have from my well respected Elmer's K7NCG and KN7T
Jim writes:

There are two ways to tune the amp.

  1. 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.
  2. 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.
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.

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.

Keying the amp from Solid-State Rigs

My rig is a Yaesu FT-897D and so I needed to make a cable from the CAT/Linear jack to the RCA jack on the amplifier. I already put the soft-key on the amp so just a few volts is all I need to take to ground when I want to key the amp. The Rig supports a open collector circuit up to 50V 500ma for keying linear amplifiers. I wired a Mini-8DIN connector to RCA putting pins Tx GND and Gnd from the Rig to the RCA jack.

Now I can key the amp safely with the Rig PTT switch and the amp's Soft-Key modification.