The Electronics Universe

I acquired an old Heathkit SB-200 Ham Radio Linear amp from a friend a couple of years ago. This posting is about the cleaning and maintenance required on an old amp like this. They were first offered in 1964 as a "kilowatt amplifier" and sold in kit form for $200. See the attached PDF flyer for the original advertisement from Heathkit.

The amp includes two 572B/T-160L tubes in parallel. They are mounted horizontally and cooled by a small fan under the tubes. Back when this amplifier was popular the FCC required ham operators to measure their power as "input to the final amplifier stage". Today it is a bit different. Today we rate the power of an amplifier as "output power in RF".

My amp looked very good when I bought it. It was in almost perfect condition with no dents or scratches on it anywhere. The paint is original and is very slightly faded but for as old as it is that is no problem. See the attached pictures.

When I got it, I couldn't resist opening her up and checking out the innards. A couple more attached pictures are below. Notice the DANGER sticker in bright RED. They are not kidding. This amp has over 2300 volts DC under the hood. That IS lethal if you get zapped. I do NOT recommend you go poking around inside an amplifier like this unless you have considerable training and understanding about how to handle this type of gear. It should be unplugged and the power supply capacitors should be fully discharged before you even open the lid.

All the capacitors in the power supply have replaced with new ones. The Harbach modifications for inrush current and soft-keying are installed.  You can see the little green circuit board for the soft keying modification in one of the pictures. This is needed to key the amplifier with one of the modern rigs that cannot handle the 120 Volt AC relay that was used in the original circuit for transmit/receive switching. It would fry the amplifier keying circuit in your new solid state rig.

The inrush current modification gives you much longer life for the tubes. It clamps down the current to the tubes when the amp is first turned on while the power supply voltage comes up to full reading. At least, that is how it was explained to me.

I added a drop of Singer Sewing machine oil to the fan bearings because it was overdue for maintenance. The wife was very understanding about letting me borrow her sewing machine oil. I put it right back where I found it when I was done.... before she got home from the grocery store. Just don't tell her I borrowed it LOL.

Anyway, that's what I've been up to. I love this old amp. She is a real workhorse and puts out a very clean signal on the air. She will do 750 Watts output on CW with 400 MA plate current and 1900 VDC plate voltage, and 1200 Watts output PEP on SSB. On sideband that is 12 DB of gain over my 100 watt transceiver. S-units are generally 6db gain from one S-unit to the next. So the amp gives me an additional 2 S-units for the receiving station I am in QSO with.

Just remember, regardless of how tempting it is, keep your fingers out of the large power amplifiers unless you have a LOT of experience with them. Get someone with experience to help you and show you around the insides of one instead. Your friends and family will be glad you did.

Regards and 73  de K7RMJ  Frank

This antenna is often called a "Windom" antenna. However, that is not quite correct. The original Windom antenna was fed by a single wire rather than an open wire feed line or by coax feed line, both of which have two conductors.

The regular dipole, which most of us are familiar with, has two halves that are equal in length. They are both a quarter wave in length and total a half wave in overall length for the antenna. It is just two quarter wave lengths of wire or whatever kind of conductor you desire, and the feed line from your radio transmitter is connected to the center of the antenna with one conductor soldered to each half of the antenna.

With the off center fed dipole it is connected the same way as a conventional dipole except that the feed line connections are center. For an 80 meter antenna You should make one leg of the antenna 45 feet long and the other leg about 90 feet long for a total of 135 feet. This changes the impedance at the feed point from 72 ohms to about 200 ohms.

On my 80 meter OCFD I made a 4 to 1 balun and put it right at the feed point. Then connected my Coax to the balun and ran it into the radio shack. I thought I was done then but discovered that I had a lot of RF floating around in the shack. So I made a coax choke balun and installed it right outside the shack.

I made the choke balun by simply coiling about 20 feet of coax into a 12 inch coil. That removed virtually all the RF from inside the shack and did not change the tuning of the OCFD.

Now the big question. Why go through all that fuss when a simple regular dipole would work on 80 meters? It's because It also now works on 80, 40, 20 and 10 meters with a 2 to 1 or better SWR which the tuner in my rig can take care of. It doesn't cover ALL of 80 or 10 meters with that low SWR but it does cover a significant portion of both and on 40 and 20 meters it covers all of the band with 2.5 to 1 or less SWR. So far It has worked into several states including Hawaii and Alaska as well as about 30 foreign countries. For DX stations it works into South America and Europe the best but that's because those areas are broadside to the antenna.

Regards and 73  DE K7RMJ

The mailman was really good to me today. I bought the Heathkit HD1250 on eBay and it arrived. It is in great shape, has all the coils and it works! I had to put a new 9V battery in it but the old one did not leak. There was a number if pieces of foam tape inside it for padding the battery to keep it from rattling around that turned to dust when I touched it. So that will need to be replaced too as soon as I find some. I am really tickled about this dip meter. It will be a huge help with my antennas. Building and restoring old antennas is my favorite part of ham radio.
73, K7RMJ

One of my favorite activities in ham radio is building antennas. The antennas in the document attached have both been built and proven to be very good antennas. The 2 element wire YAGI will surprise you how well it performs.

OOPS!!!  There are a couple of typos in the attached document.

1 = The length of the "B element" in the 30 Meter 2-element Yagi should be 51.25 feet and NOT 41.25 feet.

2 = The C distance (element spacing) for the 12 Meter 2-element Yagi should be 9 to 10 feet and NOT 1 to 10 feet.

Sorry about that. I think I need to clean my proof-reading eyeglasses LOL

Greetings all. Just joined this site and thought I should say hello to everybody. Real name is Frank and my Ham Radio call is K7RMJ. Original call was KN7RMJ back when the 'N' in the call meant you were a novice... more years ago (1959) than I want to admit LOL. Pleased to meet you all. Remember, if you keep one hand in your pocket, you might still get bit, but you won't wind up on your back on the floor. Don't ask how I know this LOL