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
