I felt that it was time to put some “Fire In the Wire”. However I wanted to learn about amplifiers since I have no clue how one really works other than giving my signal a boost. The original plan was to build one from scratch but after some attempt at collecting parts I decided that building one from scratch was not going to work out since I didn’t have knowledge to even start one. The next best thing was to buy an amplifier and rebuild it.
I ended up getting a Heathkit SB200 Amplifier. The reason I went with the SB200 is that it appears to have a huge following and (some) parts are still being sold for it. There are also many articles written about this amplifier and it’s still being used in a lot of stations to this day.
What is an amplifier and what does it do? Or what does an RF amplifier do since we’re dealing with RF (Radio Frequency). A RF amplifier is a electronic device that takes a low-power radio frequency signal and turns the signal into larger signal with more power without changing the characteristics of the signal . This can be done by at least a couple of different ways. It can be done using Vacuum Tubes or Field Effect transistors (FET) which act very different but produce similar results. Vacuum tubes use High voltage with low amperage and FETS use low voltage with high amperage. There are many different types of amplifiers that are divided into classes depending on the type of circuitry used in the construction and its final use.
Do you really need an amplifier? This question could lead to a lot of debate between hams. There are hams that take pride on making all their contacts by only using the power provided from the transceiver (and some using under 10 watts/QRP) and there are hams that prefer using amplifier at almost all times. It all depends on your situation and needs. I would prefer a nice antenna setup that can be directed and have gain over a amplifier but at this point in my life, it’s very unlikely that it will happen. I went with an amplifier as a “Pile-Up” buster. An example for me is when 9K2UU (Barrak in Kuwait) was on the other day. There were so many people trying to contact Barrak that it felt impossible that I was going to establish contact with him. The last time he was on I tried for over an hour to make a QSO without luck. Mostly due to the other operators using Amplifiers. This time I had the amplifier so after a couple of times of trying to contact him, I turned on the amp and made a QSO with Barrak with the first try.
So how does the SB200 work. We know it takes the signal from the transceiver, amplifies it and sends it out to the antenna. But how? The SB200 has three areas that make amplification possible. You have the power supply, input circuit and the output circuit. There is a relay that is controlled by either a switch or the transceiver that activates the amp. The signal comes out transceiver into the “Input Circuit of the amp. The input circuit consists of coils and capacitors that are adjusted and controlled from the band switch on the front panel that provide the tubes with the proper impedance. The signal then passes through the tubes where amplification takes place using the components in the tubes and the very High voltage provided from the power supply to power the tubes. After the tubes do their work the signal passes through the “Output Circuit” consisting of more capacitors and Coils to clean up the signal even more and is fed to the antenna. This is just a basic summary of how the SB200 works. There are websites that explain how amplifiers work in great detail and I’ll link to them at the end of the article.
Now to my SB200. I found this amp on the for sale section on QRZ.com. The amp needed work and thought that this amp would be the perfect project to get my feet wet in amplifier building. Thing about Heathkit amps is that they are kits. They are as good as the person that built the amp. If the original builder had no clue what they were doing (examples are cross wiring, swapping components and poor soldering to name a few), the amp will perform poorly if it performs at all.
The seller did a poor job at packaging the amp for shipping and my pants almost turned brown when I saw the box come off the truck. The seller then blamed me since “I didn’t pay enough for shipping” when it was the seller who gave me the shipping quote. The amp showed some signs of damage from shipping which appears to be cosmetic but I was worried about the High voltage power supply so I hooked it up to find that I was getting the 2300-2400vdc that I needed. A bit of relief. I’ve learned to inquire how the item is going to be shipped from now on.
When I got this amp it was in the middle of being restored. There were no tubes, the components were missing on the tube sockets and there were no parasitic suppressors (Which I was well aware). It appears that previous owner replaced the original cooling fan with a couple of PC type fans and installed a “Soft-Start” circuit as well as “Soft-Key” circuit but removed the “Soft-Key” before it got to me. The power supply that provides the High Voltage also appears to been replaced/upgraded. Since this amp was designed back in the 1960′s where the circuit that activates the amplifier used -110vdc. It would ruin modern transceivers keying circuits by putting high voltage into the transceiver. To combat this people install a “Soft-Key” which basically converts the -110Vdc to around 1Vdc which plays nice with the modern transceivers. Another modification that was done before I got the amp was the addition of a “Soft-Start” module that prevents a “Rush” of current hitting the tubes. when you push the on button, the amp is supplied with a load that is restricted for a couple of milliseconds and then switches to the full load using resistors and relays. Even though I’ve read on many sites that the “Soft-Soft” is not really needed for this amp but since it’s installed, I’ll keep it. It can do no harm.
The previous owner also shipped some components that will make bringing this amp back to life a lot easier. I received the filament choke and a couple kits from AG6K (Not sure if he still sells kits) with detailed instructions on installation.
The only thing I have to do is obtain the rest of the parts. The tubes were available from RF parts. The tubes were about $120 for the pair, A new “Soft-Key” kit for around $30 from Harbach electronics and the rest of the components were about $20 from Mouser. I have around $350 invested in this project.
After all the parts came in. I found a manual with a schematic online and used it to install all the missing parts and installed with “Soft-Start”. After installation of the parts I went back to the manual and double checked the entire installation and everything seemed to check out.
Before turning the amplifier on, I preformed some checks to lower my chances of a smoke and/or light show. The manual states to perform a couple of “Resistance” checks. The first one is to put a multimeter on the anode clip (nipple of the tube) and the chassis. When the meter stabilizes, it should read around 180k ohms. However I was seeing 240K ohms which made me a little bit worried. After searching google I learned that this is due to the replacement of the High voltage board. The other resistance test is to place an ohm meter on lug 3 (V3) of the tube and place the other lead to the chassis. The resistance should be somewhere between 5k-15k ohm. I ended up with 10K which is right in the middle.
After performing all the checks I can do with the amp off, I was still hesitant on turning the amp on since I never really messed around with an amplifier before. I wanted the amp to be checked out by someone else but I got impatient one weekend and decided to go for it. I removed the tubes from the amplifier and powered the amplifier. To my relief there was no smoke/light show and was still seeing around 2400v on the meter. I turned the amp off and waited till the caps were discharged. Placed the tubes in and turned the amp back on. To my amazment, the tubes sprung to life and produced a comforting glow. I left the amp on for an hour and nothing happened in that time so I decided to put a signal through the amp to see what happens. I went on the 20m band, put the radio into CW and did a tune. I was glad to see around 600W on my watt meter.
I decided at that point to make some contact with the amp on. Spinning around the dial I hear a Northern Ireland DX station and threw my call out. He responded to my call the very first time and gave me a report of 59 20+. That made me a very happy operator to know that my amp was working.
The only place where I have 220V that I can get to was in the basement. So that’s where my equipment was until I had the chance to get 240v wired to my office. Once in my office I wanted to really use it so I turned it on and went through all the check to find that I was not seeing any grid current show up on my meter. That started to scare me so I shut the amp off and went searching for the answer to “What’s causing this to happen?” I asked the HeathKit Amps Yahoo group and I also asked help from my local club. There was a major block in the road and it seemed that the help I was getting was leading me to nowhere. Some people said I have shorted tube(s) and others said that something is wrong with the wiring. Both can be plausible since I never really worked on an amp and I’ve heard of people receiving bad tubes. But then someone ask me if I had a dummy load. Of course I don’t! So another member of the club let me use his Oil can dummy load. When I hooked up the dummy load I noticed everything is now looking good.
I now have a working amplifier! Just in time for the ARRL DX SSB contest. I was happy that I was going to get to use my new amplifier and put it to the test. On Sat morning well into the contest I decided to start. Amp on and 20m is quite busy. I was making a good amount of contacts while only searching & pouncing until I started to hear a sizzle sound coming from the amp. Found out that one the resistors in the parasitic suppressor kit has become broken and was arcing to the resistor it broke from. That kept me out of the contest till I had the nerver to solder it back together and get back on towards the end of the contest.
I still have some issues but I am leaning toward my antenna setup to be the cause. For some reason the tubes get very hot on 80 and 40m. I would have to either get a tuner that can handle the power or get a resonate antenna before I can continue on other things. I am also thinking that the PC type fans are not doing their job. The chassis and case get hot to the point where I can’t touch it. Tubes are not glowing but I want to keep it that way.
Future plans for the amp are the following
- New HV board – The one I currently have is newer that original but I think if I’m going to keep this amp that it should have a new board
- Glitch Resistor – This will save the tubes if anything goes wrong with the HV board.
- Meter Protection – Since the meters are rare and get expensive when they come up for sale, I’d rather spend a couple cents on some diodes.
- LED Meter Conversion – I want some nice white/bluish LED to light up my meter to match my FT-950
- Various upgrades – There are websites out there that have a ton of things to do to this amp to make it much better. I already got the parts for most of the mods but want to make sure the amp is fully functional before I even attempt modifying the amp.
- New Chassis, Cover and face – I want to design a more modern looking case around the same chassis and incorporate some more RFI shielding and better ventilation and the use of a thermostat for fan operation.
- Clean Up – When the new chassis is made I would like to DE-oxidize all connections, coils and plates.
Some cool sites I cam across when rebuilding this amp
Rick Measures (AG6K) - Boat loads of AMP info
Yahoo groups – HeathKit HF Amps - Helped me out a couple times, worth joining
Robert Norgards (KL7FM) SB200 page – I plan on doing almost all his suggestions