KB3TTP 800+ WATT LINEAR AMPLIFIER
This project is presented by me for all you HAM's out there either who would like to build your first amplifier or for the veteran who's seen it all. It covers 40 - 10 meters including WARC bands (12 and 17 meters).
And what am I?
I'm one of the guys who wanted to build my first amplifier and here it is in as much detail as I can describe without writing a novel. I must say to you before I start to use the Boy Scout Motto and
Be prepared for this to take up to a year to build. Be prepared to work with something that with a slip of the hand will kill you! Be prepared not to take too much time off from work. Be prepared to not to take too much time off from your XYL. Do take full time to ENJOY doing a project like this. Do not rush it, because when your done and everything finally works just the way you want it to, you will not know what to do with yourself next. You can take these plans and use them straight up and everything should work just fine, or you could take the time to make changes to suit your taste and uses. I took the latter and wanted to build something to suit my needs. Be prepared if you do this to make many changes in your designs as you go along. I made at least 20 or 30 changes in mine, maybe more. This amplifier started out as ZL1AXB's plans for his amplifier that you can find on the web and I have modified those.
I AM NO ELECTRICAL ENGINEER AND THIS POJECT PROOVES THAT ANYONE CONTEMPLATING BUILDING THEIR FIRST AMPLIFIER CAN DO IT WITH HARD WORK, READING, LEARNING, AND A LITTLE BIT OF SWEAT.
I SAY "GO FOR IT"
This is not a step by step or "how to" building guide but should peak your interest if you want to build one that will get you higher power from 40 - 10 meters!
Before I begin I want to say that this is also a lesson in
Let's start with the cabinets first since they are the base of
everything else you choose. Picture 1 may look familiar in
size to you. Notice the feet. Notice the two large knobs. Notice the Bias,
HV and PWR switch placement. Now close your eyes and picture an old
Palomar 350Z sweep tube amp. Well, this is what it was. I had this amp and
got so frustrated in trying to make it work right that this is what led me
to build my own. So, I took it all apart and went to start from scratch.
The pretty face of this unit is 1/16 inch thick aluminum sheet. Sanded to
a brush finish and clear coated with lacquer. The labels are made with
Avery clear sticker paper which turned out to be not so clear. There are
other crystal clear labels that you could use out there, but these worked
for now. The power supply, Picture 2, used to be my
microwave oven. (Yes, it was broken). All I really used was the cover from
the old nuke. The base is 3/4 inch plywood, covered with a sheet of
aluminum and 1/16 inch aluminum front and back panels. Not too much more
to say about the P/S case.
Here's an idea. What about using a two drawer filing cabinet for both parts of your project? Remove the drawers and hardware, put in a shelf, put on an access door and turn it around!!!! (Wish I thought of that one sooner than I did). I'll save that for next time.
schematic. Click Here!
(Will load in separate window - large file)
The schematic may be familiar. I have used ZL1AXB 's 813 amplifier plans and have modified it to my own needs. His input tuning circuit works perfectly. I wish I had away to thank him.
Power Supply Schematic. Click
Here! (Will load in separate window - large file)
You've had a chance to review the base project and the schematics, now let's get going. For starters all the power for our amplifier is coming from a 120V outlet. I would recommend the amp and power supply be fed with their own proper circuit breaker to match the wiring of your home or the wire you choose to install for the project. A 10 Amp circuit will do it, but I would go with a 15A circuit dedicated to just these two items. Check your local electrical codes or see a qualified electrician for the proper installation.
The 120V AC serves two transformers. The first is the filament transformer whos job it is to supply correct volt and amp ratings to the tube filaments and provide a path for the HV negative line to reach the zener diodes and resistor which effectively shuts down the tubes when idle that control tube bias. The second is a 12V transformer rectified to DC to control relays, lamps, and fans. This circuit also supplies power to the PS step-start relays that turn the PS on. The fan in the amp is controlled by a 3W rheostat to quiet things down a bit when you don't need so much air flow. (See schematic.)
Next we come to the all important LC input tuning circuit for the amp. I have had the advise from other amplifier builders tell me to run away from that design. I have had others praise it highly. After building it myself, I am praising it highly because it works so well. It consists of a coaxial coil made from 3/32 copper tubing and a 12 AWG insulated wire ran through it. The coil is tuned from a receiver tuning capacitor held above ground and uses an insulated shaft. The schematic has the specs for this coil attached to it. Most of this amp is simple in design which is the beauty of it. You don't have to be an electrical engineer to build it.
See picture 3. Yes, before you ask, that is an alligator clip holding the RF feed wire to the coil. After tuning it with that clip there is a small amount of solder on it that you can't see. I didn't want to melt the insulation of the wire inside the coil so I attached it on with a minimum of solder after I found the sweet spot. I had to anneal that copper tubing myself from a short piece of rigid tubing. Soft copper was only available to me in large rolls for lots of money. It's little things like this that you run into and slow the project down. But, I had FUN!!!!!
Valve is a more accurate name than tube. I won't get into specifics here but read up on them if you're not sure about this subject and how they work.
The valve used here are 572B's. They are a high MU power triode, tough and durable, but like all valves not indestructible. I wish I could say I can buy new American tubes like this, but I can't. There are 572B's on ebay, Cetron and the like for about $200 for 2 NOS. I chose the Shuguang tubes from China. PICTURE 4 They cost me about $85.00 for the pair. I wasn't sure I wanted to kill 2 Cetrons while tuning up this amp so I figured I would kind of sacrifice these. Guess what. I put these tubes through hell and back during the tune-up phase of this project. Now that I am done they will still produce 1.1KW on 40 meters. I normally run them between 500 and 800 watts on all bands up to 40 meters and put 2750 volts to the plate. They have a nice stable low grid current on that voltage. I can't say anything for the other Chinese company's, but these have proven them self to me. If somebody starts making theses in the US again I will buy ours first, but these tubes have been impossible to beat down. It's important who you buy them from.
THE HV SIDE OF THE AMP
Starting from the PS the HV line is 8mm stranded copper core silicone spark plug wire. (Don't use carbon core). It is fed through a HV connector into the amp where it terminates at the 8 ohm 20W non-inductive resistor near the plate choke. This resistor is an important fuse like device as it protects the tubes from voltage surge and arcs. I have used up 2 of them already by reason of human error during the tune-up phase. They are available at Radio Shack. The plate choke is a simple 3 to 30 MHZ variety with a value of 55uH. The parasitic suppressors are made from silver plated wire, 6 turns on a 1/4 inch form and paralleled by a 100 ohm 2 W non-inductive resistor ending with the ceramic plate caps. PICTURE 4 above
Here is where a lot of experimentation comes in, making the tank circuit. See the schematic for the var. cap. values. The coils, which I made about 5 different sets of before getting it right are 1st stage of 12 turns #14 plated copper wire made on a 1.25 inch form. The 2nd. stage is 11 turns on a 2.5 inch form and made from #18 wire. I placed a #12 wire that directly connects C1 to the grids of the tubes instead of letting it find its way through the chassis. This tuning process was very time consuming. I hope I have saved you time in making these here. PICTURE 5
OF CHASSIS WIRING. PICTURE 6
The most important meter you can have on your amp is a grid meter to measure mA. Forget about how many plate volts it has, let the PS tell you that. A grid meter can tell you much more about what is going on inside your amp. There are many articles written on this subject for you to study on the net as it can get quite involved. PICTURE 7
THE POWER SUPPLY
This is not a voltage doubler but it is
a full wave bridge rectifier which requires a large transformer which I,
by luck, obtained. It is the best way to go if you can find a transformer
with an output of 1930V AC or somewhere close at 350mA. This will give you
the 2750 DC idle volts at the plate. The transformer I obtained had
multiple inputs for a range of power options so I made the PS a variable
one. Shown in PICTURE 8 is the transformer, step-start or soft
start relays and input control cables from the amplifier as well as the
rotary voltage input selection switch.. Notice the quarter on top of the
transformer for size comparison. The capacitor banks are done in a pair
for safety reasons. If one component should fail in one of the banks there
is still enough in the second one to take the load. That's why I used
220uF caps to make a total of 55uF for all 16 units. For even extra
protection there is a 250K 50W power resistor to draw down the HV when
shut down or a loss occurs because of component failure.
I could have split the warc bands up on the band switch with 10m and 15m and had 80m but I wasn't satisfied with the performance and tuning. I have only 6 positions on the band switch.
So that's it. I hope you have gained some ideas for your own project or maybe make one like this. Feel free to contact me at my email address of KB3TTP@comcast.net and just leave in the message subject line, "amplifier on Ham Universe".