God Bless America!

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N1NKM's Homebrew AM TX!

This is THE NUMBER ONE of my "PRIDE & JOY" Projects! :)

(This page is only a very SHORT version. MUCH more detail can be found HERE!)

N1NKM's Tech Stuff!
HOMEBREW! "BOB" (Bucket O' Bolts)

Please note:This transmitter is NOT a "standard" design! It is very unorthodox, due to my limited budget and having to work with the stockpile of junk parts that I have on hand. Therefore, even though this unit works quite nicely, please understand that it is NOT a recommended transmitter design, and I would NEVER use this kind of "engineering" for any kind of commercial use! This is strictly my HOBBY. :) It's something I do FOR FUN. This information is merely presented for those who are curious... I don't recommend you try this at home! ;)

NOTE to modem users: this page has several 80K+ pictures, which may take a few moments to load. Please be patient.

This page contains photos of my homebrew, tube AM transmitter (called "BOB"- Bucket O' Bolts) which is still under development... but is at a stable point right now.

This puppy now has a little brother! Check out "BOB Jr!"

Click here to see the detailed progress toward V4.3! (V4.0 is shown above) It's now 55 watts and includes a built-in RF driver, push-pull modulator, and even boasts a homebrew, PLL-controlled exciter/receiver!
Yeah, "he's" come a long way from that V1.0 "Litte Linear"!

Back in fall of 2006, I added a band switch to allow use of this unit on 60 meters, where it functions as a linrar amplifier. With the RF generator providing a nice, 2.5Khz band-limited SSB signal, this is perfect for those 5 channels we presently have. I get 40 watts PEP so I'm well within the limit. :) The audio stage was modified for SSB use by adding a DPDT switch to open the cathode connection to gnd, and to short the secondary of the transformer.

The 1 ohm resistors in the audio cathode ckts is for curent measurement, only. (Grid bias set for 30mv = 30 ma current flow.)

Because this unit uses relay switching, it has that satisfying "Click!" when switching between TX & RX. ;)

The RF is supplied by a homebrew PLL frequency generator, which provides about 100mw of RF. The audio comes from the salvaged audio chip (& support components) of a fried computer sound card. (BTN!) I used tinsnips to trim off all of the unneeded portions of the card, then installed the audio amp inside a small box, shown in photo, above. That box also contains a 500hz sinewave oscillator, switch, level control pot, and PTT relay. It runs on 13.8vdc. So, this transmitter is actually 4 seperate boxes! It's amazing that the blooming thing works at all. ;) (The 4'th box is the D-104 mic preamp, which also runs on 13.8vdc.)

As I said at the top of this page, THIS IS NOT A STANDARD DESIGN!! It is built and adapted around my limited supply of parts and abundant supply of JUNQUE! ;)

WHY did I do this? Several reasons... #1, simply because I COULD! ;) I had all these parts laying around, and decided it was time to make something USEFUL out of them! Point #2 is because I love to build stuff! ESPECIALLY when it doesn't cost me a dime! :) Point #3 is just to PROVE that you really DON'T need to run "a full gallon" to have good conversations! 50 watts is PLENTY to maintain solid communications when the band is cooperating! All of the signal reports coming back are good, and the guys are quite surprised that I am running only 50 watts! Yep... 50 watts of PLATE-MODULATED power! "Class E" is cool, but THIS glows in the dark! Like a REAL radio! ;)

Get some good, HV rated diodes. I have some that are rated for 1Kv that I use, but I use two, in series, with 1M ohm resistors across each diode. A .002Uf cap goes across each set. (You treat each pair of diodes/resistors & cap as if it were a single diode.) This does several things- it balances the reverse voltage across them and bypasses RF.

You ALWAYS want to put ceramic disc capacitors across the rectifier diodes in ANY transmitter! Failing to do this will result in a nasty buzz on the air. Why? Because as the diodes do their job of rectifying the AC power, that causes the RF to find a slightly different path for each 1/2 cycle of the 60Hz power. The caps cause the RF to bypass the diodes, so that their switching action doesn't do that to the RF.

Usually, to really get rid of hum, you want to use a choke. Several Henries, minimum. Make sure it can handle the current you'll be drawing. (figure 150ma per 6146 as a maximum) BTW, you DO NOT want to be running the 6146's with their plates glowing! I know they'll "handle it" OK, but they will not last very long! You want to be fairly GENTLE with them, to get as many years as possible... they don't exactly "grow on trees" anymore, after all. ;)

So you have the power transformer. That feeds the rectifer unit. A small capacitor there (a few microfarads at the required voltage) then the choke, then more capacitors. You want at least a few dozen uf, up to no more than a couple hundred. The more, the better, within reason. (You would NOT want 10,000 uf!) A good general value would be around 200uf for a 600v supply. You always want the caps to have a MINIMUM of 50% higher voltage rating than your supply delivers. (Safety margins! You do not ever want these things exploding!)

Another benefit to using solid-state diodes is that you're not wasting power to heat more filaments, and they will drop far less voltage under load. Putting a small resistor (47 ohms or so for ~200 ma supplies) between the rectifier and that first cap is a good idea. At 200ma current, that will drop just under 10v, which is insignificant. The main point of this small R is to reduce the SURGE CURRENT when you first apply power. It also reduces the PEAK power through the diodes which happens with each cycle of the AC mains. If you're going to use three 6146's, you want to drop the value of that R to 33 ohms. Four 6146's, drop it to 22 ohms.

You should be able to find PLENTY of schematics out there to help you with the actual circuts needed. Hopefully, what I've give here will get you going down the right path. BE CAREFUL, because those voltages are potentially LETHAL! (Not to mention QUITE PAINFUL!)

There's this little section of PART 97 that applies:

§97.313 Transmitter power standards: An amateur station must use the minimum transmitter power necessary to carry out the desired communications.

If you're one of those guys who would call this "PW", save your breath! Please. If you don't want to talk to me or hear me, don't answer my CQ or join my QSO... and please, for Pete's sake, don't jam me, OK? Just dial your VFO to another frequency and have a QSO with someone else. Keying up on me & those I am in QSO with doesn't prove anything. (Does it somehow make you feel "manly" to be able to clobber a 50 watt signal?! Get a life, man. Get a life.)

- The Wouff Hong - It once struck terror into the hearts of LIDS, everywhere... Whatever happened to this instrument of law and order??

That's it for my soapbox.

*ALL* Information presented here is done so without warranty or guarantee of any kind. Author assumes no responsibility for the use or inability to use this information. Author also assumes no responsibility for the ability or inability to complete the projects, above. This project uses potentially *LETHAL* voltages! If you are not sure of what you're doing, ask an experienced friend to help. ALWAYS "pull the plug" and ground the caps to make sure there is *NO HIGH VOLTAGE* when working with this unit! Keep it well out of easy-reach when operating, or attach a cover to keep hands away from dangerous voltages.

This information is presented as educational information only. No guarantee is made as to its fitness for any purpose. All risk is assumed by the person who choses to use this information. While the author's experience indicates that this proceedure was effective, any attempt to build/modify these devices IS AT YOUR OWN RISK. Extreme care must always be excercised, this is at the builder's SOLE RISK.