Homebrew AM station planning and design

[KI4YAN]

Ok, so here's my dilemma:

I have no cash. None.

I have a big junkbox. BIG. takes up my entire bedroom. I sleep on the couch.

I have a powerful need to get started on my homebrew AM rig.

Antenna: possible to get an 80M dipole about 25-30 ft off the ground, definitely able to get a 40M dipole up there. Receive antenna undecided.

Receive: 6SN7 GG RF amp, 6SN7 regen detector, 2 stage 6SN7 audio. Plug-In coils (Complete, works good)

Transmit: 17"x8"x3" used chassis, lots of octal sockets, two 5 pin sockets, plate caps of all shapes and sizes, 600W plate + filament transformer, 2x 5H 200mA chokes, SNC 5P352 Multimatch mod transformer, ratios from 1:1 to 1:2.5, Stancor 12w 1:2 interstage transformer, P-P plates to P-P grids. Rudimentary machine shop with some skills. Ability to scrounge.

Plate transformer(s): 450-0-450 HV winding, good for some 300mA. (ran 5BC3 rectifier, 28 6FQ7, a pair of 7868, 14 12AU7, in an old organ) has 50V bias tap. Core is 4.25"x3.312"x2.25" wasteless pattern. Also 400-0-400 200mA toroidal on the shelf. Can get another similar transformer if "I Need Mo Powa, Cap'n!"

Modulation transformer: originally ran PP class B 807's for modulation, RF final originally modulated is unknown. Core weighs ~30lbs, dimensions of core are 4.5"x3.75"x2.25", and is gapped with a single sheet of thickish paper.

Interstage transformer: 1:2 winding ratio, Stancor rated at 12W. Used to have a 6L6 driving half the primary into PP class B 807 grids.

Tubes I got that might be useful: 10EM7, 12AT7, 6AG7, 6CL6, 12BY7, 6BQ6, 6DQ6, 6DQ5, 807, 826, 829B, 5Y3, EY88, 1N4007 . Others as located at the upcoming Greater Louisville Hamfest.

As far as the chassis goes, the mod iron and interstage transformer came from it, along with two broken meters and a blowed up filament transformer weighing in at 40lbs. Can rewind that transformer core to power RF stage if needed.

Over time, I want to be able to run 100-200W of carrier. Can I accomplish this from what I've got listed above, or do I need to build an intermediate level TX and then run an amp later? I have a 1941 and a 1962 ARRL handbook, I'm just looking for your opinions and ideas, what would you do given the above and no other radio?

[KB2WIG]

Beg fer the parts......  or,

 Maybee this is of interest...

Five Watt Series Modulated Rig

http://www.amwindow.org/tech/htm/series.htm

The antenna is the thing........ get her up as high as you can


klc

[N3DRB The Derb]

build big. dont screw around building a 6146 rig. use the biggest and most scroteful tubes you have iron for. look for tubes not used in "collectable" gear and audiophools. . Use triodes for the rf final. The 812A is a good choice, usually cheap, under 10 bucks. makes a good final. Medium mu, needs some drive power, about 30 ma per tube. 

based on what you have, 2 807's modding a 812A would give you some strap. What voltage & current is the plate + filament  transformer? .

above all, dont build a PW rig. if it hasnt got ->75 watts carrier  modulated >90%,  you will get nothing but sorry OM gotta go comments.

[Opcom]

pair of 807's could do 88W ICAS with 600VDC on plates (or use three in parallel.., and you could modulate them with a pair hooked up in class AB or with the zero bias class B circuit shown in the handbooks and the sweep tubes you mention would be very good modulators as well, and will stand high operating voltages.

A pair of 829's in pp-par. will do a good 80W audio, should be enough to overmodulate a couple 807's. I'm assuming you have a few of them.. I have sockets for 829's, I think, if you need a pair I can help. but anyway, you have enough to start with, like Derb said, build the biggest you can. Even if the power supply is on the short side, you can beef it later. 

The sizes of the high power tubes you mention calls to mind the Collins 618S-1 transmitter that used a pair of 6146's for mod and three in parallel for the PA. It put out 90W carrier and the one I had working a few years back has no issues modulating 100%.

So you can build decently big with what's on hand if'n you cannot spend a cent. Remember all the CB linears with 4 or 6 or even 8 tubes in parallel? They worked.. So don't despair about the power, just parallel up a few tubes.

Use bridge rectifiers to get the most out of the xfmrs if they'll stand having the CT hot, otherwise you can put a doubler on each leg so they work out of phase and get the most from the XFMR that way because you will need the 700-900VDC to strap with the smallish tubes. Put the chokes in the negative lead too, for extra margin. Just some random thought on how to lash those parts up to make the most juice.

[WQ9E]

OK,  The 600 watt plate transformer-what is the highest voltage secondary on that?  I am assuming this is different than the 450-0-450 transformer you list in the next paragraph.

A Johnson Viking 1 or 2 would get you close to the low end of your target range, a Heathkit DX-100 or TX-1 a bit more and a Valiant will do around 140 carrier out on 160-40 and a bit less higher up.  All of these rigs use a pair of tetrodes for modulators and 2 or 3 additional for the final (yes the V-1 is a single tube final but the 4D32 or sometimes subbed 829B is really 2 sections in one envelope).  I would suggest that you think about these rigs since you can download the manuals for any of them from the Bama website which would provide you with schematics, layout ideas, and descriptions for your homebrew project.

A big part of homebrewing, especially these days, is being able to choose reasonable substitutes in place of originally specified parts.   I am going through and scanning a lot of the amateur oriented construction articles out of some old Radio and Television News magazines (1945-1959 era) I recently added to my collection and if I come across anything that appears to fit your parts and needs I will send you a copy.

Rodger WQ9E

[N3DRB The Derb]

and dont say, I'll build a PW rig now and get a amp later. dont even bother unless you can get 8 to 10 db in yr cb.  Just build for max blowsmoke now.

you cant raise 50 bucks for parts? For 50 bucks if you are smart you can get a 200 watt carrier OTA. for 50 bucks. You just have to go with stuff the gear collectors dont want as spares and stuff that the audiophools dont want.

buy yer chassis used and full of holes. cut a new top and it looks brand new. Just turn th junk into elbow greased goodies. real homebrew.

[W1EUJ]

Ok, so here's my dilemma:
I have no cash. None.
I have a big junkbox. BIG. takes up my entire bedroom. I sleep on the couch.
I have a powerful need to get started on my homebrew AM rig.

No dilemma. Save your money, buy a bed. Move the junkbox/storage into a room that doesn't start with bed-, bath, living- or dining-'. Save more money, then ask your original question.

With that said, somebody else's incomplete/failed project can be the cheap basis for your homebrew. Ask around...

[w3jn]

As you said you have NO money, so here's my suggestion on how to do this with only what you have on hand::

That 400-0-400 toroid xformer should be fine if you ignore the CT for 800V.  Bridge rectify it and you have a scroteful B+ supply for the final.  Use 807s for the finals and mod with an 829 in P-P.  This is gonna give you a bit less than 100W.

Oscillator, 6AG7 or 6CL6 (reluctantly, I don't like designs that use these, too much power in the osc stage), buffer another 6AG7 or 6CL6.  Much better would be a 6U8 or similar triode-pentode as the osc-buffer, then a 6CL6 as a post-buffer.  Don't mess with xtals, you CAN build a stable VFO if you pay attention to mechanical design.

Audio chain, 12AT7s pre-amp and driver to the insterstage xformer.

If you need parts for this project, post.  I and many others have overflowing junkboxes and more than willing to donate same for a worthy project like this. 

The big chassis you have, what style "5 pin" sockets does it have?  Sounds like most of the work is done for you on that chassis.  See what the PS voltage is, and what toobs fit those sockets.  I'm pretty sure we can come up with the appropriate toobs for ya.

[K1MVP]

If this is your first time building a rig,--a single 6146 rig might be great,--plate modulate it,
and put up a GOOD antenna, and you might be surprised what you can do(and fun you can have)

I have always believed,--ya got learn to "walk" before you can "run", and you will get some
experience, with circuit layout, nuetralization, spurious oscillations etc, and getting rid
of undesireable "phenomena" with a lower powered rig.

The experience you will  acquire building a PW rig will serve you well when
you decide to build a "big rig" such as a pair of 813`s running 2 kv or more and much more current.
                                       
There is such a thing as "biting off more than you can chew", and the net result is ya might
get discouraged and never attempt to build anything after(a bad experience).
                                             
                                                73, K1MVP 

[k4kyv]

build big. dont screw around building a 6146 rig. use the biggest and most scroteful tubes you have iron for. look for tubes not used in "collectable" gear and audiophools. . Use triodes for the rf final. The 812A is a good choice, usually cheap, under 10 bucks. makes a good final. Medium mu, needs some drive power, about 30 ma per tube. 

based on what you have, 2 807's modding a 812A would give you some strap. What voltage & current is the plate + filament  transformer? .

above all, dont build a PW rig. if it hasnt got ->75 watts carrier  modulated >90%,  you will get nothing but sorry OM gotta go comments.

A single 812A would not make enough difference over a 6146 to be worthwhile 90% of the time.  You need something more on the order of a pair of 810's at minimum or better still, 833A's, plate modulated at a couple of KV.  A suitable modulation transformer could be expected to weigh more like 75 lbs and up, unless you use one of those little lightweight jobs from the Gates BC-1T or later in the series.

[WBear2GCR]

 I tried looking back out through the internet to see what you've got in that room...
couldn't do it. 

My vote is find an 813 for the final. They're cheap, rugged and easy to find. Free on a good day...

Fwiw, 4 x 807 in AB2 with a bit of extra snot on the plate (>700vdc) will do a solid 200watts RMS. There's ur modulator?

The 30 lbs gapped mod iron is interesting - have you looked at the primary? What does it have there? Usually gapped iron is for SE primary drive. Take a good look. Otherwise that sounds like it will be a really good mod xfmr. But are you sure it weighs 30 lbs? At that size core it shouldn't be that heavy, imho. More like 15lbs max I'd expect...

Otoh, build whatever you can, that's the best rig! 

                              _-_-bear

[KI4YAN]

Ok, I didn't give a complete tube inventory. 6U8 types abound. After spending a bit of time on the subject, here's my preliminary setup ideas:

The 600W transformer is indeed the 450-0-450 unit. bridge rectify for 1200V, yeesh, that's a bit high. even my 826's a limited to 1KV. a pair of 812's should hack it though, and I can probably trade for some. The 400-0-400 toroid can run the modulator, a par of 6DQ5 for max scrote, or an sextet of 6BQ6. (BWAHAHAHAHA! 20W plate 6V6's...)

Anyway, here's a more complete listing of the junkbox:

4 6dq5
8 6BQ6
2 6DQ6
3 807
2 6CL6
1 12BY7A
2 826
2 829B (with sockets, but no plate pin connectors)
0-250mA DC meter
0-150V DC meter
2 ~150pf air variables, 1/16" plate spacing
1 60pF air variable, 0.170 plate spacing
a few broadcast air variables
small magnet wire, 12g bare copper, 600V insulated, some short sections of HV insulated (3kv, 10kv, 40kv)

Can fold steel chassis, but it's thinnish metal, 22g or so. I have some 18g as well. fresh out of aluminum at the moment, time will tell if I can get more.

So what do you guys think of this lineup:

6U8 mic amp/phase splitter, 6CL6 voltage amp, 12W interstage transformer, 6DQ5 class AB2 at 800V plate and 250V regulated screen, Mod transformer, using 400-0-400V toroidal transformer for power. Should make 120W audio easy, or sub 807's in for the 6DQ5's and make 120W in class B2.

and

812A PA, 12BY7A master oscillator, using the 450-0-450 transformer bridge rectified, 1200V PA plate, 250V master oscillator plate.

I agree with some here that I've got plenty to get me into trouble, with the exception of the output network capacitors...I have some plexi and a good hacksaw, should be able to make some caps with that.

Now to find a cabinet to put it in....

[w3jn]

That 450-0-450 V transfomer shouldn't be bridge rectified.  The CT is designed to run at ground potential and you're gonna run the risk of shorting it out.

The toroid, though, should be fine without the CT at ground potential.

[WU2D]

Does everybody get a chance? Here are some ideas..

Ok I like the modulator  - that is on the right track for a small rig. AB2 pentode with a pair of 807s's or 1625's works great. I made a modulator back in the 80's with a TCS mod tranny as the driver transformer and a bigger chunk of military iron as the mod transformer and easily got 50 W RMS out. My lineup was 12SJ7, 12SN7, 2/1625's.

The 6CL6 can be strapped as a triode and you can wrap some feedback from its plate to the cathode of the triode of the 6U8. I would use a pair of the 6DQ5's as the modulators since you have spares. This should have enough juice to modulate the pair of 807's.  This will get you into the "Ranger" class.

This modulator is not really enough to plate modulate an 812 unless you double up on the modulators -  4 6DQ5's (or 4 807's).

And in the RF section I would add a driver tube like :

12AT7 VFO Oscillator/Doubler, 12BY7 Driver, 2 / 807 finals or the 812.

Operate the VFO at half frequency and regulate the DC on it.

For Xtal control 12BY7 oscillator, 6CL6 Driver, 2 807 finals.

Good luck on the mini-ranger.

Mike WU2D

[W3SLK]

I think the simplest transmitter is the good ol' 813 by 811's. Both tubes are cheap and easy to find. Heck an ART-13 mod transformer, 900~1200 VDC you're all set and you don't have to fart around with the screen grid. Now if you want to use just what you have on hand, judging by the list may want to go with those 829B's modulating the 807's. I think I read that a single 829B can put out ~45 watts, (dual plate). So a pair are good for 90. Being you don't have the iron. I would still go with the 813/811's. Damn hard to beat and if you look in most of the old handbooks, you'll see this was a popular combo.
Your milage may vary. Offer not good in Utah, Read store details..............

[w3jn]

All good, but let's keep in mind he can only use the parts he has on hand.

12BY7 master oscillator NFG.  Use one of your 6U8s - triode section osc, pentode section buffer, THEN drive your 12BY7 with that.

The more power you run in your osc the more it's gonna drift and be unstable.

You also oughta regulate the osc B+.  You don't have any OA2s or OB2s do you?  If not you could use a string of zeners to regulate the base voltage of a power transistor that has a healthy BVCEo.

THe ONE thing you ought to spend money on for this project is filter capacitors.  Using old buzzard filter caps is a recipe for disaster, unless they're oil filled.

[K1MVP]

All good, but let's keep in mind he can only use the parts he has on hand.

12BY7 master oscillator NFG.  Use one of your 6U8s - triode section osc, pentode section buffer, THEN drive your 12BY7 with that.

The more power you run in your osc the more it's gonna drift and be unstable.

You also oughta regulate the osc B+.  You don't have any OA2s or OB2s do you?  If not you could use a string of zeners to regulate the base voltage of a power transistor that has a healthy BVCEo.

THe ONE thing you ought to spend money on for this project is filter capacitors.  Using old buzzard filter caps is a recipe for disaster, unless they're oil filled.

Excellent points Johnny,--as far as the 12BY7 being used as the oscillator(I would agree) but he
might be able to get away with using the 12BY7 as a "buffer" stage if he feeds it with a good vfo.--
would he not?

As a "stand alone" crystal oscillator, --I would agree he is asking for trouble.
                                                   
                                                73, K1MVP

[WU2D]

I did an opposite setup on a 10M rig, that is 829 modulated by a pair of 807's with a TCS XFMR- that was a nice system with "plenty -audio". 50 Watts out.

Mike WU2D

[Opcom]

My apologies to the 60 watt 826. Each will do 50W carrier in plate modulated service and they will also go to 220 MHz should you be in the mood for fancy bandswitching or a later use for the rig on higher frequencies. The 7.5V filament is center tapped for convenience. There are ratings of 22W carrier for class B amplifier but it's for RF, not modulation service. Still they might do 150W as a modulator. They might be hard to find replacements for but I think the socket is the same as an 829. so all's not lost there for the future. It might be better in the end to find someone who is already using them (needs spares) and trade for other tubes.

[KI4YAN]

Pat, I guess i just read your mind, because i just listed the 826 as tradeable for a pair of 812a's on the board here...

Filter caps? what filter caps? NOBODY TOLD ME NOTHIN BOUT NO FILTER CAPS! heheh...I have some New (not NOS) CD 500V 64uF electrolytics, can stack three in series for 1500V ratings.

Only thing got me real worried now is a case...a half-rack cabinet would be nice to have. I'll keep my eyes open for one.

[Ed/KB1HYS]

I am in your boat too, but-
you can find a goldmine of parts for free at the local landfill. (yes I am a dump-picker and dumpster-diver and proud of it).   I've pulled a couple of old Tube type TV's out of there (yea with 6SN7s and stuff in em...)
Good for tranies and some tubes etc.  Cabinets can be had if you want to strip old appliances for the raw materials.  The old Cathode ray type computer monitors contain usefull parts also, HV caps and such if you want to salvage them (they are almost always free as EVERYONE seems to be going flat panel now).

Option 2 -- sell some stuff for cash to buy needed stuff.  If you find one Bugleboy 12AX7 you can put it on epay and make money. Likewise those worthless black beauty caps.  One mans junque etc.

Good luck OM.  just think outside the box... that and with help from the folks here and you can do almost anything!!

[KI4YAN]

I'm sitting here tonight, looking at the junkbox, and listening to some rude 80m net. It's been almost an hour now, and I haven't heard a single ham callsign. Aren't we supposed to ID on the air?

I can only hear half of the net conversations anyway, so I guess primary goal for this upcoming hamfest is to buy a good receiver. There goes any money I was planning to spend on the tx project. Oh well, if you can't hear em', does it matter if they can here you?

[w3jn]

Don't be shy about putting forth parts you need in this thread.  I'm a firm believer in radio karma - helping others with parts otherwise moldering away in my garage builds this up 

[Opcom]

Hear Hear! or post in 'wanted' and indicate 'for free' or 'for shipping'. Got too much stuff and will die before using it.

[Carl WA1KPD]

I think the simplest transmitter is the good ol' 813 by 811's.

I in the final paper stages of pulling this combo together. Have been for about 4 years!

Thinking about the exciter- to be HB with HB VFO. What tube would you drive the 813 with? I want something that loafs along

I'm considering a 6L6. It can do it, but will it have enough scrote to do the job comfortably and overcome all general losses on the way to the grid?

Second choice is an 807 or 6146. Both seem way too much overkill or do I just run them real low?

I keep feeling there is a common tube in betweeb that I am overlooking. Was gong to break out the transmitting tube manual this weekend but might as well pick brains here.
Thank
Carl
/KPD