The AM Forum

So I acquired a AMBC Transmitter today, not a monster by any means, but not quite a PW either...

A J.R. Cunningham CM 30-50 Transmitter along with the LPB ATU-100SF Antenna Tuner

A friend of mine is in the AM radio business and he had it stuffed in storage for any number of years. He had removed it from a Transmitter site in Brookshire, TX when they upgraded. It was being used as a backup Xmtr and currently has a 1050 kHz Xtal installed

The Tube line up is:
12BY7, 6V6, 6DQ5 (broken in socket)
I'll need to find a 6DQ5 tube to replace the broken one...

Interesting note, it has a IRF820/TIP100 Modulator.

Photos:
http://riley-music.com/BowsStuff/CM3050/01Front.JPG
http://riley-music.com/BowsStuff/CM3050/04Rear.JPG
http://riley-music.com/BowsStuff/CM3050/07Top.JPG
http://riley-music.com/BowsStuff/CM3050/10IDTag.JPG
http://riley-music.com/BowsStuff/CM3050/18GutShot.JPG
http://riley-music.com/BowsStuff/CM3050/15AntTnr.JPG

I searched for "J.R.Cunningham Transmitters" and found this site:
http://groups.msn.com/gospelradio/ouramtransmittersfeatures.msnw

I emailed them tonight to try to get some tech data on this thing.

Anyone else ever heard of them?

If it is 50 Watts output (like the website states), is it worth trying to put on 160 or 75 meters?

No. Time and money better spent elsewhere.

Unfortunately that would have to be considered a low power (PW) TX.
It looks like a night time low power transmitter for AM B'cast. The swirl marks on the aluminum is intersting.
That would be a good driver for a linear. No audio mods needed. It should be b'cast quality audio.
If you already have a linear you're ready to go.
You need a good antenna and at least 150 watts min for 160M and 80M. Others will disagree, but that's why we have interference problems during a QSO. You have to try to be a tall ship on these bands. They are crowded because of the low sunspot cycle we are in now. And SSB QSO's zero in, right on top of an AM QSO OR start chatting 2 kc away.
Low power is good before the highway gets busy. Early eve or after 10PM.

The secret is to have a 1 kw rig at the half power setting and a nice flat dipole about 60 feet in the air OR a real vertical and that's the beginning of taking command of the airwaves.

I'm doing what I can with my 250W Raytheon or legal limit (375W) linear
Fred

Unfortunately, Mr. Cunningham passed away several years ago.  I have an RCA BTA 1R1 that he was restoring shortly before his death.  I purchased it from his widow a couple of years ago.  He built those transmitters for use by missionaries in several countries in South and Central America. 

Well at that rate, I'll never get on 160 from my current QTH.

A 60x120 lot doesn't lend itself to anything decent for a 160M antenna, the 75 Meter is about all I can get I guess...


And if 50 watts isn't enough to do it, well.. I'll guess I'll look at moving it on.. it was free.. free is usually "You get what you pay for".

Thanks for the replies

Bow .... this cathode mod rig is interesting ... I for one would like to hear it on the air, if possible ... I ran into Cunningham's work while researching cathode mod ... this way possibly gets around needing big mod iron ... he passed away recently ...73 and sk OM ....John

Thank for the reply Beefus,

The site has says:
For
LOW POWER AM RADIO
Contact:
David Martin, Eng.
Phone: (805) 239-1932
email: martronics@tcsn.net

I also found out David Martin's call is: WA6TYJ
So I sent him am email tonight to see if I can get any documentation on this transmitter. If I get any response, I'll let you know, if you are interested.



Anyone have a 6DQ5 they want to part with?

Dear 5EFR
I was not trying to discourage you from 160M. I had the same size lot and you just get that 120 feet of wire around the best you can. Zig-Zag, droop the ends down, whatever, and even if it's only 35 feet off the ground you can get on 160M.
You can get a linear with 3-811A's and that is the beiginning of something to enjoy. Join the crowd. Around 7PM there's a nice group around 1880 plus or minus and then there's another good group around 1980 plus or minus. Twist the dial a few turns. The exact freq changes to avoid the QRM.
The freebie low power transmitter is a great start!! We want to hear from 5 land.

Fred

Thats just about the right power for a Part 15 rig if you have a big property to cover ;D

At least mine is plate modulated ;)

Carl
KM1H

What is yours?

Home brew: 12AT7, 6AQ5 RF; 12AX7 audio, 6N7 modulator. Pretty much derived from an old ARRL Mobile Manual. Runs at low B+, nice and cool. Driven with feeds from Live365. I prefer 50-60's rock, doo wop, soul, R&B, country/bluegrass.

Sounds real good, strong enough to cover up all the noise and digital crud anywhere on the property I want to listen.

Carl
KM1H

If you want to play around with it, you can likely dump a 6146 into the final position... I'd just check what the B+ voltage is... doubt that it is super high (like a CB linear w/sweep tubes) and also the power out is not very high, so the 6146 ought to work ok. Or almost any other tube of sufficient plate capability, given the low frequency that it works on...

Just a thought...

           _-_-bear

I would think that any of the older horizontal output tubes in the 6Bxx, 6Cxx, and 6Dxx series would be fine. Even something in the 6L6 family if the voltage is within spec; the lead length to the plate pin wont be an issue at those frequencies.

Carl
KM1H

Thanks for the suggestions and replies,

I actually picked up 5- NOS 6DQ5's today on Ebay, for $35 shipped, this morning.

So I now have $35 invested...

Edit... decide to keep it for now...

Sell it. Get a better transmitter with MOAR power.

I'll have to cast my vote again  for you to keep the low power rig and glom onto a linear amp and you are home. B'cast quality audio from the little guy and a big push into the Aether from a linear. Very simple.......no mods or re-designing what is already working with another final RF tube. Prolly easy to move to 160M, too.
Carl, I think W5EFR wants to use this on the Ham bands, not as a part 15 device.

Fred

Thanks for the reply and support Fred, yes, I intend to put it on the 75 meter band (I don't have enough real estate for a 160M antenna)

After a chat with the XYL, I have decided to keep it. It was given to me by a friend who intended for me to use it.. I think that would be the right thing to do...

However, I did chat with Mr Martin on the phone today and told him that I will provide him with any information I can get from him to aid him in reproducing these units for his use...

That being said.. I remember that I have a home-brewed 75/40 meter amplifier that I picked up a couple years ago in the closet. It runs 2 - 813's in grounded grid format. The last time I used it, pushed by my IC-718, it put out 750 Watts on SSB with 65 watts of drive.. well it did that for about 10 seconds, then it set the house alarm off in Panic mode (The alarm was not "armed)  :o

Could this amplifier be used in the AM service with reduced power output?

I know that. My comment was an attempt at humor that apparently flew a bit too high. ;D

Carl
KM1H

I Caught it.. The thought crossed my mind for a quick second as soon as the guy told me he had it... but I though 50 watts is a Little more than the legal 100mw limit...

but that is 100mw "effective radiated power"... so if I ran it into a 2 meter mag mount on the beer fridge...   :P

As to using your 813 amp for AM, consider the math. It is generally accepted that the 1500 W PEP level equates to 375 W of carrier. Then your amp that can do 750 W PEP would have an AM carrier level of 187.5 W. Since your gain seems to be slightly over 10dB, you would need around 18 W of carrier from the new exciter. The limiting question has to do with power dissipation in the 813's. For continuous service, they are rated at 100 W plate dissipation each. You will need to run  them up to say 190 ior 200 W of carrier output and measure the DC power unput to the plate. DC plate voltage times current. If your amp uses a "plate" meter in the cathode or negative side of the amp, remember to subtract the screen currents from the cathode current to get the plate current.  Ignore that comment if you are grounding the screen and the grid, as in a triode connection. You then subtract P out from the DC power input to get roughly the plate duissipation. There will be some loss in the tank circuit, but that would make your tests conservative.

So with the power dissipation calculated, and if under spec, then you will be fine. You may want to make sure there is enough air flow to keep the seals from failing. Some tweaking of the bias point may be necessary to optimise performance for AM.

Sounds like a lot of fun to get this rig on the air!

Rob

Thanks Rob!

I'm waiting for my 6DQ5's that I picked up on Ebay yesterday.

I have a FCC-1.FCC-2 NorCap QRP DDS that I should be able to use instead of crystals.

I'm a hurrying!  :)

What's PEP?

Swing....  ;)

-- Federal Standard 1037

Pretty simple stuff.

So, those of us that can muster up 125% pos peaks are in whyolation???
oh My Gawd......will the FECES catch me??/
Fred

Only if the 125% modulation causes the PEP to exceed 1500 watts.  ;-)

oops - -I did not see the posys where you decided to keep it. Good!!


well, if you don't want it... I think it's pretty interesting.

Thanks for the offer...

I am going to try and put it on 75 meters... I have enough room in teh lot, and I already have a 75 meter dipole up.. the Apex is at about 25'... better than nothing...


If i decide to sell it, I will sell it to Mr. Martin, he would really like it as a working example of Mr Cunningham's work, and they would be built for Missionary service.

Hey Bow,

Welcome to the Cunningham Owners Club!  I'm betting you and I may be the only members but at least you're not alone ;-)

I have one just like that, or at least very similar with the solid state modulator.  It actually works pretty well, considering the fact that I would never hook it up to my 160 meter dipole and run full power and go driving around in my car just to see how far it transmits, or anything like that!

Drop me a PM and I'll get you a copy of the manual.    I had heard that Mr. Cunningham passed away, as you can see from his website I'm sure he's modulating from a better place now ;-)

73 Bob W9RAN

Thanks for the reply Bob!

You have a PM.

Is your unit as scary looking as mine underneath?  ;D

UPDATE!

Thanks to Bob's (W9RAN) Hard work and efforts of scanning, I now have a Manual for this thing!

THANKS BOB!

Reading the Manual, it is actually rated for 36 watts, but can push 50 watts with out 100% modulation.

Here is the Schematic:

http://riley-music.com/BowsStuff/CM3050/14CunninghamSchematic.jpg

Tuning information for the AMBX Band:

http://riley-music.com/BowsStuff/CM3050/18CunninghamTuneUpChart.jpg

And Bottom Layout Drawing:

http://riley-music.com/BowsStuff/CM3050/20CunninghamWiringDiagram.jpg

Any suggestion to get this thing on 75 meters, or possibly 40 meters... or both?

Looking at that LPB Antenna tuner in the first post shows that this was a carrier current setup that injected RF into the 3 phase power of a large building. This type of AM broadcast was common in the 1970's, and would fill a building of RF, mixed with multiples of TV horizontal oscillators. The reception usually was poor. The balanced system usually could barely be heard 100 yards away from the building.

I used to work at WFRS in Michigan where I maintained 17 Carrier current transmitters located in college dormitory boiler rooms. The LPB (low Power Broadcast) company was well represented at this college along with some home brew transmitters.

Jim
WD5JKO

Actually, this was pulled from a transmitter shack where it was a backup for a Broadcast station in Brookshire, TX, when they upgraded.

I saw a date on a drawing in the manual for 1992, so it isn't that old...

printed and looked over schizmatic ... looks like L2 is not too critical and may work as is ...L3 looks to be resonated with cap in 6DQ5 grid and will need to be reduced roughly to 1/4 for 80 mtr ....could look at most any working low power xmtr and get an idee ... reduce L5,6 about the same ....put in variables for C12,13 ... the grid dip osc is your friend here ...BEFORE changing anything verify resonance for buffer and final plate at assigned freq and then ease them on up ....mucho fun ...73...John

MAN,

I used to live in the Waller area...  Bought a car from someone in Brookshire.

Wish I coulda landed that TX... Looks to be somethin cool.

--Shane

There used to be a lot of little broadcast transmitters like that in Canada.  The stations were unmanned using a phone line audio feed to relay CBC to small isolated communities.  I remember seeing several of the antennas in Nova Scotia years ago.  Typically a 150' or so (total length) inverted L held up with 40' or so wooden utility poles, with the call letters of the station printed on a sign attached to the building.

I talked to a ham in NS last winter on AM and this topic came up.  He said that those little relay transmitters have about all been taken out of service now.  He told me he had one of the transmitters he was considering putting it on 160m.  It uses a single 6146 in the final.  About the same power output as a Ranger.

Well.... I have finally got some time to look at this thing with the schematic... and this critter ain't right.

My first clue was the 540V on the plates.. when it is supposed to be 450V (as per the schematic after the half wave rectifier) out of the transformer.

It seems the power transformer was changed out from the original. The schematic calls out a 300ma transformer, this is a 750V 175ma transformer.

hmmm...

Then I see the extra diodes and resisters that were not in the schematic, so it has been hacked pretty good... so I need to find a 450V 300ma power transformer for starters...

Any suggestions?

450 V @ .3A ....maybe an old TV .... they are still heading to the landfills ... my wife sez I can not adopt any more orphans or recycles .... the 6dq5 is probably ok at the higher voltage just remember the rated plate dissipation ( uh 18 W as I recollect) probably good for a bit more ...73 ...John

"Under normal operating conditions"

Interpretation:   If operating conditions AIN'T normal - like when Cooter and Bubba try to take over the channel - then you can you run whatever PEP (munky swing) you want.

T

Thanks John...

I'm a looking for a Transformer better rated for this thing...

Hell, a 12BY7, 6V6GT and a 6DQ5... I wonder what I can get away with for ma rating...

I have a major update on my transmitter restoration project.

I ended up rebuilding the power supply circuit on the Transmitter back to stock, and only used 1/2 of the installed power transformer secondary windings. The put my B+ Voltage level down to a more manageable level, and reinstate the previously bypassed Neg Voltage Bias supply. this allowed the power level control to actually control the carrier from 0-50 watts,  and I was actually able to modulate it using the "internal" modulation circuit (TIP100).

It got about 60-70% modulated before the circuit started to clip, squaring off the positive peak of the envelope,  but I am a lot further than I have been with it in the few years I've had it.

Now once I get the modulation circuit figured out, I'll have to find a empty frequency, locate a crystal for that freq, and build a leaky Coax....  ;D

I am happy to report:

I have Carrier:
(http://riley-music.com/BowsStuff/CM3050/Outputs/Carrier.JPG)

I have Modulation:
(http://riley-music.com/BowsStuff/CM3050/Outputs/Modulated.JPG)

This was done with an 8 watt mono amplifier feeding the External modulation connection (through the modulation transformer) with a 1kHz sine wave, into a dummy load.

Now that I know it will modulated properly, I can work on the internal modulation circuit more.

I'm not sure how it sounds "On-Air", but it looks pretty good...