homebrew am tx from drive in

[KA2DZT]

Get rid of the 1000pfd cap on the output terminal.  You're grounding your output signal.

[N1BCG]

I agree that it makes sense to focus on the RF circuit and get the transmitter transmitting. First, you'd be troubleshooting a single tube vs several audio stages. Second, the RF signal will give you a convenient way to check the modulator section of the audio circuitry.

Given the "7AC" base connection and measured plate voltage of 300, the 6L6 or 6V6 seem the most likely as the oscillator tube. Try swapping tubes to see if either will oscillate. An oscilloscope connected to the antenna connector will give you an instant indication of success.

It's also possible that the output may need to be loaded in order to sustain an oscillation. Try a dummy load or a long wire, although the latter suggestion could raise some eyebrows among the most reverent.

[w1vtp]

If I was doing this, I would ignore the entire audio circuitry for now and just concentrate on getting the oscillator to work. Once you get the oscillator to get some RF into a wire so that a receiver can pick it up, then go back and diddle with the audio stages. Why analyze the heck out of the audio stages now when the oscillator isn't running.

That's what I did with my 500 watt 813 rig.  I didn't even have the modulator parts when I was putting out 500 cool watts.  Used it on RTTY first and then on AM.

The modulator used a pair of 811a with a CG59AX line input driver with a Thordarson T11M77 modulation transformer.  I used a Bogen M-60 as a driver. My only mistake with that AM transmitter was not having either a hi Z mic preamp for my D104 or an EQ for my RE15.

Worked just fine.

[KA2DZT]

He has a .001mfd cap across the output which is probably shorting to ground the output signal.

[N1BCG]

A 0.001 uF cap across the output would certainly be an issue, even at BCB frequencies since the oscillator output is relatively high Z.

Is it possible that the cap is a smaller value? I can't make out the writing so it would be good go see an updated schematic posted.

[KA2DZT]

A 0.001 uF cap across the output would certainly be an issue, even at BCB frequencies since the oscillator output is relatively high Z.

Is it possible that the cap is a smaller value? I can't make out the writing so it would be good go see an updated schematic posted.

I asked him about that cap in an earlier post, he says it's a 1000pfd.

Fred

[kd1nw]

Hi All,

I removed the cap across the output connector and the 100pf cap from the cathode of the 6L6 and its not oscillating out of control now. The power output seems very low. The tx is on 540 and its harmonic on 1080 is just about as strong. I didn't hear anything on 1620. It wasn't modulating unless I turned the gain up and even at that it just barely modulated. I saw that there was a .05 cap coming of the plate of the modulating 6V6 to ground. It didn't seem right to me. Kind of like the cap across the output connector. I removed that and still the same cruddy modulation. So, suspecting the modulating 6V6 was bad, I swapped that and the 6V6 that drives the speaker output. So, that did it. The 6L6 is being modulated and the the audio in headphones was now cruddy. So, I guess I need to pick up a 6V6 or 2. The audio is still somewhat distorted so that will need to looked into, replace electrolytics, change some of the resistors as suggested earlier and see if I can get the output from the 6L6 to increase a bit. I need to remeasure the voltages on the 6L6. The last time I measured I think plate was 250 and the screen was 270 which doesn't seem right.

And it was great to catch you on 40 meters Clark, sorry I couldn't get on 80 that night. Probably some Friday night I'll jump in and say hi. That was quite a fade that took me out just before Tim jumped in. I actually couldn't hear you for awhile either. After a bit you were back in strong again and I could hear the other station you were talking to as well, Brian I think?

I'll post the updated schematic with the removed components and probably a fix or 2 in a few days.

Thanks again everyone.

[Pete, WA2CWA]

So You Know:
Unlicensed operation on the AM radio broadcast bands is permitted for some extremely low powered devices covered under Part 15 of the FCC's rules.
On the AM broadcast band, these devices are limited to an effective service range of approximately 200 feet (61 meters).  See 47 CFR Sections 15.207, 15.209, 15.219, and 15.221.
The Commission considers unauthorized broadcast operation to be a serious matter.  Presently, the maximum penalty for operating an unlicensed or "pirate" broadcast station (one which is not permitted under Part 15 or is not a Carrier Current Station) is set at $10,000 for a single violation or a single day of operation, up to a total maximum amount of $75,000.

[kd1nw]

Yes, thanks for the reminder

[DMOD]

I'll post the updated schematic with the removed components and probably a fix or 2 in a few days.

And you might want to review the actual circuit again.

At about the time of this website failure, I posted that the circuit in the 6SN7 area didn't make much sense.

Also, the 100pF cap at the cathode of the oscillator doesn't seem right. I can see the 100 pF at the grid to form a capacitive voltage divider, but the 390 pF cap should be the only cap resonating the variable inductor.

Phil - AC0OB 

[N1BCG]

Unlicensed operation on the AM radio broadcast bands is permitted for some extremely low powered devices covered under Part 15 of the FCC's rules.

True, but, it is completely legal to feed RF power into loads that provide limited radiation such as leaky coax (aka radiating cable) and "carrier current" (http://blogs.telosalliance.com/discrepancy/found-in-the-attic-february-2014) systems. The former was quite popular with drive-in theaters and the latter with campus radio stations.

Even 20 watts into these limited area systems could fall well within those FCC rules.

[Pete, WA2CWA]

Unlicensed operation on the AM radio broadcast bands is permitted for some extremely low powered devices covered under Part 15 of the FCC's rules.

True, but, it is completely legal to feed RF power into loads that provide limited radiation such as leaky coax (aka radiating cable) and "carrier current" (http://blogs.telosalliance.com/discrepancy/found-in-the-attic-february-2014) systems. The former was quite popular with drive-in theaters and the latter with campus radio stations.

Even 20 watts into these limited area systems could fall well within those FCC rules.

Rules probably might have changed over the years. Looks like 200 feet is the magic number unless you want to dance with the FCC rules.
https://www.fcc.gov/guides/low-power-broadcast-radio-stations

[kd1nw]

latest schematic

[KA2DZT]

Raise the 5K grid resistor to 50+K and the oscillator should work.  The 450K B+ resistor going to the 6SN7 is way too high, probably something 10-30K. The cathode resistors on the 6SN7 also seem way too high, something less than 2K.

Looks like 1/2 of the 6SN7 feeds one 6V6 for the spk output and the other 1/2 of the 6SN7 feeds the other 6V6 for the oscillator modulation.  Should work,  whole schematic starting to make sense.  There may be other changes needed, but first try to get the oscillator working.

Fred

[kd1nw]

Thanks Fred, I'll try those resistor changes. That 5k grid res is a 7 watt, I only have some 1/4 or 1/2 w resistors around. I see if I can pick up somthing 7W or greater the next time I get up to the electronics store.

[Opcom]

The FCC gripe is about a certain number of Microvolts measured at a certain distance from the property or antenna.

There is also a rule about antenna height.

The rules are pretty simple. I read them before and they made sense.

The one thing you may wish to use, in case of worry,  is a calibrated frequency-selective RF Microvolt-meter. It's one thing if you know the field strength and another if it looks like you have to guess at it.

Carrier current may radiate quite a bit, but it is a good solution for home BC and music play because the best signal will be right there in the walls near your receivers at home not on a 9 FT pole in the back yard etc. therefore you can be satisfied with more QRP-ish levels and safer from trouble.

Personally I sort of gag on the pablum I am spewing, because I would like to run a Watt or ten and be a real pie-rat, but if everyone and their dog did it, then it would be heck. So I am content with the FCC rules.

8-)

[N1BCG]

Personally I sort of gag on the pablum I am spewing, because I would like to run a Watt or ten and be a real pie-rat, but if everyone and their dog did it, then it would be heck. So I am content with the FCC rules.

I read that and then thought your location was "Clandestine, Texas" LOL...

Just goes to show how expectations can steer perception. I used to listen to "pirates" that were abundant above the AM b'cast band in the 80s. But I digress...

It was common for drive-in theater owners to replace their aging speaker boxes with a single carrier-current transmitter fed into the speaker wires that ran to each parking space. Such a system would present a very low impedance, so a transmitter with an untuned output as discussed here would need every bit of its 6L6 output.

[DMOD]

latest schematic

After you get the oscillator working, there is still a question on that lower left thingy: Is that a 2Meg potentiometer?

You already have two local feedback networks at the 6V6's for lowering distortion and for "rolling-off" audio response, so I don't see a reason for that additional 0.047 uF cap and the 2Meg potentiometer in that outer loop.

Also, the 100k pot adjusting the audio level to left (audio monitor) 6V6 should be referenced to ground.

Phil - AC0OB

[Opcom]

I read that and then thought your location was "Clandestine, Texas" LOL...

It is a play on that same word. Gotta candelstine radio up in there. My brand so to speak, "Candelstine". "When you care enough to do what you want".

[VE3LYX]

I use a 9 volt battery and clip leads and a voltmeter to sort such stuff out. (With my old mil sets I have no schematics or cabling so I had to learn to safely figure it out before I gave it a chance to smoke.) Easy to do and no damage takes place.
My instinct would say a 7AC socket diagram which could be a 6v6 or a 6l6. That makes pin 3 plate. (Feed plate of known tubes ad see if any comes to which pin on the unknown. 2&7 heater (heater wires are often twisted together or are indentical wires so are easy to spot.) 6 is unused, grids are 4(screen)&5(control) and so on. A few minutes will tell you. A 6sn7 or a 6sl7 have 7&8 for heaters so you can spot them immediately. Most builders would use a beam power 6v6 or 6l6 I would think but a couple of minutes testing would save a lot of wondering.
don

[DMOD]

Here is a suggested modulated Osc. Improvement. I drew it in 10 minutes using LTSpice.

You will need to confirm that your LxRes. shows continuity and is indeed variable.

You may need to borrow a scope and confirm RF at the cathode, grid and plate using a 3.3 to 4.7 pF cap. at the probe tip.

Good Luck.

Phil - AC0OB

[kd1nw]

Hi Phil, yes it's a 2 meg pot and a vu meter. Thanks for the osc improvement doc