Home AM Transmitter (again!)

[Winky Dink]

The saga continues. I picked this up again after two years on the shelf.



Going back to Phil's original schematic and instructions, I still couldn't make it work. Then I tried to build an Antique Electronic Supply kit that I bought some years ago, but I busted the graphite slug in the oscillator coil. I bought a new coil and I busted that one, too.

So I started to build a "Li'l 7" again from scratch. This time I put it on a big board so I could lay it out just like the schematic with all new components.



When it was finished I powered it up, turned on a couple of radios, and adjusted the RF coil until I busted that one, too.



I'm down to my last RF coil, and I'm convinced I'm missing something very obvious, so I'm going to ask you radio-smart people to check what I'm doing one step at a time. Here's step one: Putting in the Audio Signal. I have a stereo phono plug in a CD player. The lines from the left and right channels (tip and ring) are joined to carry the audio in, and the line from the sleeve goes to ground. This is how I have it set up with a 100K pot, and I've followed the signal with headphones all the way to the ll7L7.



So, does this look right so far?

The potentiometer shows a linear 0-100 K ohms until I plug in the stereo plug. After the stereo plug is in, I can't figure out what the potentiometer is doing--sometimes it looks like a bell curve peaking at 25K. And with the headphones, the audio cuts out within 10 percent of the range of sweep.

[jr_tech]

That looks correct, but I think that I would put a capacitor in series with the input, so that the low resistance of the headphone output of the CD player (or whatever) does not affect the resistance measured at the pot. Also it appears that you have the pot wired backwards, with ground going to what should be the high end. It will work ok as wired but the pot will need to be turned CCW for max input, rather than CW.

But back to more basic stuff... does the tube light up? does the power supply make around 125 (or so) DC? ... why do you keep breaking the coils? do you not have a plastic (nylon ?) hex tool for adjustment?

jr

Edit add:
Also check the wiring of pin 2 of the oscillator coil... schematic shows it connected to ground, not to pin 4 of the tube. Connection of C-6 would also not be correct if this is so.

[Winky Dink]

Yes, I realize the pot is backwards--and I am using it backwards--and I'll fix that.

The leads from the tube pins are:
1- dark blue
2- black
3- yellow
4- orange
5- light blue
6- red
7- black
8- green

Oscillator coil pin 2 is green wire. All greens go to ground. Tube pin 4 is an orange wire.

I started with the audio input because it's so basic that Phil's instructions do not describe it in exquisite detail. Would it be better to use the output for a 4-ohm speaker than the headphone jack?

Breaking the grapite slugs: I never had this problem before, but now the adjustment is very stiff (I am using the appropriate plastic hex). I finally figured out that I've been moving past the useful range, to the end of the threads. Then it binds, and when I try to back it up, it breaks. But it only took me 3 coils to figure that out!

Yes, the tube lights up. And the electrolytic caps hold a charge.

I'll fix the pot and see how it works with the speaker output. Then I'll do the voltage readings.

Thanks very much.

[jr_tech]

"Oscillator coil pin 2 is green wire. All greens go to ground. Tube pin 4 is an orange wire."

If that is the case, do you then have *both* ends of the 47k resistor as well as the grid of the tube (orange wire) connected to ground? Take a close look at the wiring between the two lugs that the resistor is connected to.
jr

[Username1]

Hey that looks like a cool project...... Can you post a schematic? with voltages you are getting ?

jr_tech has a point about matching the source and load.... look at this:

http://en.wikipedia.org/wiki/Impedance_matching

What you want is for the output device to "see" a speaker or earphone load, And for the next stage to see its natural input resistance, or "impedance"

The circuit you have, the output device will be very unhappy whenever the volume control gets turned down under 1000 ohms. (just because I have seen headsets in the range of 1K to 8 ohms) The input of the tube circuit would like to see around 50Kohms or more I would imagine....

I found the circuit, you should try to get it working with the volume control at mid point, so it don't cut the tube off....

Remember maximum power transfer will only be 50% not that you will need that......

[Winky Dink]

Jr,
Regarding the wiring of the 47K resistor (R1), I don't see what you see. One end is grounded; the other end goes to tube pin 4 and C5 (68pF). Here's an updated photo of my build and the original schematic:





(Note to all: This is a tried and true design. See the whole story at Phil's Old Radios. This will take you to the AM Transmitter article: http://antiqueradio.org/transmitter.htm
If I knew what I was doing I'd be tempted to improve on this design, but I need to stick with the original which others have built successfully.)

And here are the voltages I get:



This is as far as I got with the project two years ago. I looked at the old notes, and the voltages were basically the same as they are now. So, do these values or the wiring seem true to the schematic and the expected function of the device?

To Username1: Thanks for the link to the impedance-matching article. I have not yet read it, but I see its relevance and will get to it as soon as I can.

[jr_tech]

Thanks for posting the improved picture... In the old picture, it looked like the green wire under the resistor (R-1) was connected to the lugs at each end of R-1, this is a much better pix, and at first glance the wiring appears to be correct, but I will print it and study more closely.
Have you tried swapping the wires to #1 & #2 of the coil (or # 3 & #4, but not both windings) to get oscillation? Phase of the coil matters, and perhaps these coils are reversed?

jr

[Winky Dink]

In my previous (two years ago) efforts, I tried switching the coils--someone had suggested the primary & secondary might be reversed on the schematic. I tried that without success, and errors in this long-used schematic are unlikely.

At this point, I haven't even tried the transmitter. I had been asked previously, "Does is oscillate?" I assume that means do I receive any kind of audible signal from the transmitter, and the answer is, "No, I have no audible signal."

Here's what I'm going to try, and I'm explaining this so any ridiculous errors will come out. I'm going to turn on my 1931 Airline and my Atwater Kent model 37, tune both to a quiet spot in the expected frequency range, and turn up the volume. I can receive local stations on those sets without an external antenna.

Then I'll plug the transmitter into a CD player with the volume turned up high, and I'll set the transmitter's potentiometer at mid-range. The receivers are 15 feet away from the transmitter. Phil's instructions say to DISCONNECT the transmitter antenna for this trial.

Here's a question that may seem silly, but it's important. When tuning the coil, where should the graphite core be in relation to the windings? Fully within the windings?

I try that the way it's set up now. If it doesn't work, I'll try reversing the coils.
Thanks for the help. I'll let you know what happens by tomorrow afternoon.

[jr_tech]

I guess that I would start with the core nearly falling out of the top of the coil and adjust in from there until it nearly bottoms out... cover the entire range.
With the transmitter antenna disconnected, trying to get a signal to a couple of vintage sets 15 ft away might be pushing the distance limit somewhat... do you have a nice sensitive little portable radio that you can put *very* close to the transmitter?

jr

[bob91343]

Pin 5 on the tube is the screen grid. It shouldn't be such a low voltage; I think 100V or so would be better. I am not looking at the tube data so perhaps I am in error.

In order to get plate current in a normal pentode, the screen voltage needs to be positive, not as high as the plate. If you increase the screen voltage by a series resistor to B+, be sure to decouple the audio from the pot with a coupling capacitor. And bypass the screen for RF with perhaps a 220 pF capacitor.

[jr_tech]

Indeed the screen grid connection and voltage is suspect, but in the construction article, Phil explains that it is a "feature" not a mistake.

The pentode portion of the tube combines both the oscillator and modulator. The novelty of this circuit is that the pentode screen is returned to ground rather than to B+ as might be expected. The modulation input varies the pentode screen voltage about 1 volt above and below ground potential.

I am scratching my head over that one, however.
jr

[bob91343]

I don't think this is a good way to operate the tube. Raise the screen voltage and then modulate it with a capacitively coupled audio signal.

Maybe I will take a look at the tube characteristics to see if this is feasible.

[bob91343]

Okay, typical operation shows 105 V both plate and screen, resulting in 43 mA plate current, 4 mA screen current and about -5V control grid bias.

The tube simply will not work with such a low screen voltage, I firmly believe.

Estimating 4 mA screen current, add about 10k or so from screen to B+, then capacitively couple the audio to the screen. This will require a substantial audio signal, perhaps 20V, not easily obtained from every audio generator and certainly not from a microphone or phono pickup. So a 'speech amplifier' is needed.

[jr_tech]

A comment from the earlier thread about the very unusual operating conditions of this circuit:

old_coot88
Yeah, i thought about that too. But figured that the cathode being at `near` ground (except for the coil resistance) probably mitigates for the absent G2 voltage a little. Then with both grids at the same potential, the tube behaves electrically more like it's a triode running at lower power than a full-up pentode. Or at least that sounds sorta plausible.

Got to looking at the 117L7's spec sheet posted earlier, and it's classed as a beam power amplifier. If that's correct, it's a tetrode with beam-forming plates and there would be no suppressor grid (G3) between G2 and the plate. So with G1 and G2 "strapped" at the same electrical potential, the tube would be running as a "beam triode".

http://www.videokarma.org/showthread.php?t=251917
Still scratching my head...
jr

[Winky Dink]

Sorry I can't add much to this discussion, but it's way over my head. I spent some time checking the pin 5 voltage. With no connection to the audio source, we get 0 to -.8V.
When I plug in the earphone jack with the audio power off, we get -5V. With the power on, we get +5V. If I leave the meter on pin 5, the + or - 5V bleeds down, losing about 1V per minute. I put in a spare 117L7 and got the same results.

Tried tuning the coil with a 4-ft antenna set up 15 ft from the receiver, but received nothing at 900 KHz. I have a couple of table radios with loop antennas, but they're too noisy between stations. So, If the radio won't come to the transmitter, then the transmitter will go to the radio.



Alas, this didn't work either. We're being visited by our grandsons for the next couple of weeks, so I won't have much time to play with this. On the other hand, at 7 and 10 years old, they're both a lot smarter than me, so maybe they can figure it out.