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 Post subject: AM 800 is on the air
PostPosted: May Mon 14, 2007 8:28 am 
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Location: Saskatoon
I've been tinkering with this for the past week, and I think it's working pretty well. It's a one tube transmitter using a 6M11 compactron.
Image
I had a couple of these lying around for a homebrew receiver project that's currently on hold, so I thought, "why not make a transmitter?" It uses a ceramic resonator in the oscillator circuit. These are cheap and are readily available in a number of frequencies. I've actually run this with 640 kHz, 800 kHz and 1000 kHz resonators. And just to be silly, I plugged in a 455 kHz resonator and a 3.57 MHz crystal and picked these up on my shortwave receiver. So, this gives a few more options for areas where the 1000 and 1200 kHz frequencies are occupied.

Here is the schematic:
Image

Under the hood:
Image

Here it is fired up and transmitting audio from my MP3 player:
Image

As you can see the antenna matching network (on the right) is a separate unit. I decided to build it separately so I could use it for other things as well. I'll probably eventually build a more compact pi network into the case. I also have to get the power supply in there too. The field strength meter in the background is about an inch from the transmitting antenna (the red wire), and is at about 1/3 scale which surprised me. I didn't think these homebrew units would budge the needle on one of these cheap meters. However, I must say that having that matching network makes an amazing difference on how the signal gets out. I can pick up the signal anywhere in my yard using a 10 foot wet noodle transmitting antenna. I think you could transmit with a few feet of wet binder twine for an antenna with the right matching network. Thanks to Terry Judkins for posting about the pi matching network.

I shamelessly stole bits and pieces of the circuit design from a number of transmitter circuits that were posted here and on other websites. However, I will take credit/blame for the strange direct coupled modulator circuit with the neon lamp. The lamp has a dual function: It drops the DC level about 50 volts for the screen grid without attenuating any audio signal, and it's used as an indicator to set the modulation level.

Anyway, it sounds pretty good to me, but I don't have anything to compare it to, since this is the first transmitter I've built.


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 Post subject:
PostPosted: May Mon 14, 2007 3:42 pm 
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Location: Powder Springs,Ga. USA
I like it. Have you checked the modulation level? I found that, to get 100% modulation, the the screen had to go slightly negative.
I assume that the grid resistor for the pentode was left out in error?

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 Post subject:
PostPosted: May Mon 14, 2007 9:04 pm 
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Location: Saskatoon
Oops, yes the grid resistor (pin 2 to ground) is missing from the schematic. I had stuck a 33k resistor in there. I used a low value because I initially thought I might be injecting to much RF into the pentode, and figured that would load it down a bit. I should try a larger one in there to see if it makes any difference. I think that's the only part that I didn't tweak while I was getting it running.

As for modulation, I found that the tube was cutting off before the screen went negative. It would be good at about 90-95% modulation, and then get very touchy beyond that. I'll post some scope traces later.


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 Post subject:
PostPosted: May Mon 14, 2007 9:10 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
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Location: Sandpoint, IDAHO 83864
The grid resistor to the pentode stage sets the bias and operating conditions of the tube section. The amount of bias developed across it will vary the linearity of the tube to be modulated, so you may want to experiment with it when you get the scope hooked up to it. I would venture a guess of more like 100K ohms, but I have never played with one of those tubes before.

If you are worrying about too much RF drive to the tube, simply make a capacitive voltage divider from the plate of the previous tube to ground and tap the grid between the two capacitors.
Curt

And, where may I ask, is the B+ bypass capacitor? You need to keep stray RF out of everything else, including the power supply.

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(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: May Mon 14, 2007 10:17 pm 
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Very nice work. :D

My version of Norms TX lacks punch. Suppose I made this one but with different tubes. I could use a 6SN7 for the first two and follow up with a 6V6, 6DQ6A or even an 807. :shock:

Let me explain, If I ran the final at a low voltage I could then stay reasonable and low on the power output. Remember we are not running a matched dipole so efficiency will be poor. One could also purposly couple it to the antenna to cut ouplut. If I had some trouble getting decent coverage I would then have the option to power it up a bit. I guess I am suggesting a small block V8 in a golf cart VS. a chansaw motor in the same golf cart. It is always easiar to keep your foot out of the gas than to fight inadequate power to start with.

Good or bad idea?? Are to power limitations on how much you make in the box or how much you put in the air?


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 Post subject:
PostPosted: May Mon 14, 2007 10:48 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34326
Location: Sandpoint, IDAHO 83864
I believe all transmitters used to be rated by power input, and only in recent years did the FCC switch to power output. But I would not worry about it, as the FCC does not seem to care about the small guy, unless he interferes with the neighbors and they register a complaint.

But like I said, some tubes, you have to have enough plate voltage on them to get them into the right part of their operating curve. If this is not done, you can expect all sorts of problems with the linearity of the tube.
Curt

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 Post subject:
PostPosted: May Mon 14, 2007 11:21 pm 
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What sort of output could I expect from the 6DQ6 with, say 200- 400V on the plate, using the kind of rig we have here?


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 Post subject:
PostPosted: May Mon 14, 2007 11:38 pm 
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Location: Minneapolis, MN USA
I like the case, what is it from ?


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 Post subject:
PostPosted: May Tue 15, 2007 1:14 am 
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Location: Saskatoon
Curt,
I completely forgot about the B+ bypass capacitor. Thanks for the reminder. I'll stick one in before I fire it up again.

Scot,
The triode sections in the 6M11 fall somewhere in between a 6SN7 medium mu triode and a 6SL7 high mu triode. The 6SN7 might have a bit more oomph modulating the beam power tube. The pentode section in the 6M11 is only rated at 3 watts plate dissipation. So, you will have to experiment with component values if you use those tubes.

Dale,
I built the case. When my brain got burned out on the electronics part of the transmitter, I switched to woodworking for a while. It's MDF with dark walnut stain, and lacquer. The top is 1/8" hardboard with hammertone paint.


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 Post subject:
PostPosted: May Tue 15, 2007 10:13 am 
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Curt, your estimate of 100k for the grid bias resistor was pretty good. I put a pot in temporarily, and adjusted it to get the best signal, then took it out and measured it. It was 99k. So, close enough; I put in a 100k resistor.

Here are some scope traces. These first two were taken before I changed the grid bias resistor.

This first one is screen grid voltage versus output:
Image
This is as the circuit was originally set up. The modulating signal goes almost to zero volts, and you can see that modulation is less than 100%

In this next one, the modulation goes to zero volts, and if you look closely you can see that the output actually falls off rapidly to zero.
Image
I realized that's probably not a real characteristic of the tube. It's just the way the circuit is set up. As soon as the screen voltage goes to zero, the current dies abruptly as the neon lamp shuts off. I couldn't think of any way of smoothly driving the screen grid through zero volts and then negative without major modifications to the circuit, so I gave up at that point.

This last one is after increasing the grid bias resistor from 33k to 100k, and the horizontal input is taken right at the audio input instead of the screen grid. So, you can see the overall performance of the circuit.
Image
The modulation percentage is higher here, but you can see a bit of curvature in the envelope. The distortion wasn't audible to me though.


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 Post subject:
PostPosted: May Tue 15, 2007 3:47 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34326
Location: Sandpoint, IDAHO 83864
That non-linearity indicated by the curvature of the modulation waveform is inherant with screen modulation. It is something you pretty much have to live with. Some tubes are worse than others in this regard. It just shows that the Esg versus Ip in pentodes is not a 100% linear function.

Some experimenters have gone to great lengths to minimise this non-linearity, including feeding some audio to the control grid also. It is absolutely mandatory that a person have a scope when doing things like this and is familiar with its operation, as you well know. Just going by ear and saying it sounds good simply does not cut it.
Curt

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(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: May Wed 16, 2007 12:26 am 
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Location: Saskatoon
Yes, when I first got this fired up, the scope traces were pretty scary looking. I experimented with various forms of feedback, including a 1N34A detector feeding back to the audio input. They all worked, but cut the power or modulation too much to be useful. The best compromise I found was simply to use the unbypassed 470 ohm resistor in the audio stage cathode. The problem is that the audio stage needs a huge voltage swing to modulate the RF, and getting decent linearity at that level is tough.

So far, I haven't had particularly good music to play through the thing. But last night I hooked up a spare stereo preamp, and will be able to play a wider variety of CDs and tapes. Most of the tunes on my MP3 player didn't have great sound quality, and the MP3 player itself didn't have quite enough output to fully modulate the transmitter. The preamp works well, and I'm going to give this thing a proper listening test over the next few days to see how I like it.


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 Post subject: Re: AM 800 is on the air
PostPosted: May Fri 18, 2007 6:48 pm 
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BobWeaver wrote:
Thanks to Terry Judkins for posting about the pi matching network.


Yes I agree, Terry is a hero in my book for his implementation of the Pi Net... The folkes that are having range problems could likely improve output by using the circuit... The 6888 and 6GY6 are both Hot Rods at 150-200v B+...

BobWeaver wrote:

I shamelessly stole bits and pieces of the circuit design from a number of transmitter circuits that were posted here and on other websites. However, I will take credit/blame for the strange direct coupled modulator circuit with the neon lamp. The lamp has a dual function: It drops the DC level about 50 volts for the screen grid without attenuating any audio signal, and it's used as an indicator to set the modulation level.

Anyway, it sounds pretty good to me, but I don't have anything to compare it to, since this is the first transmitter I've built.


Congrats on the build... If any of my xmitts were any inspiration, you can probably bet I stole the idea from some one else, as my experience before ARF was zero & none... :lol:

Looks like you've added to the information pool of mini xmitt building(In other words I was wondering how I was going add a pre-amp to my 25L6(xBQ6, xDQ6) xmitt)... :roll: I champion the line level input myself, mainly used the xfomer for it's simplicity(it does work well though)...

Tom

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 Post subject:
PostPosted: May Fri 18, 2007 11:57 pm 
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Thanks Tom. Yes, I copied a few things from the schematics you posted. What I finally ended up with may not bear a lot of resemblance to what I started with though.

I was a bit surprised that the MP3 player put out such a low level signal. I thought it would be about the same as line level. If I'd realized that at the beginning, I would have used a beefier signal source while I was setting the transmitter up. With the higher level input, I should be able to go back and adjust some things to get a bit more power out now. It's a tradeoff between input signal, modulation level and output power.

I mentioned in my last post that I was a bit curious/concerned about the slight distortion in the modulation envelope, and was going to give it a good listening test for a few days. Here's what I've noticed, so far. Right off the bat, the main limitation was the receiver. I was listening though my regular stereo receiver, but it seems that the AM tuner was an afterthought on that model and the sound quality on AM is not good. However, I have a Sony AM stereo tuner with selectable IF bandwidth. So, I plugged that into the stereo and used it for the listening test. What a difference! It sounded very good with the extended high frequency response (even though it's still only up to 8500Hz). I couldn't hear the distortion. No doubt, a true A/B test would have made it apparent, but if that's what it takes, then I'll consider it to be good enough as is.


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 Post subject:
PostPosted: May Sun 20, 2007 12:29 pm 
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Location: Saskatoon
Since I can't leave well enough alone, I thought about adding a bit of automatic level control to the audio section. So, I replaced the 470 ohm cathode resistor with a #47 pilot lamp. It worked considerably better than I anticipated. It's much more forgiving of audio input levels now. But, there's a really interesting side benefit. I was watching a trapezoid trace on the scope, and noticed that the whole trace was expanding vertically with high audio levels. Thought about it a bit, and realized that this small change has turned it into a controlled carrier transmitter. At high audio levels, the #47 lamp resistance increases and limits the audio gain, but at the same time causes the V1-A plate DC voltage to rise, and therefore also raises the DC level on the pentode screen, increasing the carrier level. This boosts the output power significantly.

Are you starting to get the hint that I have this thing about using lamps for purposes other than illumination? :)

Revised schematic:
Image
Other changes in the schematic are:
- Addition of V1-C grid bias resistor which was inadvertently missed from previous schematic;
- Reduced the modulation control pot series resistor from 15k to 10k, to give a little better range of control.
- Addition of B+ bypass capacitor. (Yes, .1uF seems big, but I started with .01uF and got a serious resonance problem. So I used .1uF.)


Last edited by BobWeaver on May Tue 22, 2007 1:01 am, edited 1 time in total.

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 Post subject:
PostPosted: May Sun 20, 2007 2:39 pm 
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This is getting too interesting not to try... Likely I'll stick one in a old UHF converter with a xtal osc driving it... Need to check on my stock of 6M11s(other words, don't think I have one), I'll probably have to go with a more conventional triode/pentode... Most of the Compactrons have equalivants(less one section) in the std 9 pin tubes, need to study the specs a bit...

Any reason not to use a pot on the input, as well as the modulation controll??? None of my favorite line sources have a level adjustment...

Tom


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 Post subject:
PostPosted: May Mon 21, 2007 3:20 am 
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It would be great to get a second opinion. Like I said before, I don't have any other transmitters to compare it to. So, maybe this thing's really a pile of doo-doo.

I'm no expert on substituting tubes, and my RCA manual isn't much help, because it never seems to use the same operating conditions when giving parameters for different tubes. But, it looks like a 6JV8 (or 8JV8) might be close and they seem to have been popular in TVs. Or, maybe it would be easier to use a dual triode and a single pentode?

My oscillator section was putting out at least 30 volts peak to peak; so I don't know how well a crystal oscillator would work, unless you're still planning to have the second triode to boost it up.

I have a couple of spare resonators (assorted frequencies) that I could toss in the mail to you if you PM me your address.

No reason why you shouldn't be able to add a volume control. I didn't bother to put one in, because my audio sources all have their own controls, and I already had enough knobs to twist.


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 Post subject:
PostPosted: May Mon 21, 2007 2:04 pm 
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Bob,
Nice job on the transmitter. What kind of range are you getting from this transmitter?

Regards,
Sal Brisindi

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 Post subject:
PostPosted: May Mon 21, 2007 2:54 pm 
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BobWeaver wrote:
I'm no expert on substituting tubes, and my RCA manual isn't much help, because it never seems to use the same operating conditions when giving parameters for different tubes. But, it looks like a 6JV8 (or 8JV8) might be close and they seem to have been popular in TVs. Or, maybe it would be easier to use a dual triode and a single pentode?



Yeah, I breezed through a GE manual and there doesn't seem to be an exact equiv ti the M11... The only other Compactron listed with the same sections is equiv to a 6GH8 with second triode... I have a couple thousand tubes, so I can find something... There are 6-8 other tubes similar to the 6JV8 that have the same basing...

With 30v out of the osc I suspect it has a healthy output, the xtal osc I've seen make just 5v...

Tom


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 Post subject:
PostPosted: May Tue 22, 2007 12:58 am 
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Sal Brisindi wrote:
What kind of range are you getting from this transmitter?


Just checked again a few minutes ago, with a 10 foot piece of wire for the antenna (more or less vertical), and running at 1000 kHz. (When I run at 800 kHz, there's a station about 120 miles away that starts to interfere as soon as the signal drops off a bit.) I estimate it's a clear usable signal up to 100-150 feet. After that, it fades into the static pretty fast. But the signal is clear everywhere in my house and everywhere around the yard. On the car radio, it's still audible at the end of the block (about 250') but just barely. It's pouring rain, and the power line insulators are buzzing merrily.


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