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 Post subject: Lil 7 AM Transmitter
PostPosted: Jan Sat 07, 2017 7:12 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 104
Location: Independence, KY
I just completed the Lil 7 Transmitter from the schematic on Phil's Old Radios website. It is working and has quite a strong signal. The only issue I have is I cannot seem to get rid of all of the heterodyning. I receive the signal around 820 on the dial. which is somewhat clear around here. No matter how much I adjust the slug in the RF coil or adjust the receiving radios tuning dial It's still there, not drowning out the signal, but there in the back ground. Is this just inherit with this circuit? Also the volume control on the Lil 7 only makes the interference worse the further it is turned. Best reception is from just on to about 40% of the rotation on the volume control.


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sat 07, 2017 2:13 pm 
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Best reception location is generally at the upper end of band that is clear... If freq isn't changing by adj coil then the slug inside coil likely isn't moving...

The phono osc circuits are far from the best choices for building a homebrew transmitter...


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sat 07, 2017 2:58 pm 
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Location: Roanoke, VA
Mike L wrote:
Also the volume control on the Lil 7 only makes the interference worse the further it is turned..

The 'Lil 7 is typical of the phono oscillator type of circuit. It is a directly modulated oscillator. At low levels of modulation the signal may be satisfactory, but above about 30% modulation distortion products can become significant, including incidental frequency modulation, excessive sideband components (aka "splatter"), and harmonic radiation.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sat 07, 2017 3:55 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 104
Location: Independence, KY
Quote:
Best reception location is generally at the upper end of band that is clear... If freq isn't changing by adj coil then the slug inside coil likely isn't moving...


The slug is movable. In fact it took quite a while to tune. I was using a digital tuned radio but later found out that the tube radio sitting on my bench was better for this. I also had the volume control on the Lil 7 turned all the way up and couldn't find the signal, not until I turned it all the way down was I able to pick up a signal and begin "alignment". How do I shift it to the upper end of the range? Same way I found it at 820 just higher receiver setting? I also do not have the RF coil terminals soldered yet. Wanted to make sure the circuit worked first. Love that feeling when something suddenly comes to life.


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sat 07, 2017 7:00 pm 
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Well lets back up a bit, is this your transmitter??? If so add a .01 to .05 cap in series with input of gain(volume) control, strange results can be had with tube grid connected directly to input source, should be isolated...

Image


Tom


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sat 07, 2017 10:56 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 104
Location: Independence, KY
Tom

Yes, that is the schematic. Well... drawn differently then the one I used but electrically the same. I finished soldered the terminals in the RF coil to eliminate that as a source. I then added a .01mF cap in series with the lead to the volume control center tap. No noticeable change. I still have a high pitched howl or screech in the background. This RF coil slug is VERY sensitive too. The output is strong and I can receive the signal up stairs, but the distortion is worse further away.

Another thing I find odd. I'm using an AM radio antenna made for my weather radio. It has a long lead with a bow tie end. The connection is an RCA jack. That's what I used in the Lil 7. Here's the odd thing, plugged into the transmitter the antenna does not work, it actually kills the signal, but if I use a jumper from the antenna center terminal tip to the SHIELD part of the female jack on the transmitter, it works just fine. The two points on the jack are not connected. Only to the center terminal. I'll post a picture of my construction.

Thinking another design is in order, but this one sure was fun even with all of the bugs to work out.


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sat 07, 2017 11:23 pm 
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Yeah touching(even being near) coil or antenna will throw this transmitter off frequency...

When you get tired of foolin' with it, come down to Homebrew and there are good designs posted...


Tom


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 08, 2017 12:18 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 104
Location: Independence, KY
Oops! I guess I have been posting in the wrong forum. Sorry about that. I'll move further post on the subject to Homebrew. BTW I'm looking at yours and Norms 6888 design. May very well be my next project.


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 08, 2017 1:21 am 
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That was my first transmitter 13 years ago. Good experience. You MUST build the 6888.


Last edited by Mike Toon on Jan Sun 08, 2017 3:06 am, edited 1 time in total.

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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 08, 2017 2:14 am 
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Mike L wrote:
Tom

Yes, that is the schematic. Well... drawn differently then the one I used but electrically the same. I finished soldered the terminals in the RF coil to eliminate that as a source. I then added a .01mF cap in series with the lead to the volume control center tap. No noticeable change. I still have a high pitched howl or screech in the background. This RF coil slug is VERY sensitive too. The output is strong and I can receive the signal up stairs, but the distortion is worse further away.

Another thing I find odd. I'm using an AM radio antenna made for my weather radio. It has a long lead with a bow tie end. The connection is an RCA jack. That's what I used in the Lil 7. Here's the odd thing, plugged into the transmitter the antenna does not work, it actually kills the signal, but if I use a jumper from the antenna center terminal tip to the SHIELD part of the female jack on the transmitter, it works just fine. The two points on the jack are not connected. Only to the center terminal. I'll post a picture of my construction.

Thinking another design is in order, but this one sure was fun even with all of the bugs to work out.



Mike
That .01 should be placed between the input and the pot. You will lose your Grid leak the other way. In any case I hope you find the problem. I would use a .47.

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If I did something right, I made a mistake.


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 08, 2017 2:48 am 
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Location: Beautiful Downtown Burbank CA
I had my input wired the same way and it worked. I should add a cap.
It might sound better?

Image
Sorry, not trying to hijack with pictures. Still learning.


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 08, 2017 3:35 am 
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Mike Toon wrote:
I had my input wired the same way and it worked. I should add a cap.
It might sound better?


Ehhh maybe, maybe not... :lol:

If there is voltage on audio source it can effect the screen grid(G2), which controls tube gain... Without voltage it's going to be fairly weak but at least somewhat predictable... Far more difficult to modulate with even a volt or two applied... With the high input impedance I don't see a large value cap making a real difference... Adding a cathode bias resistor may help, problem is I haven't tried that on one of these so don't know if bias would improve operation...

Excepting that it had a preamp, Syl's 6BM8 is very similar to this unit... May want to look at it's cathode resistor...


Tom


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 08, 2017 10:57 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1926
Location: Saskatoon
With the filament wired to the line side of the power input, you better hope you never get a filament to cathode short.


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 08, 2017 3:57 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 104
Location: Independence, KY
Here are a couple photos of my Lil 7. Still need to make a cover for it to keep nosey cat out, but the performance is not what I had hoped so maybe a parts donor for the next project. Operation is sporadic. Yesterday afternoon it was working fairly decent, then late last night I could hardly get a sound out of it.
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Quote:
Mike
That .01 should be placed between the input and the pot. You will lose your Grid leak the other way. In any case I hope you find the problem. I would use a .47.
I tried the cap suggestion, twice actually. The first time in the wrong position. Thanks for pointing that out John8750. So I corrected my mistake. The photo below shows the cap added. No improvement, but that was late last night. The antenna issue I posted earlier still has me puzzled. I'll tinker with it a little this morning, but am looking for another design.


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IMG_1144.jpg
IMG_1144.jpg [ 67.18 KiB | Viewed 658 times ]
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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 08, 2017 7:19 pm 
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If it worked as good as it looks you would not be having a problem...


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 08, 2017 8:22 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 104
Location: Independence, KY
Quote:
If it worked as good as it looks you would not be having a problem...


Thanks. Well it does work, just not as well as I had hoped. I have been experimenting with changing the value of C6. Decreasing the value shifts the frequency up in the band while increasing has the opposite effect. Currently I have a 112pF (2 56pF in parallel. I have a very limited stock) and it is operating around 860Mhz. The distortion is greatly reduced but the output strength is noticeably reduced. I tried a single 56pF at C6 and couldn't find a signal at all.

I noticed at one time there was a member of the forum offering a "kit" or parts for a 6888 tube circuit. Anyone know who it was and if it is still available? I think I'm ready to move on from this project.


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 08, 2017 8:26 pm 
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Mike

Contact Bill Hamre. He might be able to get you most of the parts.

viewtopic.php?f=12&t=132867&start=540

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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 08, 2017 8:38 pm 
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Bill Hamre stocked parts, I dunno what he has remaining...


If you'd like to experiment with a preamp on the lil7, I drew a schematic for that as well(yeah the input cap was omitted when I drew it)...

Image


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 08, 2017 11:49 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1926
Location: Saskatoon
It just occurred to me, it's the screen grid where the audio modulation goes in, and depending how the input capacitor is hooked up (or if it's missing), the screen is either biased to 0 volts, or is floating. You may get better performance by connecting the blocking capacitor between the pot wiper and the grid and then adding a resistor from screen to B+. I would normally use 10-22k but something that low might cause problems with the linearity of the volume control. In any event, you could try it, and if it improves things, you could switch to a lower value volume control.

(I'm referring to the original circuit, but the same thing would apply to 35Z5's preamp version.)


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Mon 09, 2017 12:32 am 
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What happens with voltage on G2 is the RF output becomes far more efficient, signal will increase dramatically, but there is a catch... The extra RF means it requires additional signal to modulate the tube, without increase in audio you have a nice but almost dead carrier... Using just a single tube as audio amp, this is circuit that's needed to for modulation... Still, depending on tube used, screen voltage won't be more than maybe 15-18v... To really make screen modulation perform at higher G2 levels one would need audio circuitry similar to whats used for plate modulation... Of course that is pushing far beyond part 15 specs...


Image

With just one stage of audio amplification very nice modulation envelopes can be produced...

Image


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