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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Mon 09, 2017 9:47 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 104
Location: Independence, KY
35Z5 wrote:
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)...


Hey why not. I only need a tube and socket, I'm sure I have everything else. Tube seems cheap enough.


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Mon 09, 2017 10:20 pm 
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If distortion issue is already excessive gain, the extra tube isn't going to help...


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Mon 09, 2017 10:48 pm 
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Location: Independence, KY
Well, I believe some of the distortion is local stations very close to where the Lil 7 is broadcasting. If I could get it up around 1000kc that area is clear around here. I had experimented with changing the value of C6 (150pf called for). I thought I remember an older post suggesting that. Currently I have a 112pf (2-56pf in parallel). It moved the signal up the band to around 850 from 820. Distortion was reduced but also so was the output strength. Am I chasing the wrong squirrel here?


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Tue 10, 2017 12:17 am 
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Mike L wrote:
Am I chasing the wrong squirrel here?

:lol:

Changing the 150pf to smaller value should move freq higher, I'd think a 47-68 would put freq well above 1000... Since finding the freq seems a bit difficult, maybe install a variable cap? One from a junk radio such as AA5 would be fine, just be sure the movable blades/body are on ground...


Tom


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Tue 10, 2017 1:29 am 
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35Z5 wrote:
If it worked as good as it looks you would not be having a problem...




I agree. Looks like a Museum piece.

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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 Tue 10, 2017 8:03 am 
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Location: Saskatoon
In case a person wanted to experiment, and didn't have 117L7, is there another pentode with similar characteristics?

It looks to me that a 35L6/50L6 is pretty close except that these have higher plate dissipation than the 117L7 which shouldn't matter for this application.


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Tue 10, 2017 3:12 pm 
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Bob

The major difference I've seen is the power pentodes, like 6K6, 6F6 etc require more audio to modulate than do the beam power types...


Tom


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Tue 10, 2017 4:24 pm 
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35Z5 wrote:
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...

The audio circuitry doesn't really enter into the equation. The primary limiting factors for meeting Part 15 limits for AM transmitters are the 3 meter limit for the combined length of the antenna and ground system and the limit of 100 mw DC input power to the final RF amplifier.

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 Tue 10, 2017 7:31 pm 
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Dale H. Cook wrote:
35Z5 wrote:
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...

The audio circuitry doesn't really enter into the equation. The primary limiting factors for meeting Part 15 limits for AM transmitters are the 3 meter limit for the combined length of the antenna and ground system and the limit of 100 mw DC input power to the final RF amplifier.

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

It surely does at peanut levels when one is modulating screen...

Requires far more signal to modulate tube with say 50v on screen vs 5v...

Of course you are more concerned with being hall monitor, vs actual circuitry being used...


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Wed 11, 2017 7:34 pm 
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Location: pensacola fl
The list of tetrode, pentode, and other multi grid tubes that will work is long. 6DG6 is similar as are 6W6 12W6 12L6 25L6 35L6 50L6 and this is only a small list of tubes similar to 117L7. If you have a list of tubes you have and would like to try we can help you pick a useful one.


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 15, 2017 12:28 am 
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Location: Independence, KY
I dropped the value of the 150pf down to 112pf. Added the a .047mf cap to the input. changed to just a wire antenna and moved Lil 7 closer to the radio. Works great! actually had to decrease the input signal level. Listening to UK 1940's Radio Station. Very cool. Finishing up a cover for it and moving on to the 6888 with PCB. Thanks for all the help.


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 15, 2017 1:09 am 
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Kewl, congrats...

Lacking screen(G2) voltage it should be very easy to modulate...


Tom


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Sun 29, 2017 11:47 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 104
Location: Independence, KY
Well I had some time waiting on parts to arrive for the 6888 PCB transmitter, so I decided to finish the enclosure for the Lil' 7. Here is a photo of it in operation sitting on my Aircastle console. Just painted plywood and a brass trim ring. Maybe a little over done, but since it was my first real tube project I felt it deserved it.


Attachments:
Lil 7 finished.jpg
Lil 7 finished.jpg [ 43.57 KiB | Viewed 1080 times ]
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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Mon 30, 2017 4:02 pm 
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Nice, it surely has style...

First transmitter I built has long since been used for parts, was so ugly I couldn't bring it out in daylight...


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 Post subject: Re: Lil 7 AM Transmitter
PostPosted: Jan Mon 30, 2017 5:53 pm 
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Dale H. Cook wrote:
35Z5 wrote:
The primary limiting factors for meeting Part 15 limits for AM transmitters are the 3 meter limit for the combined length of the antenna and ground system...
It seems easy to keep the antenna to 3 meters, but the ground? Measured from where? The point where the rod touches the earth? The point where it connects to a heavy wire going t the rod? Otherwise, the transmitter would have to be right beside the ground rod to have any sort of antenna length.

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