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 Post subject: Re: Two-Tube AM Transmitter (6AB4 & 6888)
PostPosted: Aug Fri 10, 2018 2:47 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2200
Location: Saskatoon
harleythatcher wrote:
Question? I notice the variable cap has B+ hooked to it. Does that create a shock hazard? Depending on how big it is I may have to put it on top of the enclosure.

That is one reason for using a trimmer rather than a regular variable cap. However, if you use a compression trimmer, then the adjusting screw will still be hot, and so you need to take care when adjusting it.

I prefer to make a minor circuit modification which has no effect on performance, but removes all high voltage from the trimmer.
The original 6888 circuit has this antenna connection:
Attachment:
Antenna01a.jpg
Antenna01a.jpg [ 14.96 KiB | Viewed 206 times ]

The modification looks like this.
Attachment:
Antenna01b.jpg
Antenna01b.jpg [ 13.73 KiB | Viewed 206 times ]

These are identical as far as the RF signal is concerned. So there should be no difference in performance.


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 Post subject: Re: Two-Tube AM Transmitter (6AB4 & 6888)
PostPosted: Aug Fri 10, 2018 5:24 pm 
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Joined: Aug Tue 07, 2018 4:11 am
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Location: Burlington, WV 26710
I would be interested in seeing a photo of the final product and schmatic.


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 Post subject: Re: Two-Tube AM Transmitter (6AB4 & 6888)
PostPosted: Aug Fri 10, 2018 5:56 pm 
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Joined: Apr Sat 14, 2018 4:44 am
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Attachment:
IMG_1594.JPG
IMG_1594.JPG [ 100.18 KiB | Viewed 186 times ]
I'm still a ways off from finishing my transmitter. Waiting on some parts kindly donated by a member of this forum. (tk u). I am using an old computer PP case for an enclosure. I am using this reference for my build. It originated on this forum years ago.
http://www.electronixandmore.com/projects/amxmitter/index.html

Power supply and pre-amp done ish.

I'll post more pics as I progress.
I am a retired Navy electronics type but I have been out longer than my career in so I have forgotten a butt load particularly RF wiring so I will no doubt have lots of hum when I get it online. I put a .015 uf Y-2 cap to ground on both ac leads coming in and an X-2 across the leads. Not sure if it is appropriate for this build but it is SOP for vintage radio refurb.
Tt
KK4KRU


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 Post subject: Re: Two-Tube AM Transmitter (6AB4 & 6888)
PostPosted: Aug Fri 10, 2018 6:00 pm 
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Joined: Apr Sat 14, 2018 4:44 am
Posts: 14
BobW Tks for the ckt mod. I like it and will incorporate it into my xmitter.
Tt
KK4KRU


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 Post subject: Re: Two-Tube AM Transmitter (6AB4 & 6888)
PostPosted: Aug Fri 10, 2018 6:06 pm 
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Joined: Apr Sat 14, 2018 4:44 am
Posts: 14
I found a 1.544 mhz crystal oscillator on ebay yesterday and bought it. I have local stations just above and below 1 mhz so I may have to change freq. If I replace the 1mhz crystal osc do I have to rewind/replace the inductor? If so what value am I looking for? TKs
Tt
KK4KRU


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 Post subject: Re: Two-Tube AM Transmitter (6AB4 & 6888)
PostPosted: Aug Fri 10, 2018 6:43 pm 
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Posts: 13054
Build is lookin' good.

The transmitter did originate her on ARF, Norm Leal designed the first version without a preamp. I added the 6C4 (or 6AB4) and the antenna output similar to what Bob posted.

You'll need somewhere around a 270-330uh for 15.54Kc

Image

_________________
Tom


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 Post subject: Re: Two-Tube AM Transmitter (6AB4 & 6888)
PostPosted: Aug Fri 10, 2018 7:51 pm 
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Joined: Apr Sat 14, 2018 4:44 am
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35z5 tks for the schematic. After looking at it I see it is different than what I have. Using this actual thread I got the info to wind my own coil. The plans originally called for a 180uh coil. I was quite proud of myself because on my first attempt I got a coil with 180uh. The plans you just shared has two coil and neither r 180uh? I looked for 1mh coils and not sure what one I need.
Tt


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 Post subject: Re: Two-Tube AM Transmitter (6AB4 & 6888)
PostPosted: Aug Fri 10, 2018 10:23 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2200
Location: Saskatoon
If you go with the mod that I proposed, you should be able to use the original 180µH coil.

The schematic that 35Z5 posted is a pi network output, which is also different from Norm's original circuit, but gives a broader tuning range to match a wider variety of antennas (at a slight reduction of output power). The coil connected to the plate in 35Z5's circuit is simply an RF choke, and its value shouldn't be too critical, as long as it's 1mH or more. The second coil forms part of the antenna tuned circuit and its value will determine the tuning range of the antenna.


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 Post subject: Re: Two-Tube AM Transmitter (6AB4 & 6888)
PostPosted: Aug Fri 10, 2018 10:30 pm 
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Joined: Apr Sat 14, 2018 4:44 am
Posts: 14
TKs Bob. I have noticed 10 different component value changes between my schematic and the latest here. I guess I will just upgrade to the latest. The biggest change is the original calls for a 50k pot and this one a 100k pot. I have a 100k pot but I don't think it is an audio taper. So I can use my 180uh coil for L2 in the ant tuned ckt?
Tks for your help.
Tt


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 Post subject: Re: Two-Tube AM Transmitter (6AB4 & 6888)
PostPosted: Aug Fri 10, 2018 10:39 pm 
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Joined: Apr Sat 14, 2018 4:44 am
Posts: 14
OK well I now own 50 1mh rf chokes.lol So I am going to upgrade my ckt to match the latest one here. It's actually a good time to do it as I am still in the process. Plus I am going to unsolder a bunch of grounds and go with the uniground idea as well as shielded wire for the audio trail. Thanks for every ones help. I'll post some pics when I get a little further along.
One question however. I have a nice ten turn 100k pot but doubt it is an audio taper? Do we care?
Tt


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 Post subject: Re: Two-Tube AM Transmitter (6AB4 & 6888)
PostPosted: Aug Sat 11, 2018 3:58 am 
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Joined: Apr Sat 14, 2018 4:44 am
Posts: 14
I have a question concerning the uni-ground idea. I have my transmitter built in a computer power supply enclosure. On the bottom of the enclosure I have a sheet of 3/8 in copper clad pcb board to mount the components on. So basically the entire sheet is at ground potential. So does the uni-ground theory apply here? Should I still have all grounds attached to the substrate at the same point? I don't know enough about this to really understand at this point. Thanks for listening.
Tt
KK4KRU


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