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 Post subject: Norm's 6888 transmitter with 6C4
PostPosted: Dec Tue 31, 2019 7:50 pm 
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Location: Baxter,Ky 40806
I build this over x-mas and the first time i try it works.

Question? On the schematic it show 165 volts going to R5 of the 6C4 tube and 165 volts going to C6,L1,R9 to the 6888.
I have 162 volts, so that ok.
But on pin5 of the 6C4 the schematic call for 75 volts and i am only getting 56 volts.
I replace the R5 resistor 82K with a 51K and now i am getting 75 volts. Any ideal why i had to replay this r5 82k with a 51k?
Will this be ok or do i need to go back to the 82k.

Also on pin #6 of the 6888 the schematic call for 100 volts but i am only getting 86 volts, I think this is ok.


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Dec Tue 31, 2019 11:49 pm 
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The 51K on plate of 6C4 is probably OK... I'd consider using 82K and increasing the 1500 cathode resistor to 1.8K or a little higher(test by seriesing a 330 to 470). That'd likely increase gain from 6C4 slightly. If you decide to switch to the 6AB4, I consider 82K minimum.

6888 screen voltage is OK. Can usually juggle it's resistor a little and gain additional output but 10K is about the minimum. Lower value usually starts dragging down B+.

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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Dec Tue 31, 2019 11:53 pm 
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Oh boy. I too just got the 6888 transmitter circuit working a few day ago and like you it works good, but have a few bugs to work out. Good luck, this is a fun project.

Jeff


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Wed 01, 2020 8:11 am 
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Location: Beautiful Downtown Burbank CA
Pictures, you two.


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Wed 01, 2020 1:53 pm 
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Joined: Feb Tue 01, 2011 2:51 pm
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Location: Baxter,Ky 40806
What is the difference between a 6AB4 and a 6888?


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Wed 01, 2020 5:09 pm 
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Joined: Nov Wed 08, 2017 3:28 pm
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No photos yet Mike. The circuit is rather embarrassing to show. Once I clean it up a bit then sure.


Jeff


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Wed 01, 2020 6:33 pm 
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Sirtar wrote:
What is the difference between a 6AB4 and a 6888?


The 6AB4 is a higher gain 6C4, does not replace 6888.

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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Thu 02, 2020 2:14 pm 
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Joined: Feb Tue 01, 2011 2:51 pm
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Location: Baxter,Ky 40806
I'm the same way as j8000,
Too many jumper wire to see the work.


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Fri 03, 2020 5:11 pm 
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What did you use for the antenna?


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Fri 03, 2020 7:53 pm 
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Joined: Feb Tue 01, 2011 2:51 pm
Posts: 299
Location: Baxter,Ky 40806
I just got a wire about 10 ft. long running to the top of the window and around.

I got a lot of hum, may be all the jumper wires.


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Sat 04, 2020 4:13 am 
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What gauge wire? Mine really thin, perhaps 24gauge? No hum in mine, but a decent amount of noise I hope will go away when I clean up the circuit.


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Sat 04, 2020 2:05 pm 
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Joined: Feb Tue 01, 2011 2:51 pm
Posts: 299
Location: Baxter,Ky 40806
Its 24 gauge Stranded wire.

Its not a hum or a buzz, but like you a lot of noise.

I'm thinking a 3 prong cable with the 3rd pin grounded to the chassis may help.


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Sat 04, 2020 4:09 pm 
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Sirtar wrote:
Its 24 gauge Stranded wire.

Its not a hum or a buzz, but like you a lot of noise.

I'm thinking a 3 prong cable with the 3rd pin grounded to the chassis may help.


My experience says don't count on it helping and could be worse. I have one transmitter with a switchable third wire ground. Since I feed mine with a computer(or cable box), I've found best results achieved with ground provided through audio cables from computer.

If antenna tuning is not peaked signal can be very noisy, only a small "sweet spot" for optimum performance..

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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Sat 04, 2020 5:22 pm 
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I know mine isnt tuned in very well. That is on the list to work on. Going to make this field meter to help. I have most of the parts already. Maybe it'll help for you too.

http://www.circuitstoday.com/field-strength-meter


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Sat 04, 2020 7:37 pm 
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Joined: Nov Wed 08, 2017 3:28 pm
Posts: 53
Although I must admit, you really don't need the meter, for me it just adds more fun to the project.


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Sat 04, 2020 8:12 pm 
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You can use a 50ua meter, 1N34 & 12-15 loops of wire as RF meter.

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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Sun 05, 2020 12:35 am 
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Joined: Feb Tue 01, 2011 2:51 pm
Posts: 299
Location: Baxter,Ky 40806
I try the 3 prong no better and no worst.

I did remove all of the test leads, and it still works. lol

Notice while playing it, if i touch the volume control the hum or noise go away.
Maybe i just need to go back and make all the leads as short as possible?


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Sun 05, 2020 2:55 am 
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Joined: Nov Wed 08, 2017 3:28 pm
Posts: 53
Good luck, I'm going to do the same. I just finished playing with it for a bit and was messing with the antenna/varriable cap. side of the circuit. I didn't have any luck getting rid of any noise either. It plays supper loud twenty feet away from my Philip radio at full power.


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 Post subject: Re: Norm's 6888 transmitter with 6C4
PostPosted: Jan Sun 05, 2020 4:34 pm 
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Here's a so so pict of RF meter I threw together. May have required five minutes.


Attachments:
Tx meter 002_zpsrwftqrmo.jpg
Tx meter 002_zpsrwftqrmo.jpg [ 159.54 KiB | Viewed 1704 times ]

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