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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Aug Fri 25, 2017 5:31 pm 
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TDane wrote:
This may belong elsewhere, but does this really draw 2 amps? If not, what size transformer will work?

I don't know where you found the spec but heater current for a 6888 is .8A and the 6SQ7 is .3A so heater will require 1.1A... For a bit of headroom a xformer with 1.5A heater winding would be fine as would the 2A..

For the HV winding something around 40ma would be plenty...

Tom


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Aug Fri 25, 2017 7:20 pm 
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Think the board has its own HV generator so that would add to the current being drawn.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Aug Fri 25, 2017 7:55 pm 
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Location: Saskatoon
40 mA plate current at about 160 volts is 6.4 watts. Even assuming 100% efficiency of the boost converter, that equals 1.07 amps at 6 volts added to the total filament amps. So, the transformer is certainly not oversized. If anything, it may be a bit on the small side.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Aug Sat 26, 2017 12:44 am 
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BobWeaver wrote:
40 mA plate current at about 160 volts is 6.4 watts. Even assuming 100% efficiency of the boost converter, that equals 1.07 amps at 6 volts added to the total filament amps. So, the transformer is certainly not oversized. If anything, it may be a bit on the small side.

But with bias used, I'm yet to see a 6888 in xmitter that drew more than 20-22ma...


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Aug Sat 26, 2017 8:41 am 
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Location: Saskatoon
Okay, that makes sense. In any event, the plate current (transformed down to 6 volts) has to be added to the filament current. So, that explains why the total 6 volt load is more than just the filament amps.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Aug Tue 29, 2017 8:37 pm 
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Location: Clayton, Ind.
I only referred to 2 amps because that is what the specs are on the sheet provided for the board available on-line.

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TDane WA9YI

Radio is a divine gift; God made us able to do one thing while listening to another.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Dec Sun 22, 2019 11:25 pm 
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Location: Springville, NY
Now that I have moved to a different part of the country, there is a station very near the broadcast frequency of this transmitter. No matter how much tuning I do with the dial I cant separate the two. Is there an easy way to change the frequency other than changing the crystal?


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Dec Mon 23, 2019 12:48 am 
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Location: Raleigh, NC area
Sorry you are having trouble...
But I am so happy this thread re-opened!


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Dec Mon 23, 2019 4:26 am 
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Mike L wrote:
Now that I have moved to a different part of the country, there is a station very near the broadcast frequency of this transmitter. No matter how much tuning I do with the dial I cant separate the two. Is there an easy way to change the frequency other than changing the crystal?

Short answer, no.

Longer answer, you may need to change value of antenna coil as well.

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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Dec Mon 23, 2019 9:33 pm 
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Location: Springville, NY
35Z5 wrote:
Mike L wrote:
Now that I have moved to a different part of the country, there is a station very near the broadcast frequency of this transmitter. No matter how much tuning I do with the dial I cant separate the two. Is there an easy way to change the frequency other than changing the crystal?

Short answer, no.

Longer answer, you may need to change value of antenna coil as well.



Interesting thing though is the offending station only appears while the transmitter was on. If I turn the transmitter off, the other station is not received or even herd across the radio dial.

Now trying it again this afternoon and the source through the transmitter is crystal clear! As the evening approaches I'll have to try again. I even went so far last night as to remove the antenna wire from the radio and still had that interference.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Jan Thu 23, 2020 2:03 am 
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I just got out my 6888 Transmitter with 1 MHz oscillator. With 8 foot copper pipe and loading coil at the edge of my grounded metal deck roof, I get strong FM-like signal within about 100 ft. Beyond that it starts fluttering, which leads to my question.

The closest 1 MHz competing stations are .25 KW, 50 and 130 miles away, respectively. I can barely here them on my DX-398 during the day. So why would I get fluttering as I move away from my property? What is the cause of that fluttering?


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