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 Post subject: Questions on another 6888 transmitter build
PostPosted: Jan Fri 11, 2019 1:52 am 
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I’d like to try building Norm Leal’s 6888 AM transmitter. I have a few questions:

Is anyone still selling 6888 tubes?

I have a Miller 42-A antenna coil. Will that work in this transmitter?

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I have some 6X5 rectifiers but no power transformer, so I’m planning to buy a Hammond 261G6, which has 250V CT plus a filament supply. My local parts/surplus store has some 9-80-120 transformers for the Music Man 1600B guitar amp at a good price,but they are 67V CT, which I’m guessing is too low.


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 Post subject: Re: Questions on another 6888 transmitter build
PostPosted: Jan Fri 11, 2019 5:39 pm 
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It should work using the winding intended for the 365 pF cap. That would be somewhere around 200-250 uH.


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 Post subject: Re: Questions on another 6888 transmitter build
PostPosted: Jan Fri 11, 2019 6:14 pm 
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Bill Hamre did supply 6888 but he's no doubt wintering in the Bahamas.

As far as power transformer, is that 67v from tap to each leg or 67v across both legs?? If so a voltage doubler would no doubt work, I used one in the 12SC6 xmit. Transformer for it had 36 & 24v windings that were connected series, then applied to a doubler. Using 1N4007 rectifiers, that 60v produced approx 160v B+. All that's needed for doubler is one additional rectifier and filter cap.


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 Post subject: Re: Questions on another 6888 transmitter build
PostPosted: Jan Fri 11, 2019 8:54 pm 
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Thanks Macrohenry!

35Z5 that’s a great idea. I believe the transformer is 67V across both legs. It’s the 9-80-120 transformer on this schematic, which produces +/- 45V after going through a bridge rectifier:

http://schems.com/bmampscom/musicman/musicman_1600b.pdf

On your schematic, I’m assuming C9 and D1 are the extra diode and capacitor?


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 Post subject: Re: Questions on another 6888 transmitter build
PostPosted: Jan Fri 11, 2019 10:25 pm 
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Yes C9 & D1 are added for the doubler. D1 & C10 would normally be connected to ground but I needed a power source for crystal osc, so used the ground end for the 5v supply.

How are you planning on lighting the toob? That transformer has no 6.3 v output.

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 Post subject: Re: Questions on another 6888 transmitter build
PostPosted: Jan Sat 12, 2019 12:56 am 
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Whoops! Yes I realized that just now. I’ll check to see if my parts store has some similar deals on filament transformers. That 261G6 is looking better and better...


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 Post subject: Re: Questions on another 6888 transmitter build
PostPosted: Jan Sat 19, 2019 12:04 am 
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Well, the good news is that I found a 12.6V center tapped filament transformer hiding away in a junk box I had long forgotten. The bad news is that I never bought that 67V power transformer and now it’s gone. :(


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 Post subject: Re: Questions on another 6888 transmitter build
PostPosted: Jan Sat 19, 2019 3:14 am 
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Electronizer wrote:
Well, the good news is that I found a 12.6V center tapped filament transformer hiding away in a junk box I had long forgotten. The bad news is that I never bought that 67V power transformer and now it’s gone. :(

Lessee.

If you can come up with a 25v transformer could use that with a tripler, supplies around 105v. A 35v xformer & tripler would be over 140v.

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