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 Post subject: Symphonic Manual Phono Pre-amp
PostPosted: Feb Wed 28, 2018 6:10 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 180
Location: Hedgesville,WV
I want to add a pre-amp to this one tube wonder.
I have an LM386 amp that I would like to use and it needs 4-12 volt DC power.
Can I add an 83 ohm resistor in series with the 90 volt motor and 25 volt filament of the 25L6 to get a 10 VAC drop to use with a full wave rectifier to power the LM386 circuit?
Thanks!
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 Post subject: Re: Symphonic Manual Phono Pre-amp
PostPosted: Feb Wed 28, 2018 9:32 pm 
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Joined: Nov Wed 27, 2013 5:59 am
Posts: 497
Location: Metzger Oregon
If it were me, I would just add a 6AV6 in series and keep it all tube, plus the extra 6 volts of filament/motor drop would not be an issue.

That said, if you do go ahead with the LM386, you could probably pull voltage from the cathode of the 25L6. There should be a resistor to ground, in the 150 to 180 ohm range. It would also be a good idea to put a cap of at least 22uF across that resistor, to give a little extra gain, if there is not one there already.


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 Post subject: Re: Symphonic Manual Phono Pre-amp
PostPosted: Mar Thu 01, 2018 4:35 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 180
Location: Hedgesville,WV
Thanks for your reply.

So pin 8 of the 25L6 (cathode) is connected to chassis ground thru a 22uF cap and a 150 ohm resistor in parallel.
Are you saying that I could connect my full wave rectifier across this 150 ohm resistor?

Am still considering the "all tube" option.


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 Post subject: Re: Symphonic Manual Phono Pre-amp
PostPosted: Mar Thu 01, 2018 5:07 pm 
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Joined: Nov Wed 27, 2013 5:59 am
Posts: 497
Location: Metzger Oregon
No, I'm saying that there should be about 6 or 7 volts DC here while the amp is operating, no need for a bridge rectifier, as long as your current draw is low, just use this as a DC supply.

If the record play is functional other than the cartridge, I would suggest running it with a DC volt meter between pin 8 and ground, and see what you get.


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 Post subject: Re: Symphonic Manual Phono Pre-amp
PostPosted: Mar Thu 01, 2018 5:43 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 180
Location: Hedgesville,WV
Great!
Thanks for clearing it up for me.


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 Post subject: Re: Symphonic Manual Phono Pre-amp
PostPosted: Mar Fri 02, 2018 3:11 am 
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Joined: Sep Mon 16, 2013 2:42 am
Posts: 2694
Location: Tucson, Arizona U.S.A.
jfish2man wrote:
I want to add a pre-amp to this one tube wonder.
I have an LM386 amp that I would like to use and it needs 4-12 volt DC power.
Can I add an 83 ohm resistor in series with the 90 volt motor and 25 volt filament of the 25L6 to get a 10 VAC drop to use with a full wave rectifier to power the LM386 circuit?
Thanks!
Image

No, that won't work. The problem is the lack of a common ground between the tube and your preamp. You could use a 1:1 transformer to isolate the audio signal but one of the other suggested methods would be better.

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Jim Mueller


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