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 Post subject: Another Little Prince- tube preamp
PostPosted: Mar Fri 08, 2019 12:02 am 
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Another Little Prince. The Little Prince is a control amp and was built to be a preamp for the Prince John. I am John, the builder of the amps.
But not at all the designer. Flip presenter the modified schematics.
Thanks to him, Peter, and others, I have so much fun building and learning.

This amp will control my 807 Williamson. Thanks much to Flip.
The original Little Prince uses the 20EZ7 tube X six. It controls volume, balance, bass, and treble. Has RIAA amp. A selectable input
switch for phono/mp3/cd.

I wanted this amp to use six 12SL7 Octal tubes to match the 807's.
Should be fun.

Here is the 807 Williamson.
http://antiqueradios.com/forums/viewtop ... 2&t=338702

The original Little Prince
http://antiqueradios.com/forums/downloa ... ?id=163072
http://antiqueradios.com/forums/downloa ... ?id=163998

The power supply.
http://antiqueradios.com/forums/downloa ... ?id=246310

The new RIAA amp
download/file.php?id=254025


The new tone amp
download/file.php?id=246313

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Last edited by john8750 on Apr Sat 27, 2019 9:56 pm, edited 10 times in total.

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Fri 08, 2019 4:56 am 
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The power transformer will be Edcor XPWR 218-120. It has two 120volt 50ma windings, and one 12volt 1amp.
The six 12SL7's need 900ma at 12volts.
Here is the basic high voltage supply. The filter and ballast resistor need to be figured. Aiming for about 250volts.
download/file.php?id=246113

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Fri 08, 2019 5:10 am 
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john8750 wrote:
The power transformer will be Edcor XPWR 218-120. It has two 120volt 50ma windings, and one 12volt 1amp.
The six 12SL7's need 900ma at 12volts.
Here is the basic high voltage supply. The filter and ballast resistor need to be figured. Aiming for about 250volts.
download/file.php?id=246113
You don't need to multiply the secondary when you have two of them. Just put them in series and use a full wave bridge. E.g. it's a 240VAC CT with the center tap unused.


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Fri 08, 2019 5:40 am 
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Flipperhome wrote:
john8750 wrote:
The power transformer will be Edcor XPWR 218-120. It has two 120volt 50ma windings, and one 12volt 1amp.
The six 12SL7's need 900ma at 12volts.
Here is the basic high voltage supply. The filter and ballast resistor need to be figured. Aiming for about 250volts.
download/file.php?id=246113
You don't need to multiply the secondary when you have two of them. Just put them in series and use a full wave bridge. E.g. it's a 240VAC CT with the center tap unused.




That's a better idea. I guess 50ma is plenty of power? Do you suggest a full wave rectifier?
What do you figure the total full load B+ current will be?

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Fri 08, 2019 5:42 am 
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Flip-
Could the 250k volume pots be replaced with 100k?

https://www.ebay.com/itm/9V-Infrared-Re ... SwjXNbESAy

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Fri 08, 2019 6:00 am 
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Here is the original Little Prince on top. And the 807 Williamson on the bottom.
The sound is spectacular.
download/file.php?id=245416

BTW- I reconstructed an old ammo case for the stereo cabinet. It had 'four' handles, and 12 places
for big shells. And, had a very old OD paint job.

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Last edited by john8750 on Mar Fri 08, 2019 6:55 am, edited 1 time in total.

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Fri 08, 2019 6:04 am 
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john8750 wrote:
Flipperhome wrote:
john8750 wrote:
The power transformer will be Edcor XPWR 218-120. It has two 120volt 50ma windings, and one 12volt 1amp.
The six 12SL7's need 900ma at 12volts.
Here is the basic high voltage supply. The filter and ballast resistor need to be figured. Aiming for about 250volts.
download/file.php?id=246113
You don't need to multiply the secondary when you have two of them. Just put them in series and use a full wave bridge. E.g. it's a 240VAC CT with the center tap unused.

That's a better idea. I guess 50ma is plenty of power? Do you suggest a full wave rectifier?
What do you figure the total full load B+ current will be?
Yes, a full wave bridge is the most efficient. Current consumption is shown on the original Little Prince power supply schematic, about 15mA (14.4 mA rounded up). I'd up it a bit since we don't know what B+ actually is yet, say 20mA. Then times 1.8 (full wave bridge factor) = 36 mA to the transformer. That leaves 14mA spare (7.8 mA to the circuit).


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Fri 08, 2019 6:46 am 
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So would I figure a resistor based on the actual voltage minus 250volts at 15ma?

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Fri 08, 2019 6:57 am 
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john8750 wrote:
So would I figure a resistor based on the actual voltage minus 250volts at 15ma?
Figure on increasing R1 and R4 (equally) based on 15mA. E.g. both at 1k would drop 30V and both at 2.2k would drop 66V, which I think would be about right. Minimum 1W resistors.

Filter caps C3 and C5 can be reduced proportionally to 62uF, say 100 uF for a little better filtering. C8 would remain at 470 uf. In actuality, they could all be reduced but it would take more work to figure out the ripple.

Revised. I checked the sim and, with C3 and C5 at 100 uF and R1 and R4 2.2k, C8 can be lowered to 220 uF. That produced essentially the same ripple as the existing Little Prince so assuming it's hum free that would be hum free in the new one.


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Fri 08, 2019 10:21 pm 
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RIAA section schematic.
Attachment:
12SL7 Phono Preamp Buffered - as built.jpg
12SL7 Phono Preamp Buffered - as built.jpg [ 124.87 KiB | Viewed 1353 times ]


Volume-Balance-Tone Control section schematic.
Attachment:
12SL7 Baxandall Tone Control w-Vol-Bal v3 - as built.jpg
12SL7 Baxandall Tone Control w-Vol-Bal v3 - as built.jpg [ 125.82 KiB | Viewed 1562 times ]


Edit: Added missing D1 to Phono Preamp.


Last edited by Flipperhome on Mar Thu 21, 2019 1:35 am, edited 1 time in total.

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Fri 08, 2019 11:04 pm 
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Thank you Sir.
I will figure on the changes. Here is the original power supply. Quite different from the new one.
download/file.php?id=165323

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Fri 08, 2019 11:53 pm 
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Here is a draft of the new Power Supply. May need changes as we go.
Attachment:
new ps.jpg
new ps.jpg [ 58.99 KiB | Viewed 1560 times ]

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Sat 09, 2019 5:41 am 
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john8750 wrote:
Here is a draft of the new Power Supply. May need changes as we go.
Attachment:
The attachment new ps.jpg is no longer available
You got it. Here's my schematic.
Attachment:
240V Xfrmr Power Supply - FW Filtered.jpg
240V Xfrmr Power Supply - FW Filtered.jpg [ 77.26 KiB | Viewed 1544 times ]


Edit: changed R3 from 500 to 2K.


Last edited by Flipperhome on Mar Sat 09, 2019 6:37 am, edited 1 time in total.

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Sat 09, 2019 5:47 am 
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Note, that 12V winding is 12V, not the heaters 12.6V. But I don't think being 5% low is going to hurt anything.


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Sat 09, 2019 5:55 am 
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Flipperhome wrote:
Note, that 12V winding is 12V, not the heaters 12.6V. But I don't think being 5% low is going to hurt anything.




I was thinking about that. And noticed on some preamp tube filaments a lower voltage is used. Like a resistor in series.
As long as you think it will work, then its a go.
I wonder why that winding was not 12.6v.

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Sat 09, 2019 5:58 am 
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Thanks for the power supply schematic. I like your better. And think I have all the components already.

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Sat 09, 2019 6:09 am 
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Is this still a good idea, for the MP3 booster.
Attachment:
20EZ7%20Phono-MP3%20Preamp%20Buffered[1].jpg
20EZ7%20Phono-MP3%20Preamp%20Buffered[1].jpg [ 93.98 KiB | Viewed 1546 times ]

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Sat 09, 2019 6:49 am 
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john8750 wrote:
Flipperhome wrote:
Note, that 12V winding is 12V, not the heaters 12.6V. But I don't think being 5% low is going to hurt anything.
I was thinking about that. And noticed on some preamp tube filaments a lower voltage is used. Like a resistor in series.
As long as you think it will work, then its a go.
I wonder why that winding was not 12.6v.
Don't know but likely it wasn't meant for heaters, rather some solid state supply or something.

The only thing that concerns me a bit is in this one the heaters are AC whereas the first Little Prince eliminated any chance of hum with it's DC heater supply. I note the Shure tube phone RIAA amp had AC heaters too but it was 6.3VAC and ours is double that. We may need to bias the heater supply up to some voltage. I need to think on that some more.


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Sat 09, 2019 6:50 am 
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john8750 wrote:
Is this still a good idea, for the MP3 booster.
Attachment:
20EZ7%20Phono-MP3%20Preamp%20Buffered[1].jpg
Ah, there it is. I couldn't find that schematic at first but it was right there all along.

Yes, it should still work.


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Mar Sat 09, 2019 7:12 am 
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Flipperhome wrote:
john8750 wrote:
Is this still a good idea, for the MP3 booster.
Attachment:
20EZ7%20Phono-MP3%20Preamp%20Buffered[1].jpg
Ah, there it is. I couldn't find that schematic at first but it was right there all along.

Yes, it should still work.



Good. I will use it. Thanks for that.
Looks like some fancy switching will be needed.

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