Power supply for 12SL7 phono preamp - diyAudio
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Old 22nd June 2008, 02:22 AM   #1
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Default Power supply for 12SL7 phono preamp

Hello,
I'm thinking of building this phono preamp, Bob's Vacuum Tube Phono Preamp. But, as I'm new to this, I have no idea what the power supply would look like, does anyone have any ideas?
Thank you very much,
Steven Paul
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Old 22nd June 2008, 03:03 AM   #2
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Granted a regulated supply would probably deliver the best performance but I'm wondering if you could get away with just a
150 volt and 90 volt regulator in series. You would end up with somewhere around 240-255 volts which should be close enough.

I am wondering why there isn't a plate resistor for the second half of the 12SN7.
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Old 22nd June 2008, 03:05 AM   #3
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Steven,

You have 2 power supply issues. Along with the 250 V. B+ rail, you also need 450 mA. of 12 VDC for the heaters in each channel.

Rectify the O/P of a R/S catalog # 273-1512 trafo using a MBR20100CT common cathode twin Schottky diode. Filter with 25 WVDC/15000 μF. 'lytic. Individual 7812 3 terminal regulators feed the heaters of each channel.

Individual TO92 case LR8 3 terminal adjustable regulators will take care of the 250 V. B+ for the 12SL7s, but you need something more substantial for the 12SN7s. A choke I/P filtered B+ rail would be OK for that job. One way or another, the 12SL7s need to be decoupled from the 12SN7s.

If you have yet to buy tubes, consider the Locktal based 14F7 and 14N7 as corresponding replacements for the 12SL7 and 12SN7. Locktal based tubes, particularly those with "12" V. heaters, are inexpensive.
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Old 22nd June 2008, 12:54 PM   #4
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Quote:
I am wondering why there isn't a plate resistor for the second half of the 12SN7.

The 2nd 1/2 of the 'SN7 is a cathode follower that buffers the preamp's O/P.
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Old 22nd June 2008, 01:22 PM   #5
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Eli

Thanks for pointing that out I took a quick look and it didn't dawn on me.

Are you putting the heaters in series for each channel?

Why is the LR8 necessary? Wouldn't a 90 and 150 gas regulator tube in series be ok? Or would that work ok for the 12SN7 ?



Wouldn't a CCS in place of the cathode resistor (R2) and possibly another one in place of (R7) help performance?
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Old 22nd June 2008, 01:30 PM   #6
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Quote:
Originally posted by burnedfingers
Is the long tail pair (12SL7) configured correctly?


Long tail pair? Are we looking at the same circuit?
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Old 22nd June 2008, 01:35 PM   #7
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Yes,

I just haven't had my cup of coffee yet sorry

Paralled 12SL7

That is what happens when looking at several designs at once.

Age figures in there also
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Old 22nd June 2008, 03:22 PM   #8
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Quote:
Why is the LR8 necessary? Wouldn't a 90 and 150 gas regulator tube in series be ok? Or would that work ok for the 12SN7 ?

Stacked gas discharge VR tubes should be just fine, as the minimum current draw requirements could be met by using a bleeder resistor. However, the "sand" parts are inexpensive and occupy way less space. If recyclable stuff is available in the "junk box", use it. I'd go with LR8s, when buying all new parts.
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Old 22nd June 2008, 03:34 PM   #9
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Eli

Thanks for the reply. I have a lifetime supply of gas regulator tubes that I need to start using up.
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Old 22nd June 2008, 03:53 PM   #10
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Will this work?
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