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Old 15th January 2011, 01:38 AM   #1
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Default Tube bass preamplifier power supply (Need Help)

I have been doing a lot of research on the methods of obtaining a B+ supply for preamp tubes. In my case, it is for a 12ax7 tube.

I have two
5H inductors at my disposal as well as many other components.

The transformer I am using is a
250-0-250 with a filament tap (I have already designed and built my filament PSU).

I would like to know a good design that would yield the lowest noise and ripple possible. The goal is to have a B+ voltage of around 300VDC.

Doing some simple calculations brought me to the conclusion that my unloaded voltage from the center tap to the 250VAC tap is about 354Vp. After the bridge rectifier this would be around 350V ripple DC at 120Hz. Since the bridge rectifier is being used in a high voltage supply, the voltage drops don't amount for much.

Thank you for the help!

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Old 15th January 2011, 01:54 AM   #2
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On my disco valve mixer I just used a CRC supply.
If I remember correctly it was 100uf, 8k2, 100uf.
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Old 15th January 2011, 02:13 AM   #3
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Quote:
Originally Posted by nigelwright7557 View Post
On my disco valve mixer I just used a CRC supply.
If I remember correctly it was 100uf, 8k2, 100uf.
Thank you for the reply. I heard a CLC filter works better for hum reduction and the inductor is usually replaced with a power resistor in low budget applications? Take care Nigel.
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Old 15th January 2011, 01:24 PM   #4
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For a preamp a CRC type filter circuit is usually more than adequate. Don't stint on the capacitor values if you have a semiconductor rectifier. You can get 470uF/450V electrolytics quite cheaply these days (they have good specs too as they are used in switched mode power supplies).

You will find that several RC sections in series will have much better ripple reduction than and single section of equivalent R and C. So for example, if you had four stages of 1K and 470uF, this would be very much better than one stage of 4K and 2000uF.

Most preamps consume little more than 10mA from the HT line so the 1K value is about right for your application.

Remeber to place a 100nF cap across the input to your bridge rectifier to snub switching spikes which will get into your preamp.

Cheers

Ian
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Old 15th January 2011, 02:41 PM   #5
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Quote:
Originally Posted by ruffrecords View Post
For a preamp a CRC type filter circuit is usually more than adequate. Don't stint on the capacitor values if you have a semiconductor rectifier. You can get 470uF/450V electrolytics quite cheaply these days (they have good specs too as they are used in switched mode power supplies).

You will find that several RC sections in series will have much better ripple reduction than and single section of equivalent R and C. So for example, if you had four stages of 1K and 470uF, this would be very much better than one stage of 4K and 2000uF.

Most preamps consume little more than 10mA from the HT line so the 1K value is about right for your application.

Remeber to place a 100nF cap across the input to your bridge rectifier to snub switching spikes which will get into your preamp.

Cheers

Ian
Just one 100nF across the input? I read somewhere that it was a good practice to place small caps across all four diodes in the bridge to snub the switching spikes. Is that not necessary?
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Old 15th January 2011, 04:22 PM   #6
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Thank you very much, great information!
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Old 15th January 2011, 11:06 PM   #7
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Quote:
Originally Posted by PWR RYD View Post
Just one 100nF across the input? I read somewhere that it was a good practice to place small caps across all four diodes in the bridge to snub the switching spikes. Is that not necessary?
I have read that too. I have built many power supplies and never yet found it necessary. Even for a 70dB gain mic pre, the single 100nF has always been enough.

Cheers

Ian
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Old 17th January 2011, 03:29 AM   #8
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I ran a simulation with the single 100nF capacitor across the AC pins of the bridge rectifier and I noticed that the signal looked very poor. I was simulating using a 3N259. I have a feeling that this bridge is not suitable?

Thank again, take care.
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Old 17th January 2011, 04:07 AM   #9
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Click the image to open in full size.Click the image to open in full size.

Last edited by fingerboy21; 17th January 2011 at 04:08 AM. Reason: Wrong link.
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Old 17th January 2011, 06:37 PM   #10
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Quote:
Originally Posted by fingerboy21 View Post
I ran a simulation with the single 100nF capacitor across the AC pins of the bridge rectifier and I noticed that the signal looked very poor. I was simulating using a 3N259. I have a feeling that this bridge is not suitable?.
That bridge should be OK. However, you need to set the source resistance of your generator to some realistic value to get a meaningful simulation - with it set a zero the 100nF will not work.

Cheers

Ian
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