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Old 13th October 2007, 05:34 AM   #1
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Default Making a Bias Voltage from B+ tape?

Hi Friends,

I have a good xformer with a 390+390v 300mA to supply TWO 6L6's in PP.

The unique problem is this:

I like ajustable bias but the xformer haven't a tap to low voltage bias ( +- 40v )

How can I take this voltage from one tap of 390v?

How I do this? Can anybody help me?

Best Regards,
Felipe
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Old 13th October 2007, 06:20 AM   #2
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Can you sacrifice 35V from 390V?
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Old 13th October 2007, 06:36 AM   #3
Colt45 is offline Colt45  Serbia
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backwards diode to one (or two to both) of the legs..

that'll give you 500v or something. filter and divide.

eg.


This is 150-0-150 instead of 390-0-390, but same premise...

Click the image to open in full size.
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Old 13th October 2007, 08:19 AM   #4
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Hi,

If you look at some guitar amp schematics, they derive the bias from tapping right off the HV. They put a resistor in series with on the AC side of the rectifier diode, rectify 1/2 wave and have a load R immediately after the diode as well as the bias pot for load. This keeps surges down to a manageable level.

If your amp runs class A or AB1, you can have a total current through the V- supply of 5mA or less. If it runs A2, AB2 or B, you should use either a proper tap or a transformer setup like Fred's circuit above. Shaver isolation transformers work really nice for this

Cheers!
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Old 13th October 2007, 12:54 PM   #5
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There are a couple interesting ideas here: http://www.aikenamps.com/BackBiasing.html
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Old 13th October 2007, 02:44 PM   #6
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Hi Friends,

I think it's very danger retifier ALL voltage of xformer to do the bias.

I'm thinking to do this ( see the attach file ). I see this on a HIWATT schematic for EL34, but without the potentiometer.

Do anybody know who is a good idea put a ZENER with the bias voltage to make sure the voltage is low than -50v?

I don't know the value of the resistor and the value of the potentiometer.

Can anybody help me with this?

Best Regards,
Felipe Navarro
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File Type: jpg biasvoltage.jpg (8.7 KB, 196 views)
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Old 13th October 2007, 03:07 PM   #7
SY is offline SY  United States
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I think what Wavebourne was getting at was the idea of putting a small resistor between the transformer secondary CT and ground. You lose some B+ that way, but a one component solution has its virtues.
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Old 13th October 2007, 03:18 PM   #8
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Using a zener to regulate bias is not a good idea unless the screen voltage is also regulated. When all voltages are unregulated, the plate current will go down when line voltage goes up, as control grid has more gain than screen, and dissipation is more or less constant.

Some other possibilities: a small transformer just for bias, or even a reversed 120:12V transformer connected to a heater winding to give 60VAC. Some amps used a capacitor as a dropping element from one side of the B+ winding - you'll want a cap of 1000V or more, though.

How about combination bias? A 6V heater winding can give about -15V with a doubler, and the rest can be cathode bias.
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Old 13th October 2007, 03:30 PM   #9
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by SY
I think what Wavebourne was getting at was the idea of putting a small resistor between the transformer secondary CT and ground. You lose some B+ that way, but a one component solution has its virtues.
Here's an example: Just adjust the resistors to get the value you want. Add additional filtering if you like.

http://www.diyaudio.com/forums/attac...amp=1113901355
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Old 13th October 2007, 05:09 PM   #10
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Quote:
Originally posted by Geek
Hi,

If you look at some guitar amp schematics, they derive the bias from tapping right off the HV. They put a resistor in series with on the AC side of the rectifier diode, rectify 1/2 wave and have a load R immediately after the diode as well as the bias pot for load. This keeps surges down to a manageable level.

If your amp runs class A or AB1, you can have a total current through the V- supply of 5mA or less. If it runs A2, AB2 or B, you should use either a proper tap or a transformer setup like Fred's circuit above. Shaver isolation transformers work really nice for this

Cheers!

Geek,

To I calcule the size of the resistors how many mA will use the grid bias of the 6l6?

Best Regards,
Felipe
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