Making a Bias Voltage from B+ tap? - diyAudio
 Making a Bias Voltage from B+ tap?
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 13th October 2007, 05:34 AM #1 diyAudio Member     Join Date: Dec 2004 Location: Rio Making a Bias Voltage from B+ tap? 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
 13th October 2007, 06:20 AM #2 diyAudio Member     Join Date: Sep 2006 Location: Pleasant Hill, CA Can you sacrifice 35V from 390V? __________________ The Devil is not so terrible as his math model is!
 13th October 2007, 06:36 AM #3 diyAudio Member   Join Date: Feb 2002 Location: Canada 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...
 13th October 2007, 08:19 AM #4 diyAudio Member     Join Date: Sep 2004 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!
 13th October 2007, 12:54 PM #5 diyAudio Member     Join Date: Mar 2007 Location: Newark, DE There are a couple interesting ideas here: http://www.aikenamps.com/BackBiasing.html
diyAudio Member

Join Date: Dec 2004
Location: Rio
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
Attached Images
 biasvoltage.jpg (8.7 KB, 231 views)

 13th October 2007, 03:07 PM #7 diyAudio Moderator     Join Date: Oct 2002 Location: Chicagoland Blog Entries: 2 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. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 13th October 2007, 03:18 PM #8 diyAudio Member     Join Date: Jun 2002 Location: Macedon NY 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.
diyAudio Member

Join Date: Dec 2003
Location: San Diego
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

diyAudio Member

Join Date: Dec 2004
Location: Rio
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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