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Old 1st February 2009, 02:37 PM   #1
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Default Need a bias supply how??

I have an HP transformer that I am using in a Dynaco stereo 70 that I am putting together. It doesn't have a bias supply tap.
Can I make one by coming off one of the AC secondaries? Its 435-0-435 AC secondary.

Can someone help me out here?
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Old 1st February 2009, 03:03 PM   #2
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This is a good starting point:

http://www.aikenamps.com/BackBiasing.html
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Old 1st February 2009, 03:05 PM   #3
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Some folks have used a small 6v or other low voltage mains transformer in inverted fashion off the heater windings - one needs only a few mA so it does not need to be expensive. You could also use a series capacitor and diode pump circuit off one 435 tap - but the capacitor had better be good and h.v. (>700V). I do not have such a circuit handy right now; there are several variations. But the inverted transformer solution might be the more elegant.
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Old 1st February 2009, 03:14 PM   #4
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http://www.aikenamps.com/BackBiasing.html

This will only give me -9--13volts if I am looking at it correctly and I need to bias the EL34's.

Diode pump circuit? Don't know what you mean.

I was thinking that I saw a circuit somewhere that used a voltage divider off the AC and then one diode and then went to the bias circuit. Maybe I'm wrong :

I don't have the room for even a small transformer on or under the Dynaco chassis.
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Old 1st February 2009, 03:30 PM   #5
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Quote:
I don't have the room for even a small transformer on or under the Dynaco chassis.

Use an AC "wall wart" and a voltage multiplier to generate the negative rail.

Another option is to SS rectify the B+ and self bias the "finals".
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Old 1st February 2009, 04:17 PM   #6
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OK

When I mentioned a small transformer I was thinking about a 3/4" by 1" square little thing we have available here; about 1VA, but of no use if you cannot obtain the same.

Yes, the bias circuits shown in the quoted piece subtracts from the original h.t. In that sense one might as well use a power zener as a cathode bias source.

Perhaps I will find time later tonight to hook up something with a series capacitor as in what I described earlier and let you know (it is now 19:18 SA time).
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Old 1st February 2009, 04:19 PM   #7
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Burnedfingers,

If you go "schematic heaven" and look up a Marshall JTM45 you'll see what you need as long as you are NOT using a bridge. The JTM45 uses a 220K Ohm resistor to knock down the voltage to the bias diode, you may need a larger one due to the higher voltage of your xfmr. What type of wire did you end up finding? I was going to offer you some Teflon coated stuff but without the correct strippers it's kind of a pain.

Craig
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Old 1st February 2009, 04:33 PM   #8
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Quote:
This will only give me -9--13volts if I am looking at it correctly and I need to bias the EL34's.
It depends on the voltage of the zener.
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Old 1st February 2009, 06:25 PM   #9
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You'll need a 25W zener - or you can use a resistor, which will effectively be cathode bias rearranged... with the voltage you have, dropping off 30-40V may be a GOOD thing...
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Old 1st February 2009, 07:17 PM   #10
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Quote:
You'll need a 25W zener - or you can use a resistor, which will effectively be cathode bias rearranged... with the voltage you have, dropping off 30-40V may be a GOOD thing...
Either way you do this, It will generate SERIOUS heat, plus it will take 30-40V off the plates. I've tried this and it's not worth it.

I've used a 25.2V, 2A Radio Shack tranny which can generate up to -66V with a doubler circuit. And it can also serve as some nice clean DC for the preamp tube filaments.

~John~
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