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Bias question

3/26/2006 9:02 PM
Bias question
Hi all.  
Can I get the bias voltage from one leg of the secondary AC if I'm using a bridge rectifier? (Like on FI a JTM45)  
Or is this just possible with a full wave rectifier?  
3/26/2006 9:22 PM

Just check that
3/26/2006 10:30 PM

Thank you for the reply.  
Do I need the .047uF cap between the bridge and the reversed diode to make this work?  
I have tried to do this the JTM45 way with a 220k resistor before the reversed 1N4007, but I can't get a negative voltage out of it no matter what I do. If I turn the diode the other way I get about +130V (diode not connected to the rest of the bias supply), but with the diode reversed I get absolutely nothing. Really frustrating this...  
3/26/2006 11:54 PM
Carl Gigun
you need the cap, you won't get negative voltage without it because the bridge never goes below ground.
3/27/2006 4:27 PM

Thank you Carl.  
I'll give it another try tonight.  
3/28/2006 2:52 AM
Reid Kneeland

On the contrary, you'll get WAY TOO MUCH negative voltage without the cap - it's connected directly to the PT secondary, not the + output of the bridge. If there weren't any negative voltage there, a cap wouldn't make any appear!  
The cap is simply being used as a resistor. Since the frequency is a constant 60 Hz, the cap has an impedance of about 56K ohms. You can replace it with a resistor of that value if you need to.  
Trust me on this - I have a V4B, and I replaced the cap with a 100K pot to make the bias adustable. Works great.  
3/28/2006 9:07 PM
Carl Gigun
Seems to me that in a FWB as soon as one side of the secondary tries to swing low it's diode to ground opens, effectively grounding that end so it can't go lower and the other side develops a positive voltage that can supply the cap. When the AC reverses the opposite end gets grounded. So no voltage below ground to tap from.  
The Cap blocks those dc references and supplies a smaller AC signal to the bias supply. The resistor to ground after the cap centers this AC signal about ground so there is positive and negative voltage to be had. The reactance of the cap and the resistor form a voltage divider that knocks the AC down to the level you want. After that it's a simple halfwave rectifier.  
That's how I read the circuit, If you see something different let me know.
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