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pros & cons Fender Brownface style bias supply

3/1/2004 2:23 AM
pros & cons Fender Brownface style bias supply
My appologies if this has Re hashed here a million times.  
What are the pros and cons of a fixed bias supply taken from the HV secondary on a power transformer that doesn't have a separate bias tap like Some Brownface era Fenders?  
I have a PWR tranny with no bias tap and wanted to put it into service so I am debating cathode bias or fixed or swithable.  
Thanks Ben
3/1/2004 11:35 AM
Chris ( CMW amps )

Hi Ben  
Altough most techs and companies do prefer and use PTs with a seperate-bias tap there are a lot of amps with the bias-circuit tapped of from one of the HT-windings .  
I also did use a HT-tapped-bias-circuit in my bf Bandmaster 'cause the bias-tap was out of order ..... no changes in sound and road-proofness .  
Hope this helps ,  
3/1/2004 1:09 PM
Chris ( CMW amps )
one more important note
it's safer imho to always have a bias-supply ( negative voltage ) connected to the output-tubes when on standby ( too ) so I do recommend using a Fender style StandbySwitch .  
3/1/2004 3:30 PM
OK how do I calculate the values for it?
Well, If I am going to use this bias setup I'd like to know how to claculate the values for the components in the circuit. I know I could probably just look at an old Fender schem and get close enough to tweak it to work but I'd like to know how to calculate it out. Any help there??  
Thanks in advance  
3/2/2004 12:58 PM
Dave H

If itís like the 6G3 Brown Deluxe schematic on the Fender Field Guide i.e. 100k + diode in series from one end of the B+ winding going to a 22k with a parallel cap to ground the bias voltage will be half the average value of the transformer voltage x the resistor ratio.  
333/2/1.11*22/(100+22) = 27V  
The 333V on the schematic will be rms. 1.11 is the rms/average ratio for a sine wave.  
3/3/2004 1:49 AM

That's exactly what I needed to know. Thanks Dave!!!  
How do you get the resistor ratio? I have never taken any dc circuits classes so I am in the dark about the rules for calculating these things other than resistance in series or parallel.  
What I see in the afformentioned circuit is voltage in series with the 100k resistor and the 22k to ground your formula calls for the resistor ratio to be:  
I am assuming that since we are looking for a ratio, the trailing zeros fron the 22000 and 100000 ohms can be dropped.  
I understand the actual calculations but how do you get the formula?  
Wish I had the time to take a basic circuits class.  
3/3/2004 3:24 PM

getting the formula is pretty simple. imagine two 5-ohm resistors in series, with 10V across the pair. at the junction of the two resistors there is 5 volts in either direction (ignoring real-world resistor tolerances, and the fact that these resistors would have to be 5W types). if you change the top one to 3 ohms and the bottom one to 7 ohms, then you have 3 volts across the top and 7 volts across the bottom one. so we get the formula: V2=(V1*R2)/(R1+R2). the bottom of the equation represents the total voltage across the divider network, and the top of the equation represents the fraction of the total voltage you get out. if the raw bias supply is 50v, and you're using the 22k/100k pair you've mentioned it becomes V2=(50*100)/(22+100)=about 41V, assuming the 22k is on the top of the voltage divider.

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