# Fender Blackface - Capacitor on the Bias Board



## Lincoln (Jun 2, 2008)

On a Fender AB763 (DR) the original schematic shows a 25uf 50V capacitor on the bias board. I probably read too much, look at too many drawings.....but when I do I'm seeing other value capacitors in the that spot on newer schematics like the Weber kit site or the Hoffman site. I've seen 47uf 100V, 50uf 100V, 50uf 150V, 100uf 100V, etc. The thinking seems to be that this is an improvement over the original design, and the more capacitor you have in the bias curcuit the better it is for noise. 

Am I ok with a 47uf 100V? or should I set my sights higher? Is there a maximum? can I go too high in capacitance?


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## jb welder (Sep 14, 2010)

100 to 150uF is generally the upper limit seen for bias filter cap. Upping the voltage to 100V as you are doing is good practice.
The larger the cap, the longer it takes for the bias to build up, which is not a good thing.


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## nonreverb (Sep 19, 2006)

Not really a problem with an amp using a tube rectifier as the cap will charge before the rectifier will start conducting. Solid state rectification is another matter as this could be a concern with cathode stripping although it's debatable.




jb welder said:


> 100 to 150uF is generally the upper limit seen for bias filter cap. Upping the voltage to 100V as you are doing is good practice.
> The larger the cap, the longer it takes for the bias to build up, which is not a good thing.


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## Church-Audio (Sep 27, 2014)

I use atom or f&t 100uf @ 100v no issues.


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## jb welder (Sep 14, 2010)

nonreverb said:


> Solid state rectification is another matter as this could be a concern with cathode stripping although it's debatable.


 I was more concerned with reduced tube life if the tube is running flat out till the bias comes up. But there is usually a standby switch to prevent this issue (if the standby is used properly).
The main reason we don't see bias filter caps bigger than 100uF is there are no real advantages to going larger (tonally or otherwise).


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## nonreverb (Sep 19, 2006)

I would think the heater warmup time would basically prevent that? As for bigger is better, I totally agree that there is no advantage to installing big capacitance in the bias circuit.



jb welder said:


> I was more concerned with reduced tube life if the tube is running flat out till the bias comes up. But there is usually a standby switch to prevent this issue (if the standby is used properly).
> The main reason we don't see bias filter caps bigger than 100uF is there are no real advantages to going larger (tonally or otherwise).


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## Church-Audio (Sep 27, 2014)

jb welder said:


> I was more concerned with reduced tube life if the tube is running flat out till the bias comes up. But there is usually a standby switch to prevent this issue (if the standby is used properly).
> The main reason we don't see bias filter caps bigger than 100uF is there are no real advantages to going larger (tonally or otherwise).


It's never an issue with tube rectifiers and even the heater warm up. I have seen issues on JMPs that were wired strangly on the power switch.


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## jb welder (Sep 14, 2010)

Amprepair said:


> I have seen issues on JMPs that were wired strangly on the power switch.


Yes, some amps have the bias circuit on the wrong side of the standby and should be corrected. Unfortunately, many amps get cloned exactly, right down to the faults.

The heater warm up time should prevent the problem of bias rise time, but I haven't done the math of the charge time nor am I certain of a minimum heater warm up time, so I defer to the previous discussion on the issue :smile-new:.
http://www.guitarscanada.com/showthread.php?49687-Fender-Bias-Circuit&p=424679#post424679


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