# Fender 5F1 basics



## Lincoln (Jun 2, 2008)

I'm just getting ready to solder up my second 5F1. Compairing layouts, I noticed that the "Weber" 5F1 adds a 25UF cap from the 12AX7 terminal #3 to ground. The original and all others only use a 1.5K resistor in this position.

Do I add it as per Weber? I'm not at a point yet where I can understand what effect adding a cap in parallel with a resistor is going to have.

I didn't use it on my first one and I find a little noisy (hum) when cranked. Will this cap effect noise?
(but then again I used all recycled very old components on my first build)

Cab anybody help me see the light or point at something to help me understand?


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## Wild Bill (May 3, 2006)

Lincoln said:


> I'm just getting ready to solder up my second 5F1. Compairing layouts, I noticed that the "Weber" 5F1 adds a 25UF cap from the 12AX7 terminal #3 to ground. The original and all others only use a 1.5K resistor in this position.
> 
> Do I add it as per Weber? I'm not at a point yet where I can understand what effect adding a cap in parallel with a resistor is going to have.
> 
> ...


The 1K5 cathode resistor does two things. One is to set the bias current for that triode stage. The other is to set the gain.

The problem is that in a more complicated fashion it also adds negative feedback to the stage, cancelling out gain at the lower and mid frequencies.

We don't want to lower the value of the resistor. This would increase the gain but also screw up the bias current from the optimum point for the triode stage. This means we would get distortion as well as more gain.

So we use a bypass cap! The beauty of this is that caps block DC, so the bias current stays the same. The DC current flowing through the tube only sees the resistor.

However, to the AC signal, the cap looks like a VERY low resistor! This jacks up the gain to those lower and mid frequencies, the same gain that we had lost from that negative feedback.

The jump in gain for the bass and mids is quite strong. If you make the cap switchable into the circuit you have the "FAT" switch used with some amps, like the VibroKing and others.

There were a number of different Champ circuits. The 5F1 is perhaps the only one that DIDN'T use the bypass cap! All the others did and it seemed most folks preferred the tone that resulted. Just look at the schematics for the 5C1, 5E1, AA764 and others and you'll see a bypass cap on that resistor, plain as day.

It shouldn't add any noise. If you used old components you probably used some carbon comp resistors. These are the best noise generators on the planet! That's why the entire electronics industry couldn't dump them fast enough! The only people who still want them are audiophiles who aren't technical but think if a part is super expensive then it must be good! The people who make their money with this mojo crap do everything they can to feed this nonsense.

So Weber added it 'cuz most folks LIKE it! Try it both ways and see which you like best or better yet, add a switch! You can label it the "Wild Bill" switch or the "FAT" switch, since sadly these days they amount to the same thing...

WB


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## Lincoln (Jun 2, 2008)

Awesome Bill! Thank you for taking the time to reply in such an informitive manner. I will put it on a toggle & label it "Wild Bill".

Any other little cool tweaks I can do at thie stage? I've been thinking of adding a tone pot, that looks really straight forward.


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## madkatb (May 14, 2009)

^Wild Bill- Have you seen this regarding carbon comp resistors? I haven't tried it but maybe in the right spot in the circuit they do have some "mojo".
http://www.geofex.com/Article_Folders/carbon_comp/carboncomp.htm


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## Wild Bill (May 3, 2006)

madkatb said:


> ^Wild Bill- Have you seen this regarding carbon comp resistors? I haven't tried it but maybe in the right spot in the circuit they do have some "mojo".
> http://www.geofex.com/Article_Folders/carbon_comp/carboncomp.htm


Yep! In fact, I posted that article here months ago myself!

The article was written by R G Keen, a bona fide amp genius guy. It's amazing how many guys cite his article to prove carbon comps are bigtime tone makers. That only proves they never actually read his article or if they did, they didn't understand it.

R G did a lot of testing and found that IF there's a lot of current flowing in a carbon comp resistor you get a SLIGHT change in its resistance value, which will SLIGHTLY change the voltage drop across the resistor!

Most places where you find a resistor in an amp you don't have anywhere near enough current flow to cause this affect. R G mentions that the only place you MIGHT find this happening is with the plate resistors in the Phase Inverter!

He further states that IF you are playing the amp loudly enough to cause enough current flow to get the carbon comp PI resistors to give a SLIGHT change in value it would take someone with incredibly good ears to MAYBE hear it!

In effect, R G is saying that while the change can be measured under loud volume conditions it is so slight that few human beings if any could hear it!

We should also understand that this slight tone variation is by definition distortion. Of course, for electric guitar this can be a GOOD thing! Electric guitar is supposed to be distorted! However, what does this mean to the claim in some hifi audiophile circles that "carbon comps give better sound"? Supposedly the game to those folks is to have the MINIMUM amount of distortion!

To recap, we have an effect that only can be measured by lab equipment when the amp is cranked very loud, that can MAYBE be heard only by a tiny portion of the population!

How does this jive with the idea of "you should put carbon comp resistors in your amp so you'll sound better"? Does it make sense to go to all that money and effort for the POSSIBILITY of having a tone change that so few could even notice? Not appreciate, just notice!

If you do, the trade-off is that along with that minute change comes NOISE! Those old carbon comps would ALWAYS start to add lots of noise when they got old enough! Hiss, spit and "frying eggs" when they got bad enough.

Like I said, it's important to actually read and understand what R G Keen is saying in that article. He is NOT saying that carbon comps give better tone, just that there is SOMETIMES a measurable effect that if you look long enough you will find someone who can hear it!

Is a factor so trivial worth getting all that noise?

WB


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## Lincoln (Jun 2, 2008)

I found my noise problem on the first 5F1!

I had an old el-cheapo cord pluggled into that amp and that's all I ever used. I was quick switching amps comparing tone using my good cord.....when I realized there was no noise even when cranked. Duh.........


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