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blend pot problem

HeartMood

HeartMood

Dec 19, 2005
10
4
Frankfurt Airport Area, Germany
soundcloud.com
Disclosures
Endorsing Artist: Fodera, EA-Amps, Wilson FX, Italialeather, R.Cocco
#1
I just replaced all pots in my 1998 Fodera Imperial six string. It was made passive, but they had a printed circuit board with soldered on pots in it and the pickups were attached with little plugs. I replaced the circuit board and soldered everything exactly in the same way with new pots.
The blend pot gives a lot of output if completly in either bridge or neck position, in the center position the (formerly equally mixed) signal is very weak.

I tried a suggestion found here. reversing in and output of the blend pot - without success: same problem.

Anyone got an idea? Maybe it has something to do with audio(logarithmic) or linear behavoiur of the blend pot? I don't know what it is. I ordered it at guitarelectronics.com. It's a 500k pot, like the old one.

TIA Hartmut
 
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#6
Problem solved!
I replaced the blend pot with another one I had ordered here in Germany (at least I think so, maybe I confused the pots, but both were sold as balance pots 500K).
The old one reads "A500K" on the top and has a round plate at the bottom for the center dent.

The "new" working one reads: "ALPS" (so it seems to be an ALPS, though it was not advertised as one) stamped into the metal on one side and has no plate for the center dent, it seems the center dent is made inside the pot.

Though its a bit tiny with a small shaft, it works great.

So I can't tell what the right pot is logarithmic/audio taper or linear, because I don't know what type the "ALPS" is.

Joey (FODERA Guitars)mailed me:"It sound like the pot that you put in has a different taper or the pot itself is not wired. I can send you a pan pot that will work. "

Very nice of them, they wanted to send it free of charge, but I don't need it anymore.

But I'm not totally satisfied, because I'm curious and like to know why and how things work...

Thank You all for Your suggestions!

Hartmut
 
#7
can anyone tell me what is the difference in using 250K and 500K pots for blend? and what happens if other value of pot is used?
I had put blend knob on my jazzbass but not sure of its value. I don't think it work perfectly as I can't have very deep bass (middle pu only) or very treble (bridge pu only) like when I had vol/vol.
Can anyone help me with these question? I would really appreciate any info. Thanks
 
#8
Hey leftdecay,
I went through all kinds of forums and places and spent about twenty hours of soldering and changing pots and capacitators the last couple of days to come to a conclusion/solution for the 250/500k issue: many people say 500k gives more treble and output. I tried both (for vol + tone) for balance/blend I just used 500k and don't know if it makes a difference to use a 250k blend pot, but for vol + tone I liked the 250k better, so I fear You have to try also both versions and decide according to Your ear. I think You can even mix them e.g. 250k for volume and tone, 500k for balance (like my actual set up). I think it depends heavily on the pickups You use.

Another issue is audio/logarithmic versus linear pot-behaviour. For me linear works best for volume and treble, for blend I don't really know (besides it reads "ALPS" on the pot) what's inside my bass now. Because the last blend pot did not work and read "A500K" (and I believe "A" stands for logarithmic and "B" for linear behaviour), I think the ALPS is a linear one.

It seems no one in all these forums has a really deep knowledge or the set ups are so different that You can't predict the sound of exactly Your bass. So You have to test it with your setup of bass and pickups, sorry that's a lot of work.

I'm going to change my vol and tone pots of both my Foderas a last time in the next days, replacing the japanese ones with CTS because the CTS fit better in my basses wood-holes and they feel better.

The original ones were DiMarzios, they are very durable, rugged and soild constructed, but I can't find a retailer to ship them to germany, so I get the CTSes from guitarelectronics.com...

Good luck for Your experiments
Hartmut
 
#9
Hi Heartmood ...
My bass is Fender Jazz V. I still use the original 250K pot for the volume, but but change the 2nd volume knob to blend and change the one for tone to 500K and it does make it sound brighter.
I forgot what I used for the blend pot. It was spur of the moment thing, so I just used what was available at the moment at my friend's house (that's where we changed the volume to blend knob).
The blend knob works but somehow I just don't feel (or hear) like it's a 100%. Guess I have to check it again and fix what needs to be fixed.
Thanks Heartmood :)
 
#10
Well, I don't know whether everyone agrees with this, but to me a blend pot should have one log track and one reverse log track. Anything else will result in loss of signal amplitude when in the centre detent.

Also, some log pots are logarithmic at all, they are simply two distinct regions of resistance that make a wide V shaped curve that approximates a log curve. Far from ideal.
 
#11
HeartMood said:
Another issue is audio/logarithmic versus linear pot-behaviour. For me linear works best for volume and treble, for blend I don't really know (besides it reads "ALPS" on the pot) what's inside my bass now. Because the last blend pot did not work and read "A500K" (and I believe "A" stands for logarithmic and "B" for linear behaviour), I think the ALPS is a linear one.
Click to expand...

I thought it was rule of thumb that audio pots are used for volume and linear are for tone, making me think audio would be used for blend pots.

*Edit*
After rereading niftydog's post I pretty much just agreed with his opinion.
 
#12
This might help;

At the centre detent the pots rotation is 50%, right.

50% rotation on a linear pot equals 50% of the full resistance, which in turn equals 50% of the volume! Not good.

Whereas, 50% rotation on a log pot equates to something like 98% of the full resistance, which in turn equals 98% of the volume. Good!

That's why log tapers are used for balance pots.

But, there's a third case - pseudo log pots are a cheap attempt to approximate a log taper. These have two linear regions of different slopes meeting at a sharp corner. It could well be the resistance value at 50% rotation is more like 90%.
 
#13
Hey nifty, I'm a little confused. Wouldn't a reverse log audio pot be needed? If it wasn't a reverse pot than would you be getting 2% of the volume instead of 98%? Not disagreeing with anyone, just trying to clear something up for myself.

*Edit*
Now what about tolerance for capacitors. Will there be much of a difference between +-10% compared to +-20%?
 
#14
Yes, see my previous post. You need one forward and one reverse log taper.

Re cap tolerances; I guess it depends on how particular you are. This is a bit of a 'how long is a piece of string' topic. Buy two caps, a 10% and a 20%, and there's a real chance that the 10% cap has a real-world value less than that of the 20% cap! If you want to be sure, the only way is to buy a bunch and measure their exact values.
 
#16
To clear up a few things:

First of all, on audio-taper pots, rotating the pot half way removes the majority of resistance between the pickup and ground. As you turn down from full volume, the resistance decreases very fast at first, then slows down. This is because the human ear hears sound logarithmically, so large changes to loud sounds are similar to small changes in small sounds (e.g. a change from 100 to 110 sounds the same as a change from 1 to 1.1 (both are 10%)).

Secondly, a blend pot consists of two pots mechanically linked. When centered (half-way through each individual pot's rotation), each pot still shows its full resistance from the wiper to ground. Then, when the knob is turned in one direction, one pot remains at full value while the other decreases, and vice versa for the other direction. So basically, each blend pot stays at full value for half of its rotation, then starts to decrease past that.
This is not to say that there aren't any blend pots that do change resistance for the full rotation, but it isn't standard AFAIK.
 

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