Log B potentiometer options?

[RockinRocket]

Im looking for Log B potentiometers for a James tone stack Im playing with in Duncons TSC. Ive looked with no luck. Im hoping for mil spec but anything would be fine. I need Log B to be able to achieve a flat response when set in the middle. The James TS is common in hifi so these pots have got to be out there right? Thanks

[rock_mumbles]

A 'Log B' pot in TSC is a 10% Audio taper ... I think CTS Audio pots measure pretty close to 10% ... Alpha's tend to be closer to 15%

[RockinRocket]

Thanks Rock. Turns out the alphas and Pecs I have do the 10% half way. About 50k for a 500k pot. When I measured the pots two things I noticed, the pecs (from a scale of 1-10 through the range of the pot) 1-2 is a few ohms to 1 K.. very little resistance. The Alphas didn't do that. At the end of the range 9-10 for the pecs the resistance floated back and forth 5k of max resistance while the Alphas had a steady rise to max.

This seems to easy to be the pots for a James tonestack after all I already have them. Its never this easy..

[RockinRocket]

Now that ive spent more time accuretly measuring alphas they are 15%. While 2 of the 4 Pecs I have measured a perfect 10% and the other 2 are at 8% and 11%

Now this is where im getting confused around. If a log B is 10% at the half way rotation why when I play with the TSC for James TS does it not matter if the pot is a 50k 100k 300k 500k ect? At 50% of all those pots its still flat response. Keep in mind 10% of 50k is 5K and 10% of 500k is 50k.

Edit- Im guessing its as simple as the relationship of 10% of what ever value the pot is to the rest of the circuit..

[tubeswell]

The difference between 10% taper and 15% taper is moot in a guitar amp. Where the mid point is depends on what guitar you plug into it, strings, pickups, pedals etc, what speaker you have connected.

[JMFahey]

when I play with the TSC for James TS does it not matter if the pot is a 50k 100k 300k 500k ect? At 50% of all those pots its still flat response.

That's exactly the point.

Attenuation depends on the voltage ratio which in turn depends on the resistance ratio.

The idea behind using 10% electrical ratio at 50% geometrical ratio (halfway rotation) is so with all pots halfway you have "perfect" 20dB attenuation (10:1) and flat response, so in theory you have 20dB boost (really 0dB attenuation) and symmetrically, 20dB cut , at the nominal bass or treble frequency (say 50 or 100Hz and 10kHz or 20kHz respectively.

Of course since action at the extremes is not abrupt but bends smoothly, actual range is not that high, but +/- 15dB is easy to achieve, and +/-12B even easier, both very useful control ranges.

The James is really a brilliant design.

That some manufacturers choose to slightly cheat by using 15% and even 30% is not the point, and definitely not the James fault.

[RockinRocket]

Thanks guys for the explanations. Im thinking the Pecs I have will work out perfectly. Anyone have an idea what the log A taper would be in the Duncon TSC? To get a comparison between the two log types in the TSC so I have an idea how the curves between the two work in this circuit. Thanks

[JMFahey]

The curves will be exactly the same.

Simply that same effect or EQ will appear at a slightly different angle or knob position.

Just to make up an example: the sound or curve with the 10% pot set to, say, "6" will be the same as a 15% pot set to "5" and so on.

The amp using 15% or 30% pots will seem to be slightly louder or have slightly more Bass and Treble than a 10% one when set to the same "number" , but end to end range will actually be the same.

In a tone control it's not as annoying as in a volume control.

[RockinRocket]

Hmm, I took a snapshot of the sweep in TSC and while close, I could not match the frequency range when switching from A to B log 100%.

I think I worded what I was really curious about wrongly. I was wondering what taper the A log was in TSC not for the frequency range but how/ where on the dial it would need to be to match it of the other log.

Example with A log I get a flat response with both dials on 2.. not much room to fine tune easily. If I only wanted to boost mids by cutting bass and treble I only have till 1 on the dial compared to 3. With an A log it seems to be a little less sensitive in this circuit

[JMFahey]

Don't worry about the "numbers" on a knob or panel scale, the actual adjustment is the same.

What you find as 2 in one and 3 in another is the same when you get to 0 or 10.

[Structo]

I wonder if using 10% pots causes some of the crap we are hearing in newer amps.

In other words, at half volume a 10% pot will trim less highs than a 30% one at the same location.

Or just forget the pointers and numbers and turn it to where it sounds good.

I am kinda serious about the different tapers sounding different.

It seems that we are always trying to squelch those annoying high buzzy frequencies.

I like the idea that some forum members are doing, ordering a batch of custom pots.

[Leo_Gnardo]

Or just forget the pointers and numbers and turn it to where it sounds good.

Best idea yet!

One of my crustomers whinged about how his HR Deluxe was too loud at 3. So I padded the pot, expanding his control range. That started a whinge-storm about how "all the power's gone, I gotta turn it up to 5 to get the same volume wah wah wah wah change my diaper." Back in we go! Round and round an round n round. Can't wait to hear the next complaint.

All the neurotic, none of the talent. Feh.

Lucky they're not all like that.

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Amp volume coming up too fast = manufacturers' ploy to sell amp in the store. If amp A is plenty loud at 2, and amp B you have to wind up all the way to 4, must be Amp A is more powerfuller - ring up another sale ka-ching! Later on the buyer may regret the purchase once they evaluate tone and learn to discard what number the volume control is pointing to. If that ever happens.

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And then there are those who discover in their bass-ackwards way that treble/mid/bass controls set all the way down take away all the volume in a Twin or similar amp. "Somethings WRONG!!!!!" shouts Chickn Little. "My amps AFU and it's your fault!"