Bass Pot Replacememnt: 250K Linear vs 1Meg Reverse Audio

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Bass Pot Replacememnt: 250K Linear vs 1Meg Reverse Audio

Postby daveplaysbass » Mon Jan 01, 2018 11:29 am

My Tribute bass pot is very scratchy. I started looking for a metric replacement that is a 1Meg reverse audio taper and there is not much out there. The G&L store is out of the Tribute 1MegC variety to make things worse.

So I looked at the circuit and realized that the bass pot is only used with the cw and wiper terminals. Since the ccw terminal is not being used, the pot can be replaced with a different value without impacting the circuit behavior. You will just be limited in how much adjustment range you have.

When the bass is "full on" the wiper to cw resistance is zero and as you rotate for bass cut, the cw to wiper resistance increases from 0 to 1Meg. Given this is a reverse audio taper, the first 50% of rotation from the "full on" position is seeing the shallow side of the reverse log taper. I usually do not rotate the 1Meg bass cut anywhere beyond the 50% point which is in the ballpark of a 100K resistance.

So I ran some PSPICE simulations to see how the 250K linear would stack against the stock 1Meg reverse audio taper. These simulations are for a passive mode model which assumes a 1000pF cable capacitance, volume full on, tone pot full on, and one MFD coil (SC).

The 0-100% settings in the plots are for pot position. I used a mathematical estimation for the resistance seen by the circuit at each position. I then did the simulation again using a 250K Linear Taper Pot. I think I will prefer the finer adjustment exhibited by the 250K linear in the -4 to -6 dB cut range. And I don't think I need more than -6dB of cut.

I also found a CTS made "metric" pot that should be a direct replacement for the metric pots used in the Tributes with the smaller mounting hole (I had to drill a bigger hole for the push pull pot I put in the volume position that uses the more standard US 3/8" bushing).
CTS Metric 250K Linear Pot
Alternative Metric 250K Linear Pot

I am somewhat surprised by these results. I see no reason to chase down a 1Meg reverse audio taper pot. And why has G&L gone through the trouble of sourcing this odd ball part for the last 35+ years? This makes me question if I have done the math right. I think I am going to order a CTS metric 250K linear pot and see if it works as advertised.

Edit: I am posting new plots that hopefully don't truncate like the originals.

1Meg Reverse Taper Stock
Image


250K Linear Taper Mod
Image
Last edited by daveplaysbass on Sat Jan 06, 2018 1:07 pm, edited 3 times in total.
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Re: Bass Pot Replacememnt: 250K Linear vs 1Meg Reverse Audi

Postby daveplaysbass » Mon Jan 01, 2018 5:17 pm

While I was ordering the 250K linear bass pot, I thought I would order a metric CTS 250K for my tone as a backup. So before ordering a tone pot, I ran simulations to compare audio and linear tapers for tone. I will stick with audio taper for the tone. Results below.

Edit to place proper labels on the plot.

250K Audio Taper Tone Stock
Image


250K Linear Taper Tone
Image
Last edited by daveplaysbass on Wed Jan 03, 2018 8:11 pm, edited 3 times in total.
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Re: Bass Pot Replacememnt: 250K Linear vs 1Meg Reverse Audi

Postby bigtone23 » Wed Jan 03, 2018 1:19 am

Great info! If you did do your math correct, the 250K Linear bass knob is much more useful. I have never taken a stock 1meg bass knob below 5 or 6, myself.
I have found that I prefer audio taper tone pots. I do prefer linear volumes on basses with the Jazz bass style Vol Vol set up. It gives a little more usable adjustment range for those finer blends.
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Re: Bass Pot Replacememnt: 250K Linear vs 1Meg Reverse Audi

Postby daveplaysbass » Wed Jan 03, 2018 8:11 pm

bigtone23 wrote:I do prefer linear volumes on basses with the Jazz bass style Vol Vol set up. It gives a little more usable adjustment range for those finer blends.


I have never thought about audio vs linear for blending but upon reflection I agree. I like running jazz basses with a 500K master volume and a 250K blend pot that I believe is a linear taper.
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Re: Bass Pot Replacememnt: 250K Linear vs 1Meg Reverse Audi

Postby TDR1138 » Thu Jan 04, 2018 9:49 am

This definitely warms up the possibility of putting treble and bass on a stacked pot and freeing up a hole for a vol-blend or vol-vol setup...
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