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 12:29 pm

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 2:07 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 6:17 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 9:11 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 2:19 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 9:11 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 10:49 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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Re: Bass Pot Replacememnt: 250K Linear vs 1Meg Reverse Audi

Postby vedi » Tue Apr 23, 2019 2:36 pm

dear all,

great posts in a great thread on a great forum; a lot of useful info, and I want to take the opportunity to thank everyone involved.

I wanted to inquire whether anyone in this particular thread persuaded the idea of replacing the bass pot, and were you satisfied with the results?

I have acquired an L2k5 Tribby, converted it to fretless and am now looking into making most of the electronics, so naturally I have read most of the threads here (once more, great stuff!). besides bringing the preamp to US specs and possibly doing some mods to it while it is out of the instrument, I wanted to do what I do to all my two-pickup basses: replace the balance pot (or second volume pot) with a rotary switch. now, if I could somehow source out a stacked pot for bass/treble cut, that would free up one hole for the rotary switch with 5 positions:
- neck pup full
- neck pup full + bridge pup %
- both full
- neck pup % + bridge pup full
- bridge pup full

(the % notates a percentage of the signal that is set internally with a trim-pot, usually 1MOhm)

I apologize if this derails the thread somewhat. I have some more questions regarding the rest of the wiring, but will search for appropriate threads. thank you for your time!
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Re: Bass Pot Replacememnt: 250K Linear vs 1Meg Reverse Audi

Postby daveplaysbass » Mon May 20, 2019 1:47 pm

vedi wrote: if I could somehow source out a stacked pot for bass/treble cut, that would free up one hole for the rotary switch with 5 positions:


You may have already finished your project. But if you were going to try a dual concentric tone / bass cut pot, I would suggest going with an Audio 250K style. The tone control would function just the same. And the bass cut would be more sensitive in the first 50% of travel by an estimated factor of 2 given theoretical calculations. The 1 Meg reverse taper does 100K in the first 50% (CW to wiper). The 250K Audio taper does about 225K in the first 50% (CW to wiper). If that bothered you, you could put a 250K resistor in parallel with the CW to wiper portion and get a pretty good match on the first 50% of travel relative to the stock bass pot. You would just loose some of the high bass roll off points on the lower 50% of travel that no one ever uses. I think it would be worth it to add the 250K parallel resistor (anything between 180K and 300K would do the job).

I still have not replaced my scratchy bass cut pot with the linear taper 250K. I may get around to doing this soon and I will measure the 1Meg reverse taper resistance curve and see how well it matches the predictions.
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Re: Bass Pot Replacememnt: 250K Linear vs 1Meg Reverse Audi

Postby vedi » Sat May 25, 2019 12:34 pm

thanks for answering, Dave! and even more for the tips - I will definitely try the mentioned resistor wired in parallel. it has been an incredibly busy month, but I will get to ordering the necessary parts next week. most of all I need to replace the scratchy volume and treble cut pots, but since it is going to be on the bench anyway, why not do some other mods as well. hope I will be able to fit it all in ;)
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Re: Bass Pot Replacememnt: 250K Linear vs 1Meg Reverse Audi

Postby daveplaysbass » Sun Mar 15, 2020 2:36 pm

Well its only 26 months since I started this thread. I finally got around to replacing the scratching 1Meg pot in my Tribute L2K with a metric CTS B250 linear pot. It took about an hour to pull everything out, reuse the two film capacitors, solder the green and white wires to the pot, and resolder 4 ground connections on the back of the pot. I was surprised that the metric CTS pot was smaller diameter than the stock pot. The stock pot had a shaft diameter of 8.7mm where the new one was 7.8mm. But everything went together with the smaller shaft just fine. And the spline and knob where a perfect fit at 5.9mm.

As for the operation, I am really pleased with the results. Now that the pot does not crackle, I found myself very inclined to use it to make fine adjustments in parallel, SC, and series. I found a nice tone with the neck pickup and OMG series using about 3/4 bass cut. At 100% bass cut, it is about the same as the stock bass pot at 60% cut. Setting the bass to 3/4 cut is about a 6 dB cut which seems to balance out the 6dB boost typical of series. It still retains the thickness but is not as boomy. If I was still using the stock pot, I would be trying to find something around 50-60% cut and the pot adjustment would have more sensitivity in that range. I might actually be trying to adjust things very close to the log to linear transition point in the pot which could make it very difficult to fine tune.
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