Switched tone&volume back-off?

[rumblefuzz]

I really like the sound of my Tokai Breezysound tele through my new Fender Vibrolux reverb, BUT:

When I roll back both volume and tone pots about 10%, the sound gets this really nice mellow jazz feel to it, without even losing that much volume. I'd like to have this sound ready at my fingertips with the flick of a switch, but I'm not much of a technician. I can solder OK and know some things, but I need help. here's what I thought:

I dial in the sound I'm after, and measure the resistance of both pots. Then, assuming a pot is just a variable resistor, I buy two resisors as close to those values as I can and connect them excactly the same as the pots are wired with a similar capacitor for the tone part. Is this the correct way to go about this, or will I read incorrect values measuring the pots when they're still connected? Also: will a resistor replacing a pot sound the same? Is it possible to find a resisor that's an excact match to the pot in a certain position?

What would the scematic look like? I'm trying to figure out myself but I could sure use your help...

I came up with this:



Does this make any sense? Will the switch cause a pop in this way?

[ashcatlt]

Welcome.

Remember what they say about assume? Well, the Volume pot in most guitars is not "just a variable resistor". It's actually a rheostat. As you turn down the volume pot you are both increasing the series resistance to the hot output AND reducing the parallel resistance to ground. That said, I'm not sure how much that series resistance has to do with what you're hearing.

Most of the action that you are noticing here comes from the fact that you're reducing overall resistance to ground thereby adjusting the cutoff of the hi-pass filter (mostly caused by cable capacitance) downward. The tone cap doesn't really play much of a role until you get down toward the very end.

I think you could probably get the same effect just by switching in a single resistor whose value was chosen to give the same overall resistance as you've got with the pots turned down. This would require an SPST switch and resistor. If you wanted to include the series resistance, you'd need another pole. The proper value could be determined, and tested, without even removing the pickguard.

Either way, the actual position of the pots will affect the performance of this switch. It's up to you whether that would be a significant problem. Wouldn't take much more to be able to overide the master volume/tone array with a preset resistor combination.

At one point, I was looking into replacing my 5-way switch with a stepped attenuator. Can't be done (well) with a standard strat switch, but a SuperSwitch, or rotary could.

[ChrisK]

I dial in the sound I'm after, and measure the resistance of both pots.

How?

Then, assuming a pot is just a variable resistor,

The tone pot is used in a two-terminal rheostat configuration. It is just a variable resistor.

The volume pot is used in a three-terminal potentiometric (ratiometric) configuration. It is just two variable resistors, one from the input terminal to the wiper and one from the wiper to the grounded terminal.

(if) I buy two resistors as close to those values as I can and connect them exactly the same as the pots are wired with a similar capacitor for the tone part....

I would buy three fixed resistors and share the same tone cap since caps have a +/- 20% tolerance. I would use a DPDT switch. A clever sort might use a DPDT push pull pot.

...or will I read incorrect values measuring the pots when they're still connected?

Yep, you betcha for the volume pot.

Now, how would one do this? Since the guitar is single coil based, the pickups likely have a DC resistance of 5 to 7 KOhms. The volume pot is likely 250 KOhms nominally.

If it's 5K and 250K, with the volume control at "10" (full-on) and only one pickup selected, the resistance measured at the output jack, from tip to sleeve, will be 4.9K.

Let's re-assume that the pickup is 5K and the pot has a +/- 20% tolerance. At 200K we would measure 4.88K and at 300K we would measure 4.92K.

Hmmm, these are all around 2% less than the pickup alone would measure for a pickup of 5K, and 4% for a pickup of 10K.

That being said, one can safely assume that the maximum effect of the volume pot in parallel with a pickup will be about 2 - 4% maximum'ish.

procedure

Remove the Tele control plate.

Set the volume control at "10" (full-on) and with only one pickup selected, measure the resistance at the output jack. Post it (R10). This is 2 - 4% less than the resistance of the pickup alone.

Set the volume control to the exact spot that you like it and with only one pickup selected, measure the resistance at the output jack. Post it (Rbot).

Also measure the resistance from the wiper to the input terminal on the volume pot. Post it (Rtop).

Measure the resistance of the tone pot when it is set to the exact spot that you like it. Post it (Rtone). This is the value for the fixed tone resistor as there is no DC coupling in a high-cut tone control that will affect the reading.

/procedure

After you post these I can do a few quick calculations.

R10 = Rpu || (RVtop + RVbot)

Rbot = RVbot || (RVtop + Rpu)

Rtop = RVtop || (RVbot + Rpu)

Also: will a resistor replacing a pot sound the same?

No, only to humans. Audiophiles will assure you that only oxygen-free hydrogenated vegetable oil-based resistors costing at least $60 each can possibly sound the same.

Is it possible to find a resistor that's an exact match to the pot in a certain position?

No, you'll be forced to settle for being within 1% of the measured value (of course, your meter likely has a 1 - 2% accuracy error).


Since you likely might want the volume and tone controls to have no effect when the "special mode" is selected, we would want to switch them out of circuit, and the new components in circuit, and sharing the same tone control cap might help eliminate any popping.

If this is true, let us know and I'll post a diagram.

[rumblefuzz]

Thanks for the replies guys!

See, it's indeed a lot more difficult than I thought, but I figure I can do it.

Here are the values I measured:

R10: 5.72
Rbot: 128.7
Rtop: 130.2
Ttone: 85.4

I forgot to tell you I changed the treble bleed capacitor: instead of having it across the volume pot, I soldered it directly across the pickups' output to filter out the tele's upper high frequencies. Works like a charm this way, but does it complicate matters? I don't know the value of this capacitor, but the letters "x 102" (or L02?) are printed on it.

To be sure, the current wiring looks like this:


I'm eager to see the schematic you suggest, it'll be a good learning experience for me. When I look at the design I did, I realized that it probably isn't going to do much, but I couldn't figure out how to remove the pots from the circuit easily...

Audiophiles will assure you that only oxygen-free hydrogenated vegetable oil-based resistors costing at least $60 each can possibly sound the same.

That's ok, I'm a vegetarian...

A clever sort might use a DPDT push pull pot.

You're right. Would something like this do (I think it's quite expensive...)?
www.thomann.de/nl/goeldo_e250p.htm

[ChrisK]

Here are the values I measured:

R10: 5.72
Rbot: 128.7
Rtop: 130.2
Ttone: 85.4

Are these all in K Ohm?

If so, since Rtop and Rbot are so similar, this would indicate that the volume pot is 500K. It also indicates that it is an audio taper if it's turned down to aboot "7" or "8". This is 20 - 30% of rotation (and not "about 10%" ).

I would use 240K fixed resistors for Rtop and Rbot.


The tone pot reading indicates that, if this is 250K, it's turned down to "7" if it's an audio taper, and down to "3.5" if it's linear.

If this is 500K, it's turned down to "5" if it's an audio taper, and down to "1.5" if it's linear.

Either way, an 82K 5% 1/8 Watt fixed resistor can be used.

Works like a charm this way, but does it complicate matters?

No.

I don't know the value of this capacitor, but the letters "x 102" (or L02?) are printed on it.

102 would be 10 plus two zeros or 1,000 pF or 1 nF or 0.001 uF.


This is a full-scale solution that completely replaces the pots with fixed resistors. Note that Rbot may be eliminated and Rtop replaced with a fix resistor of some value (perhaps 62K). It may give a similar sound.

You may also want to move the 0.001uF cap to the switch on the tone pot so it is in in both modes and to eliminate any chance of popping from switching it.

It's a Telecopy; the beauty is that it's easy to mess with the wiring without fully undressing it.

[rumblefuzz]

Well thanks Chris!

This looks easy enough, I can do this! Soon I'll have my own "smooth-jazz-tone-at-the-pull-of-a-knob-a-caster!

Are these all in K Ohm?

Yeah, I forgot to mention. Sorry.

This is 20 - 30% of rotation (and not "about 10%"

Alright alright . This is actually partially due to the cap that I put in. Before that the "soft" sound occurred sooner. Now the guitar is a little darker sounding (it's lost the upper high frequencies that made it sound quite unnatural through 10" speakers) but the pots now need a touch more rotation to have that first noticable 'softening' effect on the sound.

For the tone pot, since I have to order a new one, I have to figure out the value. It only says "DIM Ω" or "UIM Ω" on both pots and when I measure it I get 700K...
I redid the measuring on this pot and figured I had it turned a little too far down. On the new setting I read 375 K and 334 K at the other side of it. This was the difference between roughly changing it from '4' (85K) to '3' (375K). I hope this makes sense... I guess this means I should get a 500K audio taper pot to replace it right? I'm starting to see the margins here. I think I'll get a bunch of resistors between 82K and 390K and expiriment a little.

...These measurements are all done using a €5 multimeter bytheway, so I'm not too sure about the 1-2% accuracy...

Either way, an (...)K 5% 1/8 Watt fixed resistor can be used.

I could only find 0,25 watt ones. Should I get carbon or metal fim?

How does one 62K resistor make up for two 240K ones?

[ChrisK]

For the tone pot, since I have to order a new one, I have to figure out the value. It only says "DIM Ù" or "UIM Ù" on both pots and when I measure it I get 700K...

Hmmm, well, it's either a 1 Meg Ohm pot that's 30% low or a 500K pot that's 40% high....

You're likely to find a 500K push pull pot, I don't know about a 1 Meg one.

It says this on both pots, but I presume that the 700K only applies to the tone pot. The values that you posted of

R10: 5.72
Rbot: 128.7
Rtop: 130.2
Ttone: 85.4

...seem to indicate that either half of the volume pot in parallel with the other half in series with the pickup coil read about 130K in both cases. This would indicate that each half of the pot is 260K'ish and the total resistance of the pot is 520K'ish. This would not be a 1 Meg pot.

I redid the measuring on this pot and figured I had it turned a little too far down. On the new setting I read 375 K and 334 K at the other side of it. This was the difference between roughly changing it from '4' (85K) to '3' (375K). I hope this makes sense...

This makes no sense to me. How can a lower setting have a higher resistance? Are you perchance left-handed?


I guess this means I should get a 500K audio taper pot to replace it right? I'm starting to see the margins here. I think I'll get a bunch of resistors between 82K and 390K and experiment a little.

I could only find 0,25 watt ones. Should I get carbon or metal film?

A 1/4 watt is fine. The signals are so small in passive guitar electronics that it would be nearly impossible to find a resistor with a power rating low enough for it to be an issue.

How does one 62K resistor make up for two 240K ones?

It doesn't, it was a guess. One might be able to replace the two resistors with just one series one, but it would have to be determined by listening. I'd just use two.

[rumblefuzz]

This makes no sense to me. How can a lower setting have a higher resistance? Are you perchance left-handed?

I don't quite regard myself as the autority on this matter, but to me it does seem to make sense... The lower the resistance to ground, the more signal will "flow off" in that direction, right? So when I turn the 500K pot fully clockwise, the resistance will roughly be 500K, and the more I ....

Ah, ehm... I see: I had the numbering wrong. I thought of position 10 as the higgest possible influence of the tone knob on my signal. Works the other way round...

However I think you're deductive thinking was fabulous ;D : the beautiful simplicity of your conclusion made me wish I was. I'm not.