King of Tone clone - tone control help needed

[java.todd]

I need help diagnosing an issue with a King of Tone clone build. The tone control is not working properly. The cutoff frequency is too high, and I have reached the end of my ability to troubleshoot. Referencing the following schematic:

http://3.bp.blogspot.com/-fmqfL3ltHZ4/VjeHm1jj2gI/AAAAAAAAAf4/QKZa99CT-pI/s1600/schem.gif

I'm using veroboard with the following layout:

http://2.bp.blogspot.com/--5c1YY0-u...s1600/Analogman+King+of+Tone+with+trimmer.png

The only variation in my version is that the trimpot has been replaced by a switch that takes C7 in and out of the circuit. C6 is in place, and is getting the bias voltage (4.5v). There is continuity between Tone 1 and C6 and between C6 and the bias voltage node. I've replaced C6 and the one I removed is measuring 10nf.

I don't have an oscilloscope, so I'm wondering if anyone has some suggestions about what to try next. Any advice would be extremely helpful. This is my third build of this pedal, but the first that's giving me this problem. The only change between them is box film caps in this one and ceramics in the previous one.

Thanks!

[PRR]

Welcome.

The cap isn't that value, the resistor isn't that value, or they are not connected as shown.

Your second link does not work (dots in the middle).

[dschwartz]

I haven't build this.. but by looking the schem, this should have a low output level after those filters..

[GGBB]

This is my third build of this pedal, but the first that's giving me this problem. The only change between them is box film caps in this one and ceramics in the previous one.

Which means it's either a build error or the cap substitutes are not the same value. Any other differences? Did you do the switch instead of the trimmer in all three?

Just re-check the entire tone section - carefully.

There is continuity between Tone 1 and C6

Be more specific - what position is the tone pot in? There should only be "continuity" there (very little resistance) when the tone pot is all the way down. Did you measure resistance between C6 and tone pot 2 & 3 (2 at a couple of positions)? This is important for the tone control to work properly.

Also does the presence switch have any effect at all? You probably know this, but for the record, with that switch open (presence "on"), the tone control won't be able to roll off all of the high end - it works more like an upper-mid cut.

[java.todd]

The continuity check was between the Tone 1 lug and the leg of the cap, just to make sure that I had wired it correctly. The tone control does work, it just has a higher cutoff freq than the other two that I built. I think I'm coming to the conclusion that it's right. I need to disassemble one of the other ones I built to see if I'd put in the wrong pot value or something else, but I can't find any mechanical or electrical faults. The values are all correct.

The presence switch does work - when the second 10n cap is out of the circuit, the transients are more prominent in the tone. For darker guitars, this sounds really good.

I'm really trying to get my head around the tone control - this looks like a second-order low pass filter, but the way the tone pot is placed into the circuit, it looks like it functions as a voltage divider in the middle of the filter. Am I right about that?

Thanks to all for the advice and help.

[GGBB]

I'm really trying to get my head around the tone control - this looks like a second-order low pass filter, but the way the tone pot is placed into the circuit, it looks like it functions as a voltage divider in the middle of the filter. Am I right about that?

I think you are generally right. The way I see it  - in addition to the voltage divider, the first stage is a shelving low-pass where the tone pot wiper position determines the shelf level. The cutoff is constant formed by (R9+25k)_C6. If tone is all the way up, the shelf is almost all the way up. All the way down you have no shelf - just a standard low-pass RC. This is interesting in that instead of the usual shelving low pass implementation (where the shelf resistance is variable and therefore not only sets the shelf level but lowers the cutoff as the shelf level goes up), the cutoff never changes, only the shelf level.

The second stage (if not switched out) is a standard low-pass, with the RC cutoff formed by C7 with R9 plus R7 plus the resistance of the tone pot from 3 to wiper. A presence pot would turn it into a shelving low-pass and serve to raise the shelf level and lower the cutoff.

Without the presence control or switch, the tone pot works so that as the tone is turned down, the first stage lowers its shelf level while the second stage lowers its cutoff. With the tone turned down below ~25%, the second stage's cutoff becomes lower than that of the first stage, so it actually starts cutting frequencies below those cut by the first stage. When you add a presence pot to the mix, it lowers the cutoff of the second stage even further but adds and raises the shelf level, so there is quite a bit of interaction going on between the two controls. (Note that in the linked schematic the presence pot is connected backwards.)

I have built a couple of these with an external presence pot - at some settings it's hard to tell which one is presence and which one is tone, and sometimes it seems like you can get a bit of a scooped mid effect. Takes a bit of getting used to, but really useful for dialing in just the right amount of hair.

[java.todd]

GGBB - That's incredibly helpful information. Thank you!