Pedal not cleaning up when turning down the guitar volume pot...

[isildur100]

Hi,

I am working on a tube pedal based on the Valvecaster. I am very satisfied with the sound but there is only one problem. I like turning down the guitar volume pot when I'm playing to obtain a less distorted tone. But this pedal gets very very dark as soon as I turn it down. How can I prevent that? Is there something I can do (except leaving the guitar volume pot to maximum ) ?? What makes one pedal clean up properly and another, not at all?

[jacobyjd]

What kind of guitar are you using and do you have a treble compensation cap & resistor on your volume pot? Having a treble bleed circuit mod keeps your guitar from losing brightness as you roll off your volume.

The Valvecaster is very dark as it is. It's probably the combination of having no treble bleed and a dark circuit.

[isildur100]

Hi,

My guitar (American Standard strat, single coils) has a treble bleed and with other pedals it cleans up very well. It happens only with my valvecaster type pedal. In fact, I have modded the valvecaster to obtain a sound that is a lot less darker than the original. But it stays bright only with the guitar volume at maximum. I was wondering if something simple could be done with the pedal circuit to prevent that.

[earthtonesaudio]

Make the gain depend on the input resistance.

If you check out a typical inverting op-amp gain stage (or CMOS inverter, or Fuzz Face, or...), the overall gain is determined at least in part by the ratio of feedback resistor to the input series resistor.  If you make the input resistor small (or zero), then adjusting the guitar volume pot will make a big difference in the gain of the circuit.  This is because the guitar volume pot is adding series resistance to the circuit, which is equivalent to increasing the value of the input resistor and reducing the gain.

Simultaneously, rolling back the guitar's volume increases the input impedance in such circuits so that turning the guitar volume down (if the values are right) also results in a brighter sound.  In my opinion, this is the key to getting that jangly bright clean tone when you turn the guitar down before a Fuzz Face.


Adding the treble bleed cap to your guitar's volume control will preserve more treble when you turn down, but won't do anything to adjust the following circuit's gain.

[jacobyjd]

Good suggestion--my first inclination was that without treble bleed on the guitar volume pot, there weren't enough high-freqs to attain much distortion--everything running through the LPF-ing that usually comes with any distortion circuit.

[isildur100]

Ok, I see what you mean but I don't know how I can apply that to the circuit I have. I know I should post a schem but I don't have one right now. The pedal is on the breadboard.

Basically, I am using a bs170 mosfet in front of a valvecaster at 12V. The plate voltage resistors are tweaked by ear. The gain is controlled similar to a pepper shredder, that is, a pot between the 2 triodes. The mosfet stage makes the first triode clip and the signal is clipped even more going in the second triode. This gives a pretty heavy overdrive and provides Angus Young tones.

Do you think there is something I could do at the mosfet stage to help the pedal cleanup?

[earthtonesaudio]

Probably.  How is the MOSFET set up?

[Ripthorn]

Yeah, there is something you can do at the MOSFET stage to clean it up, bypass it.  What happens is you have three gain stages now, instead of two.  So even if you aren't slamming the first section (rolling down guitar volume) your first stage boosts the signal up to what it might be had you neither rolled of the guitar volume nor had the MOSFET stage.  That puts you kind of at square one.  Now, if your MOSFET is just a unity gain buffer, then ignore me, but if it has any gain to it, that is your main problem, as the Valvecaster does clean up nicely when rolling off guitar volume on its own. 

So my suggestion is to maybe make the MOSFET stage footswitchable or something, since that is obviously the problem (3 gain stages distorts more than 2 and 4 more than 3).

[isildur100]

Thanks Ripthorn,

I feared it might be that 

The mosfet stage is indeed a gain stage and is what makes my modded valvecaster distort and sound like that, which I really like. If I remove the mosfet stage, I don't have enough gain for my needs. I guess I can't have the best of both worlds...

However, thinking about it, I have a thunderchief by ROG which uses 4 jfet stages and still cleans up very well...

[Ripthorn]

Your thunderchief could well clean up depending on how much gain each stage supplies.  If the first stage provides more gain than the following ones, getting rid of that gain stage does more than rolling down the guitar volume.  The devil is in the details, they say.  Again, if you have the option, I would just make the Mosfet stage a separate footswitch.

[isildur100]

I tried adding a buffer before the mosfet stage using a tl071 and it really made a big difference, it now cleans up a lot better.  I am no longer losing all the highs when rolling down the guitar volume. I guess the mosfet stage was very sensitive to input impedance.

Thanks for all your insights guys!

cheers