Fab Tone Mod Issues

[Dulouz]

Hey guys! This is my first post here. This seems like a great, friendly community with a lot of knowledge. I'm looking forward to getting to know everyone, learn some things about pedal building and hopefully contribute to the discussion.

So I'll start with a question about the Fab Tone that inspired the recent post on the blog. Here's what I did.

R25: 22k to 10k
R27: 100k to 47k
C5: Removed
R15: 10k to 1k
C19: 0.33 µf to 1 µf
D4: Changed to 1N4148 + LED (5mm UV)  in series
R9: 2k to 4.7k
Changed the indicator LED (5mm UV)

I'm having a couple issues that I'm hoping you can help me with. First, is the treble. The pedal is still very fizzy sounding with the treble anywhere except 0.  Any ideas how I can make the treble more manageable and useable?

Second, is the volume. The volume is very touchy. Unity gain seems to be just under 9 o'clock. But if I move it a couple of millimeters up or down, the volume drops or jumps dramatically. Any way to make it less sensitive?

Thanks in advance!

[IvIark]

C11 is a filter at the opamp gain stage.  As you increase the value the bandwidth in the higher frequency range is reduced and so that may be a good way to cut some high end before the signal even reaches the tone section.  Common values in there are 100p, 220p, and I've even seen 1n caps used in the past.

[Dulouz]

Awesome! Thanks so much! I'll give this a shot.

Any idea what might help the volume control?

[IvIark]

The level pot is a classic voltage divider with a portion of the signal being dumped to ground to attenuate.  It is a linear pot in the schematic and so would probably benefit from being 100K log instead.  But the way you describe it sounds like the pot may be faulty anyway, the controls shouldn't be that tight and sporadic.  So I'd say swap it out for a log pot.

[Dulouz]

Ah yes. A change to a log pot. That was my first thought. Thanks for the confirmation.

These look like pc mount pots. I'm not super familiar with these but I'll give it a shot.

Thanks for all the help!