Quick Distortion+ question

[erick4x4]

I have built a dist+ clone. I used 1n34 diodes so it is much quieter than it was with 1n914. I also increased the cap parallel to the diodes to help tame the high end.

I am wanting to get more output, I increased the Volume to a 100k, though it made very subtle difference from my 50k, and added some hiss.

So I have seen some guy out there on the internet change R10 from 10K to 100 ohm.

How is this resistor affecting final output. If I can lower it this much, why not just jumper it? Can someone help me understand its role, and why the 100k helped a little?

Thanks!

[H S]

Changing the pot won't make it louder.  The reduced output is due to the reduced forward voltage on the 1N34 Ge diodes compared to the 1N914 Si diodes--they clip at lower voltages.  If you like the softer knee of the 1N34's, try replacing each 1N34 with (a 1N34 in series with a 1N914).

[Mark Hammer]

You'll forgive the anthropomorphisms but the output pot "thinks" the fixed resistor before it is part of the pot.  If that resistor is 10k and the output pot is 10k, then the pot "thinks" it is a 20k pot that can never be turned up more than halfway.  If the pot is increased in value then that assumed maximum pot setting goes higher and higher, such that with a 100k pot, the 10k+100k combo behaves as if it is a 110k pot turned down just a bit.

While changing the value of an output normally does not make a pedal louder, in this case it is the interaction of the series fixed resistor and the pot that produces a change in the maximum output level from this pedal.

Note that the diodes "think" purely in terms of the 10k resistor preceding them.  If that resistor is changed, it will have an effect on the behaviour of the diodes, but will also interact with the output pot.  Worth dickering around with IMHO, but keep in mind the interaction of the resistor and pot.

[erick4x4]

Thanks Mark.

So I will try this tonight.

So because of the interaction, I could also go back to my 50k and mess with R10, and get different results from the diodes, but still possibly increase volume right?

I think I will socket R10 in and see where it takes me.

[H S]

If that resistor is 10k and the output pot is 10k, then the pot "thinks" it is a 20k pot that can never be turned up more than halfway. 

Hmm, but the diodes connect between the series resistor and the pot, so regardless of the resistance values, the max output is set by the diode drop, no?

[Mark Hammer]

You're right in one sense, but the further attenuation of what you can extract from those diodes is impacted on by the R/pot combo.  So, if one switches to silicon 1N914 types, the max volume will be increased, but if one switches from 10k/10k-pot to 10k/50k-pot you will still here a volume increase.  The role of the diodes is such that you will not get screaming volume boosts like you would on some of Joe Davisson's high-gain monsters.  But you will be able to get an output greater than unity with less than max distortion/gain, which was sometimes very hard to do with a 10k pot.

As RG was fond of pointing out when Fuzz Face units were all the rage: when there aren't many parts involved, every one of them becomes more influential.

[H S]

The max p-p amplitude is essentially fixed by the diodes; the extra rms amplitude you get by switching from 10k/10k to 10k/50k is due to more squaring-off of the signal, i.e. more distortion, yes?  If you adjust the gain to keep the amount of distortion constant, isn't the max volume set by the diodes?

[Mark Hammer]

Yes.  But the diodes simply set the ceiling at +/-250mv or +/-550mv (typical Ge and Si values).  The signal can be lower than that ceiling if max gain is not dialed in, and evenb when max gain is dialed in it can still be attenuated below that ceiling by the pot. 

You can do the experiment yourself.  Install a diode lift switch so that the pedal simply has the 10k resistor and output put.  And select between one of two output pots: 10k vs 100k.  You WILL hear a noticeable difference in max volume.

Alternatively, use a pair of 10k pots, one wired up as a variable version of the 10k series resistor, and the other wired up as the stock volume pot.  With the diodes out of circuit, as the series resistance is decreased, the maximum output will increase, as if you were turning the volume pot up.

[erick4x4]

Just for people's searching sake I want to follow up.

I did try socketing everything but the pot, and found that I liked 1 silicon/1 germanium best tonewise, and that added a fair bit of volume putting 1 silicon back in.  With this combination the 100k pot makes a significant difference in increased volume over a 50k pot. But with the 2 germanium diodes, the 100k pot made less difference.

Also, past 50% the 100k pot does actually "seem" to add a touch more gain as volume increases.

[Mark Hammer]

I don't think it will add more "gain", but certainly the added amplitude of the output signal pushes the amp a little harder, likely extracting more harmonic content from the amp itself on top of whatever the diodes do.  Indeed, I would say this is the main reason to want to change the pot: it permits the user to play off the diode clipping and amp clipping in the manner they like the best.  If the output level is too low, then you're stuck with the diode clipping alone.  That's not exactly punishment from God, but it does limit the tonal palette.

And yes you are right about the Si vs Ge diode difference.  Since the volume pot can only take away from what gets past the diodes, having a higher ceiling (with either different diodes or more diodes) allows the higher value pot an opportunity to "strut its stuff", where hard clipping with Ge diodes at a low ceiling will downplay whatever the pot change might contribute.