Tonebender MKII with Q2 and Q3 Trimpots - Breadboard/Veroboard Layout

[audiobalm]

Hi All,

I've put together a Tonebender MKII on breadboard and replaced the 100K and 8.2K Resistors with trimpots to get the best sounding collector voltages. (See Image)

But I can't quite find a Veroboard layout online that would be suitable to build this. Does anyone know of a veroboard layout for the MKII that works in this fashion? Even with only a trimpot on Q3 Collector only would be great.

For those Interested, attached is a pic of my breadboard. Interesting to see it in this way.

Thanks!

David

[Big Monk]

You really just want the trimmer on Q2.

Changing the collector voltage of Q2 does most of the required adjusting for Q3 as well.

[audiobalm]

Oy yeah? That's cool. So usual 8.2K Resistor on Q3 and a 100K Trimpot on Q2?

What voltage am I looking for on Q2?

Thanks
DP

[Big Monk]

Oy yeah? That's cool. So usual 8.2K Resistor on Q3 and a 100K Trimpot on Q2?

What voltage am I looking for on Q2?

Thanks
DP

That should work.

What you really want to do is concentrate on Q3c voltage by tweaking Q2c's resistance, i.e. tweak the trim on Q2 and measure Q3. I always go back to Electric Warrior's values as he has been immensely helpful in schooling me (and many others) on what the vintage units were running at:

https://www.diystompboxes.com/smfforum/index.php?topic=117101.msg1086660#msg1086660

Depending on the transistors, you'll maybe want to tune by ear. If you use some of the vintage units, you can't go wrong with his values. Somewhere around 8 volts or so on Q3c and < 0.2 v on Q2c, ballpark of course.

I have some old General Electric NPN units with low leakage that seem to be louder, have more sustain and not bloom at around 7.2-7.69 volts on Q3c, so it really depends on devices. Definitely not 4.5 volts as seen in other places.

[audiobalm]

Nice, Thanks for the link.

I've got some Texas Instrument CV7003s that Analogman once used in his Surfaces, and they're pretty tasty:

https://www.analogman.com/fuzzface.htm

I'll try that with setting the voltage on Q3 by adjusting the resistance on Q2

[Electric Warrior]

Instead of a trimpot on Q3, consider using one for Q1b - if you really need trim pots. You might as well figure out which values will work with the breadboard and then commit to resistors that will bias the circuit right.

For leaky transistors such as OC75s, Sola Sound usually used a 10k on Q1b and 47k on Q2c, lower leakage OC81Ds used 100k in both spots. Once the voltages are in the right ballpark, it's all about transistor selection.

CV7003s may not be a good choice for this circuit, as some of these leak extremely little. As the first stage relies on leakage to bias, it may be hard to get it right with these.