Why does a Tone Bender Mk1 sounds good? 2cents analysis

[Steben]

Hi guys.
We all know tone benders. We know they sound good. We know some look like a fuzz face with a boost stage in front. Some are completely different.
A Mk1 is for example a maestro on 9 volts right?
But if those 3 trannies are straightly blown, why does the pedal sound that good?

Some will say: it's the germanium man. It is like the steak but with the salt and pepper.
Well... there are silicon versions of all Tone Benders which seems to be doing rather the same. Just as silicon fuzz faces are just as edible.

I found out there might be a simple explanation which perhaps is out there already but I've never seen it in explanatory sites or analysis.
Just remember the old school thesis on drive circuits: filtering, filtering and filtering.

Let's have a look at the Mk1 circuit:

Two of the stages are non-biased. In theory the base is shut down. It's well known these germanium designs rely on leakage.
High leakage transistors will result in higher bias, giving less gated sound, low leakage will be sputtery especially with guitar volume roll off. Some pedals have clean up, others are sputtery.

Now let's look at the characteristics of the devices.
I know there is some tolerance here, but let's say on average these numbers are somewhat correct:

Internal emitter resistance is somewhat different than silicon (normally even lower) but it should come down to +/- 26 ohms / 1000*[Ie].
With these hFe's one can somewhat address the equivalent base impedances. Now we can calculate the stage input impedances.
Note how tricky and specific these numbers are with each different transistor.

Now we have the impedances, we can point to the high pass filter characteristics of each stage (cap followed by impedance to ground).


2 elements really stand out:

• input stage has relatively HIGH input impedance compared to fuzz face for example.
• third stage that clips first has medium input impedance which forms an important high pass with the 100n cap. A -3db frequency at 724Hz puts it in tube screamer drive stage territory!

The second stage has very low input impedance, but given the first buffer stage this doesn't load that much. The coupling cap has such a high value, no audible filtering is happening.

Conclusion: the Mk1 tone bender, albeit with very high influence of components, has pre gain bass cut in the third gain stage. Which is not what classicaly is thought about fuzzes.
Changing the 100n cap to 1µ for example will have a HUGE effect on the character resulting in much more "farting" low frequency distortion.

Makes me think of my FAT Germs fuzz where transistors are simply clipped with rangemaster like pre filtering:
https://www.diystompboxes.com/smfforum/index.php?topic=119424.0

Next is the Mk2, my attempt can be seen here:  https://www.diystompboxes.com/smfforum/index.php?topic=125397.0

[John Lyons]

Interesting. Thanks.

[cdwillis]

Thanks for this. I've just started learning about Tonebenders and Fuzz Faces. The differences between the different benders are so interesting to me. I didn't realize this one had basically a buffer (?) in front of a fuzz face type transistors section.

[Steben]

Thanks for this. I've just started learning about Tonebenders and Fuzz Faces. The differences between the different benders are so interesting to me. I didn't realize this one had basically a buffer (?) in front of a fuzz face type transistors section.

Second part is not a fuzz face typology 😉