This Week on the Breadboard: The Germanium Filter

[Chuck D. Bones]

In the right circuit, they can impart a smoother tone, fewer high-order harmonics.

And then there's the MOJO factor.

 

[jwin615]

2025 Component Abuse Challenge: Reverse Biasing An NPN BJT

For the Component Abuse Challenge our hacker [Tim Williams] observes that N-P-N reads the same way forwards and backwards, so… what happens if we reverse bias one? (Note: this remark about N-…

hackaday.com

 

[Chuck D. Bones]

I have seen Si transistors operated in reverse beta mode in a precision synchronous rectifier where the extremely low Vce,sat is highly beneficial.

 

[giovanni]

For my own understanding, the Ge transistors work with reverse beta which is completely undocumented? Now that is punk.

 

[HamishR]

Are you sure about the wiring of the Filter pot in your schematic? I had to swap pins 1 & 2 to get the pedal to behave like the clips and then the audio taper makes more sense.

I used a pair of MP38s and the first two I used were best. Funny how things work out. I think putting the volume knob outside is a no-brainer - what a pain if it's inside. And the sound gets so thin with filter above noon! At zero (flipped) it sounds lovely and sludgey.

 

[Chuck D. Bones]

I am sure the schematic is correct. My breadboard works just like the youtube demo. It does indeed get trebley when the filter knob is above noon. You're free to make mods, but please don't pass them off as corrections. BTW, do you have C5 installed?

What voltages do you get on Q2-E & Q3-E?

 

[HamishR]

Whoops - my bad! I wired it backwards originally in my build - sorry! As soon as I flipped the filter it works properly - I don't know why I got confused! Other than the usual reasons...

Q2-E: 4.73V, Q3-E: 4.95V. It sounds good, but I only really like the first quarter of the filter's rotation. No reflection on you or DBA, just that I don't really like those sounds particularly.

 

[Chuck D. Bones]

Look for a different tranny for Q3 to produces a lower voltage on E. Shoot for 2.5V to 3V.

The top end of the FILTER knob is only useful with low GAIN settings IMHO.

Also, try my mod of installing 1nF from Q3-E to GND. It mellows out the top end slightly.

 

[comradehoser]

I breadboarded the circuit with some ge 2n169, about 60hfe, but it sucked. Unusably gated and spitty so I can definitely attest to the importance of the transistors. I know I could have futzed with the resistors to get the gain up, but I decided to do the easy things and sub in some higher gain transistors, which made the circuit come alive properly.

NOS metal can silicon, 146 hfe q2 and 188 hfe Q3. Q2 hit 4.5v on the nose, and Q3 sounded the best out of all that I tried.

Is there something against combining germanium and silicon? I'm guessing Q2 is the crucial one for germanium love.

I can't say that it sounds a lot like the demos, although it is very responsive to guitar inputs. Gain up, filter down sounds like it is getting into octavey fz-2 territory. Gain up does impart some middy ugliness to the distortion to my ears. I'm guessing octavey and gross distortion are the aforementioned harmonics that germanium transistors are more demure about. Have some mp38s on order to compare.

I wonder if Dba auditions/adjusts each circuit to account for the inevitable differences in each build and transistors given that the transistors seem pretty important to the nature of the pedal sound? Seems kinda less likely with a bunch of SMD on tap.

 

[Chuck D. Bones]

Ooooooh, clear protoboard

Did you measure the emitter voltage on Q3?

 

[comradehoser]

Clear is not common? Asking out of ignorance. I think I've been abusing it by shoving my potentiometer legs in there.

I did not measure Q3, just auditioned by ear, but for you, dearest Charles David Bones, I will.