EHX Germanium OD analysis

[patrick398]

Hello all,
Could do with some help on this one. Would be interested to hear your insights on the workings of this circuit.  There's a few elements i don't recognise which is why it has piqued my interest. I've removed the power section, bypass switch, and indicator led parts just to simplify it.
Schematic:


Edited for clarity:


I get that the base of Q1 is biased to be always on by R4 and R5 but the 'bias' and 'gain' controls have me slightly confused. I'm used to seeing a bias control adjusting voltage on transistors collector but i'm not really sure how the two pots interact here. C5 removing DC from the wiper of the 'gain' control has me perplexed. Is that so the two pots don't both affect the bias of Q1?

With the 'volts' control, i can see that it's a potential divider biasing the base of Q2 and presumably adjusting how much current is allowed to flow from Q1's collector thus affecting gain but why not just use a pot to ground like we're used to seeing?

I've got this circuit on the breadboard at the moment with some low gain russian MP20s. It's a pretty mild OD currently and i'm not really a fan of the way these controls are set up so i'd like to find another way of adjusting gain. I tried using shunt feedback diodes but that didn't yield any change that i could tell. I think i'll also add a tone control at some point but first i need to get a better handle on exactly how this circuit is working.

Thanks 

[MaxPower]

Partial answer: the gain pot works like the fuzz face gain pot. Cap on the wiper means DC resistance/gain/bias is unaffected by the wiper. Only the AC signal/gain is affected.

[patrick398]

Thanks for the reply, that makes sense. When i first saw it i thought i recognised the set up to be kind of 'push pull' amplifier but doesn't that normally use a mix of npn and pnp trannies? Also for negative ground aren't these transistors in backwards so to speak? I know vaguely that this is a reverse beta setup and results in less gain? Would it be possible to use positive ground and flip the transistors

[Eb7+9]

The circuit is based around Q1 (pnp) so has to be biased in that direction ... the old way (battery only) would have +9v replaced by gnd and gnd replaced by -9v

Q1 is the only voltage gain device, so not too much to be expected ... Q2 and Q3 are voltage followers, Q2 acting as a variable DC source and replacing the role of a variable top rail in typical SE gain stage and hence providing variable clipping point ... Q3 as an AC follower buffer

some ideas other than replacing Q1 by another device type:

disconnect C5 from Bias pot wiper and try going to top rail or gnd instead, maybe up its value ... 1u, ... 10u ... making controls independent ....

For a bit more sensitivity try replacing R3, R4 and R5 by 220k each ... R6 sets the amount of gain/output

[j_flanders]

I have had this pedal for a long time but never liked it much.
A couple of days ago I tried it again and, probably contrary to the the other times I tried it, this time I used a guitar with single coils and plugged straight in going to an amp on the verge of breaking up.
It's awesome, and cleans up exceptionally well with the guitar's volume knob. Without a buffer(ered pedal) in front it's also less bright.

So I decided to check what circuit it was and how it actually works.
The circuit you posted is correct but I've redrawn the circuit so it's a little easier to understand (for me at least).

As said in the previous replies, Q3 is just an output buffer and Q2 is there for the variable voltage.

If you leave these out, you end up with a Rangemaster style circuit but with different component values:

There are some important differences though.
The EHX Germanium OD
- has a much larger input cap and thus is no longer a treble booster but full range.
- it's set up for negative ground which is why you see 9V going to the emitters instead of to the collectors.
- it has a gain control the same way a fuzz face has
- it has no volume control, it's always 100% output.

Here's the redrawn circuit:


EHX added a voltage control, but as far as I understand you could actually call this a ground control.
The circuit is setup for negative ground with 9V going to the emitter and normally you'd expect the collector thruogh a resistor connected to 0V ground but they replaced 0V ground with a variable 'ground voltage'. At 'max voltage' ground is around 0,5V and at minimum voltage ground is around 7V.

Another thing they added is a bias control.
Bias is set by 4 resistors: 2 voltage divider resistors for base, 1 for the emitter and 1 for the collector.
Changing any of the 4 will change the bias.
They made the emitter resistor variable.
Compared to a FuzzFace fuzz pot this is like you could change the pot value (from 1k to 100k) on the fly, without changing the gain (cap from wiper to ground or 9V in this case)

One thing to note: changing the ground voltage(and thus total voltage) changes the bias point as well.
Here are two examples of voltage settings in a simulator. It doesn't have Ge-transistrs, so I had to use Si, but for the general idea this will work.
Check the voltages at the testpoints (TP)
Maximum voltage/lowest ground:


Medium voltage / middle ground:


I'm interested to hear what the experts think and hope they correct me where I'm wrong.

Here's the circuit in an online simulator to play with:
https://tinyurl.com/vhd9chb

Here's a trace of the pcb (red lines are the traces on the front, white traces are the ones on the back of the pcb)
Here's a link to the super large version:
https://i.postimg.cc/Q8xdR4CS/EHX-GERMANIUM-OD-TRACED.jpg