PPCB Soul Vendor vs. MBP Patsyface

[Locrian99]

I have a couple of Soul Vendor boards and a couple pasty face boards. Seems like the bias trim pot opens up a wider option of Transistors that are usable for Q3 to my ear at least, having some leakage in Q3 seems not as crucial.

I was thinking I could modify my build on the soul vendor board to make them similar fairly easily, but I just was wanting to make sure I'm thinking correctly here.

My plan is to breadboard this first so I can measure where I want the trim pot resistance to be and then just use a fixed value resistor for it.

So I would on the soul vendor board. Omit r9, on the solder side of the board attach my "trim pot" resistor value to the base and collector pads of q3.

Less concerned about the differences in the output section as my main goal is just to be able to use more of my pnp germs I have that are very low leakage.

Or would just playing around with the value of r9 affect that voltage divider for biasing the collector q3 and accomplish the same thing?

Thanks

 

[Chuck D. Bones]

The PF's bias control allows one to compensate for insufficient leakage in Q3. If Q3 is leaky enough, then you don't need the bias trim. That's one of the reasons I prefer to build leakage-biased circuits with NPN transistors. It's easier for me to find leaky NPNs. The other benefit is no charge pump req'd.

 

[Locrian99]

The PF's bias control allows one to compensate for insufficient leakage in Q3. If Q3 is leaky enough, then you don't need the bias trim. That's one of the reasons I prefer to build leakage-biased circuits with NPN transistors. It's easier for me to find leaky NPNs. The other benefit is no charge pump req'd.

Yea all my Soviet transistors don’t leak!

 

[PedalBuilder]

Yea all my Soviet transistors don’t leak!

Have you tried the GT115x and MP21x series devices? Both of those families of Soviet PNP transistors have enough leakage to work well in a leakage-biased circuits like the Tone Bender Mk. 3 or Buzzaround. As an example, I used a GT115D with ~300 µA of leakage as Q3 in a recent Buzzaround build:

But as you’ve probably figured out, that’s more the exception to the rule for Soviet transistors families, which tend to have less leakage (Q1 and Q2 were GT309’s with no measurable leakage)

 

[Chuck D. Bones]

The П28 transistors I have are all low leakage. The MП38A's I have are all fairly leaky.

 

[Locrian99]

I haven’t, the pasty face works well with the low leakage though