Good circuit for testing out NPN / PNP transistors? (Newbie)

[I_Am_Jon]

I'm a Bass player. Have been playing around with making a few NPN based Fuzz Face & Muff circuits.

Have just acquired a bag of old Germanium transistors and looking for a simple boost / distortion circuit to use for testing out their sound.

I have a mix of AC125, AC128, OC45, AC188 / AC187 pair, a few other unmarked PNP's and some "Top Hat" style NPN's (Could be 2N167's) to play with. Have already sorted through the bag with a cheap TC1 tester.

Thinking is to solder up something like a rangemaster boost test board. Use sockets for transistor, input & output caps. Maybe also add an extra trimpot on the emitter resistor (like on Fuzz face) and use bipolar caps so can reverse polarity for PNP / NPN?

Any suggestions on what works well for testing. Want to hear what they sound like on their own if that makes sense before moving on to integrating them into more complex stuff.

[GibsonGM]

I think the Rangemaster would work well for what you want to do; it's a very simple circuit.   That, or a 'vanilla' gain stage.  At the least, you'll get an idea of 'go/no go', and I'd think major gain differences would become apparent.    But be prepared to use trimmers to bias the swapped transistors for best performance!

[Steben]

Hmmmm you seem to get my attention again on the same transistors I have in the stash....
Be aware they are quite valueable.
GibsonGM is right: boosters are great projects.

[I_Am_Jon]

Hmmmm you seem to get my attention again on the same transistors I have in the stash....
Be aware they are quite valuable.
GibsonGM is right: boosters are great projects.

Was more than I wanted to buy but think I got a real deal on the bag. Included about 20 AC128's with hfe testing in the 80-190 range, about 60 Ge trans altogether plus a few useful silicon ones.

Trying out a breadboarded rangemaster with AC128 now. Being careful with current limited supply etc, but prefer to solder a few useful test modules so less crackle and risk of accidents.

[I_Am_Jon]

My Rangemaster (AC128) test sounds kinda cool but not sure it's doing quite what I wanted.

Was expecting more of a clean boost with some clipping when I push the volume. But I'm still getting some distortion at all volumes. Have changed input and output to caps to .1uf as I don't want to lose too much bass and added a 200k bias trim (R2).
https://www.electrosmash.com/dallas-rangemaster

Could be the impedance from my active bass? Think I might need a pull down resistor on the input (1M to 0V rail)?

Edit: I think this is closer to what I’m trying to make: https://www.diystompboxes.com/smfforum/index.php?topic=126534.msg1210328#msg1210328

[GibsonGM]

Too bad you don't have a scope to view the waveform (?)  Then you'd see exactly what's happening. 
It clips at any bias point?

I'd expect an active bass to have very low output Z, a good thing...the pulldown won't hurt, but probably won't do too much I'd think.

If the transistor has high enough gain, I'd expect some clipping.  And a bass is cranking out quite a punch on its attack, and lower frequencies 'have more power' (demand more from power supplies and so on).   

[Steben]

Rangemasters are not ment to "stand alone" though but rather add to a driven amp or pedal.

[GibsonGM]

Yes ^    And to add some 'grit' to the signal, as well as treble boost. They're not clean...but not REALLY dirty either...

[I_Am_Jon]

Too bad you don't have a scope to view the waveform (?)  Then you'd see exactly what's happening. 
It clips at any bias point?

I'd expect an active bass to have very low output Z, a good thing...the pulldown won't hurt, but probably won't do too much I'd think.

If the transistor has high enough gain, I'd expect some clipping.  And a bass is cranking out quite a punch on its attack, and lower frequencies 'have more power' (demand more from power supplies and so on).   

Think that's all it is and active bass output is a little high, volume out of my rangemaster is very high with the 10K pot turned up. Test AC128 is measuring ~100 hFE. Have added at 100k pot across the input to give me a little more control.

Rangemasters are not ment to "stand alone" though but rather add to a driven amp or pedal.

Not really looking to create an effect pedal with this one, just a basic circuit for testing out sound of different transistors on.

[antonis]

Not really looking to create an effect pedal with this one, just a basic circuit for testing out sound of different transistors on.

The only way for "testing sound" (other than via an amp with speaker) is waveform monitoring (via an oscilloscope..)

Anyway, Rangemaster's output (as for any CE amp) is strongly dependent on next effect input impedance..

The main difference between a classic Rangemaster and the one posted by Iain (https://www.diystompboxes.com/smfforum/index.php?topic=126534.msg1210328#msg1210328) is that trimpot sets total Collector resistance, hence Gain, where Boost pot sets Output Volume (amplitude)..
(that's the reason for getting distortion at all volumes..)

[mozz]

Those 2N167, are npn germanium.  I have a bunch but the hfe is pretty low.  Haven't figured out what to use them for. 

[I_Am_Jon]

Those 2N167, are npn germanium.  I have a bunch but the hfe is pretty low.  Haven't figured out what to use them for.

I've stuck them in my NPN fuzz face. Darker sound than the BC108c's, sounds pretty good on bass.

They just look cool, got to do something with them

[I_Am_Jon]

Still not used to working with PNP's, was getting a bit confused by negative vs positive ground setups.

This link seems to explain it well:
https://www.amplifiedparts.com/tech-articles/pnp-positive-ground-pedals

Think for my rangemaster positive ground makes more sense so long as powered from isolated supply / battery.

[I_Am_Jon]

I think this should work ok:

PCB:


Put extra sil sockets on as measuring points and make it easy to plug in different transistor packages.

Mockup:


Not sure if should move C1 infront of Input pot so its AC coupled which should allow for positive & negative ground wiring?




Edit: Doh, looks like i got R2 & R3 muddled. Wanted R3 3.9k and R2 to be the 200K trim. Back to the drawing board 
Fixed:

[antonis]

Think for my rangemaster positive ground makes more sense so long as powered from isolated supply / battery.

Not necessarily..

[I_Am_Jon]

Not necessarily..

Not seen it wired with the load on the emitter before, does that work? Also isn't C2 inverted?

[antonis]

Not seen it wired with the load on the emitter before, does that work? Also isn't C2 inverted?

How came that monstrosity..??

T1 should be flipped 180^o and +9V & GND should be exchanged..

[duck_arse]

^ he needs coffee.

[amptramp]

Reversing emitter and collector is known to give transistor action but at a lower hfe.  But I agree that C2 should be reversed.

[antonis]

Reversing emitter and collector is known to give transistor action but at a lower hfe.  But I agree that C2 should be reversed.

+1 agree..

But in the above (decaffeine!!) schematic, BJT works as a poorly biased Emitter follower..