I would like to convert this circuit to use 3AX31Bs that I have quite a few of.  I have it on the breadboard and I marked the values I am using in red.  With those I get 4.7V on the collectors but it's really noisy.  However, the fuzz tone underneath has promise. What value resistors should I be using to get these to bias correctly?

Part Number:3AX31B
Part Type:PNP TransistorCollector Maximum Dissipated Power (PCM):0.125 W
Collector maximum allowable current (ICM):-0.125 A
Collector-base breakdown voltage (BVCBO):-30 V
Collector-emitter breakdown voltage (BVCEO):-18 V
Collector cut-off current (ICBO):-12 μA
Collector-emitter leakage current (ICEO):-600 μA
DC current gain (hFE):40～180 mine are around 115
chip material:germanium

What have you done about the change from NPN to PNP? Did you flip the supply?

Quote from: ElectricDruid on May 15, 2025, 04:46:27 AMWhat have you done about the change from NPN to PNP? Did you flip the supply?

Yes, the power supply is correct.  I don't have an output capacitor yet so the noise might be DC. 

Quote from: idsnowdog on May 15, 2025, 08:59:30 AMYes, the power supply is correct.

That's not *exactly* answering the quesion!

Tell us step-by-step what you've done to convert the circuit, and then we might be able to spot something you've missed and help out.
Better still, draw a circuit diagram of the circuit *exactly as built*. You'll spot several things you hadn't thought of while you're doing it, I bet.

QuoteI don't have an output capacitor yet so the noise might be DC. 

If you've got the two caps by the tone control in place, there's no need for another one on the end. There's no DC at that point. Notice that the Tone control and the output volume are separated from the rest of the circuit by those two caps.

HTH

Quote from: ElectricDruid on May 15, 2025, 02:39:53 PMTell us step-by-step what you've done to convert the circuit

You don't like schematics Tom, do you..??

This is how things are laid out.  I notice that I don't have a ground to the base of Q1 like in the schematic which may cause noise.  However, I don't see how that helps me figure out which value resistors to use for the collector and feedback?  I used this to try and figure out the resistors. With 1.7K on the collector of Q1 and a 216K feedback resistor I get 4.7V on Q1.  I haven't been able to figure out what Q2 should be?   

https://www.guitarscience.net/calcs/cecb.htm

It is likely that you're fighting a couple of unknowns.

Note that the transistor calculator page you linked does not have the string "leak" occurring, even once. That suggests to me that the calculator presumes an NPN silicon device, in which the leakage can almost always be ignored.
Germanium junctions leak about 1000 times the leakage in a silicon junction for the same voltage conditions. Not only is the leakage much higher, it drifts more with temperature changes. Both of these make biasing a trickier business than biasing silicon.
As an aside, germanium devices were common before humans learned to do precise semiconductor processing and passivation of semiconductor dies, so they were notorious for high levels of hiss. Not all of them, but it was common to find really noisy devices.
There are reasons why germanium devices were bypassed in favor of silicon.
There is another issue with germaniums. The mythos of germanium pedals has turned a horde of pedal makers loose on the world's remaining pockets of germanium devices. It's common - and necessary - to sort the batches you find, not relying on the datasheet numbers to be right for whatever devices you get. In some cases, big users find batches of germanium, sort the for the best ones, then sell the rejects back into the market.
All of that is to say that there are valid reasons why any given germanium transistor may be noisy or wildly varying in leakage.

The collector-feedback circuit used here counts on both the collector voltage, converted to a current by a large resistor to the base, to turn the transistor on just enough. It works, but leakage variations do not get servo-ed out as well as with other bias circuits. This is especially true for circuits with no emitter resistors, as in this circuit.

If you're set up on a breadboard as you show, try subbing in a number of your stash of devices. I would think that they would each be noticeably different unless they have been pre-sorted.  Also try setting up your voltmeter on the collector to ground, and watch the collector voltage swing as you place your fingertip on the transistor.

I also think that it would be fun and instructive to compare the noise of several different transistors for Q1.

The collector feedback circuit with no emitter feedback and germanium devices presents a number of challenges, and a lot of them don't have much to do with resistor values. Another aside: a series resistor before Q1 adds its thermal noise to the input to be amplified, especially if you have made a special effort to get carbon composition resistor, and especially-squared if you're using new old stock carbon comp.

I don't know why everyone seized on the noise as the primary issue?  When my question was which resistor values to start with.  I know breadboard circuits with a lot of jumper wires are noisy. I used this site and tried 45K for the collector and 3.9M for the feedback resistor for Q1 alone.  It sounds great with low noise.  I reduced the collector to 33K which gives me 3.7V and I will try a few smaller values to see if I can get it closer to 4.5V and then add Q2 into the circuit.  I hope the two don't end up throwing each other out of whack if I change values.   

Quote from: antonis on May 15, 2025, 03:22:37 PM

Quote from: ElectricDruid on May 15, 2025, 02:39:53 PMTell us step-by-step what you've done to convert the circuit

You don't like schematics Tom, do you..??

No, I don't. I *LOVE* them! 

Quote from: idsnowdog on May 15, 2025, 10:12:27 PMI used this site and tried 45K for the collector and 3.9M for the feedback resistor for Q1 alone.  It sounds great with low noise.  I reduced the collector to 33K which gives me 3.7V and I will try a few smaller values to see if I can get it closer to 4.5V and then add Q2 into the circuit.

I presume you set 250 to 300 mV for V_BE..

For Q2, you'd have to set Q1's Collector resistor value for R_S..
Actually, it should be lower by [R_B + β*r_e] / [R_B + β*(r_e + R_C)]..

Quote from: idsnowdog on May 15, 2025, 04:32:01 PMThis is how things are laid out.

if so, you need to move the -9V wire one row right. also, if as shown, you do need an output cap.

Quote from: duck_arse on May 16, 2025, 11:40:36 AM

Quote from: idsnowdog on May 15, 2025, 04:32:01 PMThis is how things are laid out.

if so, you need to move the -9V wire one row right. also, if as shown, you do need an output cap.

See? Isn't it amazing what even a basic schematic like this can do?!? 

Although how the hell you're reading that and seeing that, Duck, I can't fathom. Fair play.

Here are the values I settled on.  I can get 3.7V on the collectors and it sounds good but I don't know if those values make sense.  If anyone can tell me other values to try that would be appreciated.  These are the changes I made to the circuit and the layout I would like to use.  Do the diode and capacitor on the voltage input need to be reversed?

Quote from: idsnowdog on May 17, 2025, 03:41:43 PMI don't know if those values make sense.

"Sense" from what point of view..??

3.7V on Collectors might seem OK but you have to realize that Q1 Collector resistor is severely dominated by Q2 input impedance (especially for GAIN set at max) so DC and AC load lines might be remarkably divergent..

Quote from: idsnowdog on May 17, 2025, 03:41:43 PMDo the diode and capacitor on the voltage input need to be reversed?

Yes, if you've swapped the supply around. Without swapping the diode around, you won't get any voltage to the circuit. And once that's sorted, the electrolytic will be in backwards, which generally makes them go POP!

Quote from: idsnowdog on May 17, 2025, 03:41:43 PMI can get 3.7V on the collectors and it sounds good but I don't know if those values make sense. 

if you are using 3AX31 pnp with a positive ground supply, then no, a positive 3V7 reading does not make sense.

Quote from: antonis on May 18, 2025, 04:02:43 AM

Quote from: idsnowdog on May 17, 2025, 03:41:43 PMI don't know if those values make sense.

"Sense" from what point of view..??

3.7V on Collectors might seem OK but you have to realize that Q1 Collector resistor is severely dominated by Q2 input impedance (especially for GAIN set at max) so DC and AC load lines might be remarkably divergent..

Quote from: duck_arse on May 18, 2025, 10:34:07 AM

Quote from: idsnowdog on May 17, 2025, 03:41:43 PMI can get 3.7V on the collectors and it sounds good but I don't know if those values make sense.

if you are using 3AX31 pnp with a positive ground supply, then no, a positive 3V7 reading does not make sense.

I asked for suggestions for values for the collector and bias resistors for Q1 and Q2.  Nobody has offered me any values to start with.  Is it forbidden to suggest resistor values? 

forbidden - no. helpful - ?

if you think it sounds good, if it doesn't catch fire constantly, if it isn't noisy, it doesn't matter your circuit diagram or your voltages or what we say. it sounds good is the only thing you need worry about.

Quote from: idsnowdog on May 18, 2025, 11:41:10 AMI asked for suggestions for values for the collector and bias resistors for Q1 and Q2.

For exactly which circuit..??

All schematics posted concern positive supply and n-p-ns, where you are talking about p-n-ps and negative supply..

Post your own schematic and we'll gladly suggest you appropriate values..

This is how I have it configured.