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
(https://i.postimg.cc/yg59mLVC/EQD-Spires-PCBWay-Community.jpg) (https://postimg.cc/yg59mLVC)
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
(https://i.postimg.cc/56mRvxpJ/rosac.png) (https://postimg.cc/56mRvxpJ)
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 (https://www.guitarscience.net/calcs/cecb.htm) 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 PMQuote 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! :icon_lol:
Quote from: idsnowdog on May 15, 2025, 10:12:27 PMI used this site (https://www.guitarscience.net/calcs/cecb.htm) 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.. :icon_wink:
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.
(https://i.postimg.cc/56mRvxpJ/rosac.png) (https://postimg.cc/56mRvxpJ)
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 AMQuote from: idsnowdog on May 15, 2025, 04:32:01 PMThis is how things are laid out.
(https://i.postimg.cc/56mRvxpJ/rosac.png) (https://postimg.cc/56mRvxpJ)
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?!? :P
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?
(https://i.postimg.cc/V5FhhfjH/EQD-Spires-PCBWay-Community-revised.jpg) (https://postimg.cc/V5FhhfjH)
(https://i.postimg.cc/6TzH0HXk/EQD-Spires-Green-Ch-Rosac.png) (https://postimg.cc/6TzH0HXk)
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.. :icon_wink:
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 AMQuote 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.. :icon_wink:
Quote from: duck_arse on May 18, 2025, 10:34:07 AMQuote 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.. :icon_wink:
This is how I have it configured.
(https://i.postimg.cc/qzLQ0DTc/EQD-Spires-PCBWay-Community-revised.jpg) (https://postimg.cc/qzLQ0DTc)
That said, I presume you get -3.7V (negative) readings..
Not bad but they could be a bit lower (-4.5V ideally)..
Try 39k for R4 & 4k7 for R6.. :icon_wink:
Of course, you may retain Collector resistors values and alter feedback bias resistor values..
e.g. for Q1, R3 should be about hFE times R4..
Quote from: idsnowdog on May 18, 2025, 01:50:13 PMThis is how I have it configured.
(https://i.postimg.cc/qzLQ0DTc/EQD-Spires-PCBWay-Community-revised.jpg) (https://postimg.cc/qzLQ0DTc)
I'm assuming from that schematic that Vcc is +9V, because you're *still* not telling us clearly what you've done or haven't done. Ground is at the bottom so that's clear, at least.
In which case, you've got a lot of PNP transistors reverse-biased in a circuit designed for NPNs. So your biasing voltages are correct (in the sense that what you measured is what you've got), but completely wrong for the transistors you're using.
.... and, if as shown and positive ground, both electros are backwards.
Quote from: ElectricDruid on May 18, 2025, 07:36:01 PMQuote from: idsnowdog on May 18, 2025, 01:50:13 PMThis is how I have it configured.
(https://i.postimg.cc/qzLQ0DTc/EQD-Spires-PCBWay-Community-revised.jpg) (https://postimg.cc/qzLQ0DTc)
I'm assuming from that schematic that Vcc is +9V, because you're *still* not telling us clearly what you've done or haven't done. Ground is at the bottom so that's clear, at least.
No you are *still* hung up on the schematic symbols when I already told you that I have reversed the +/- leads and that the circuit does work. It's not like the circuit magically started working just by changing the symbols on the schematic. I asked for resistor values to start with. I didn't ask anyone to critique the physical layout of my breadboard or to diagnose noise issues.
This isn't the first time I have used PNPs. I have converted between NPN/PNP many times without problems because I knew the bias scheme and the resistor values were at least in the ballpark. I regularly go to alltransistors.com and check for compatible transistors. I can change si to ge and NPN to PNP in the search criteria and get a list of compatible substitutes. The difference this time is the bias scheme was completely
new to me, the documentation for the transistor was lacking, and the stock resistor values for the circuit were so far out of whack that I didn't know where to start.
Biasing a transistor is little different than adjusting timing and carburation of an engine. Get it right and things are nice and quiet. Get it wrong it adds to the inherent noise of an unshielded circuit on a breadboard where everything is an antennae.
I also don't see the point of quibbling over whether the voltage at the collector reads -4.7v or +4.7v when all I need to do is reverse the leads on my multimeter. It's not like it reads 12v one direction and 3v the other direction.
(https://i.postimg.cc/2LQVkw7W/EQD-Spires-PCBWay-Community-revised.jpg) (https://postimg.cc/2LQVkw7W)
Here's how I resolved the bias issue.
1. I looked up transistor biasing and I saw that the bias method is called collector-to-base feedback. I hadn't used that before. That led me to this website https://www.guitarscience.net/calcs/cecb.htm . This gave me a ballpark for the resistor values.
2. I spent hours trying different resistor values and recording the voltage at the collector. What I noticed was Q1 and Q2 were too interdependent to allow me to zero in on a specific value.
3. Instead of using the 100 hfe average in the online calculator I used the actual hfe values of both transistors that I got from my transistor tester. This allowed me to find collector and feedback values that worked for both transistors.
4. After playing for an hour I noticed that the transistors degraded when they got hot. I already knew this was an issue and it was skewing my measurements. So I looked for methods to mitigate temperature drift. I found this site where they were talking about using a germanium diode to stabilize the transistors. https://web.archive.org/web/20090122112519/http://www.geocities.com/SunsetStrip/Studio/2987/britface.html . After adding 1N34As between the base and emitter of Q1 and Q2 the temperature sensitivity lessened and I could now change resistor values to fine tune voltage without skewing the values on either transistor. Noise also decreased.
> Q1 and Q2 were too interdependent to allow me to zero in on a specific value.
How? They are not DC-coupled. Combined with your remarks on breadboard, I wonder if it is oscillating.
Quote from: idsnowdog on May 19, 2025, 06:03:47 PMNo you are *still* hung up on the schematic symbols when I already told you that I have reversed the +/- leads and that the circuit does work. It's not like the circuit magically started working just by changing the symbols on the schematic. I asked for resistor values to start with. I didn't ask anyone to critique the physical layout of my breadboard or to diagnose noise issues.
Don't get me wrong but I think WE try to help you and not YOU try to help us..!!!
It should be much easier for someone to post their very schematic than for many others struggling to understand what's really going on.. :icon_wink:
P.S.
As per your last statement, you've found resistors proper values so vivere in pace e serenità..
Quote from: idsnowdog on May 19, 2025, 06:03:47 PM(https://i.postimg.cc/2LQVkw7W/EQD-Spires-PCBWay-Community-revised.jpg) (https://postimg.cc/2LQVkw7W)
both the electros are backwards.
Quote from: duck_arse on Yesterday at 10:58:53 AMQuote from: idsnowdog on May 19, 2025, 06:03:47 PM(https://i.postimg.cc/2LQVkw7W/EQD-Spires-PCBWay-Community-revised.jpg) (https://postimg.cc/2LQVkw7W)
both the electros are backwards.
and if we believe the notations, it's running on 18V. But I don't.
Quote from: ElectricDruid on Yesterday at 12:52:05 PMand if we believe the notations, it's running on 18V. But I don't.
So you didn't see her face, yet..
P.S.
Sorry guys but OP clearly stated that he only needed resistors values..
(irrelevant to circuit's configuration, polarity and supply voltage..)
He also stated that he doesn't need any comment about his "mysterious" circuit, so plz, don't be rude against such a polite/noble/gentle/gracious person.. :icon_redface:
Quote from: PRR on May 19, 2025, 09:39:00 PM> Q1 and Q2 were too interdependent to allow me to zero in on a specific value.
How? They are not DC-coupled. Combined with your remarks on breadboard, I wonder if it is oscillating.
If it is it isn't audible. If my termites can hear it they can move out.