epicyclic,
To have done this when you don't even own a guitar! Above and beyond the call of duty, take a bow!
On distortion, maybe something like this:
Q1's distortion is asymmetric, and present at low signal levels, generating a complete harmonic series, odds and evens. Q2's distortion is symmetric, and kicks in with bigger signals, generating odds only. Just like a valve guitar amplifier.....
Once again epicyclic, damn good show!
To have done this when you don't even own a guitar! Above and beyond the call of duty, take a bow!
On distortion, maybe something like this:
Q1's distortion is asymmetric, and present at low signal levels, generating a complete harmonic series, odds and evens. Q2's distortion is symmetric, and kicks in with bigger signals, generating odds only. Just like a valve guitar amplifier.....
Once again epicyclic, damn good show!
I built a breadboard of the Fuzz Face today with sockets for PNP transistors. I used a 10K trim pot for the 8.2K. I rounded up 4 Germanium transistors, selected the one with the lowest leakage (40uA) for Q1, it had the lowest beta (80). The trimpot was set at max (9.6K) to get the collector voltage of Q2 (beta 160, leakage 200uA) to -5V. Next I selected a PN2907 with beta of 80 @ 200uA for Q1 and a 2N2905A with beta of 160 at 800uA for Q2. The trim pot was set to about 4.9K to get the collector of Q2 to -4.5V. Last I tried a pair of 2N5087 with beta about 450. The trim pot was set a little higher, about 6K to get the collector of Q2 to -4.5V.
They all sounded about the same once the collector voltage is tweeked in. I think the whole thing about selecting transistors for beta is just to get the operating point set correctly without changing the resistors. This thing can sound pretty bad when the operating point is off to one side.
They all sounded about the same once the collector voltage is tweeked in. I think the whole thing about selecting transistors for beta is just to get the operating point set correctly without changing the resistors. This thing can sound pretty bad when the operating point is off to one side.
Very interesting and useful info Loudthud, good stuff.
I'm a bit unclear about the transistors you were using, you mention Ge devices, but the only type numbers you give seem to be Si. Myself, I've only built these things with Si devices (and a positive supply rail) though they've sounded good and authentic.
I selected for gain, as per the folklore, though the transistors easily available to me now mostly have much higher beta than the 'classic' 70 & 115, so this is very useful information for me, especially as I've got someone wanting one sometime soon.
This is a negative feedback circuit, which normally should reduce sensitivity to device gain, but it is operating in non-linear areas. Small signal analysis is thus useless, and whilst I've seen a thread on here that looks promising for good spice models of Ge transistors, I've yet to start trying them out. There's certainly a lot of rubbish elsewhere.
If I could keep on building the occasional Fuzzface without having to wade through testing batches of transistors for gain and leakage, or taking part in the germanium gold rush on ebay, I'd be a happy bunny.
I'm a bit unclear about the transistors you were using, you mention Ge devices, but the only type numbers you give seem to be Si. Myself, I've only built these things with Si devices (and a positive supply rail) though they've sounded good and authentic.
I selected for gain, as per the folklore, though the transistors easily available to me now mostly have much higher beta than the 'classic' 70 & 115, so this is very useful information for me, especially as I've got someone wanting one sometime soon.
This is a negative feedback circuit, which normally should reduce sensitivity to device gain, but it is operating in non-linear areas. Small signal analysis is thus useless, and whilst I've seen a thread on here that looks promising for good spice models of Ge transistors, I've yet to start trying them out. There's certainly a lot of rubbish elsewhere.
If I could keep on building the occasional Fuzzface without having to wade through testing batches of transistors for gain and leakage, or taking part in the germanium gold rush on ebay, I'd be a happy bunny.
I don't remember where I got the Germanium devices I used. They were made by TI with the number RT146, date code 7334. Probably a military part. Not much information online. I'm trying to contact some people I know that worked at TI back then.
It's interesting that you can use a silicon transistor for Q2 and it doesn't change the bias very much. What can change the bias is the drop across the 100K resistor so the beta of Q1 changes the operating point. I measured an input impedance of about 8K on the Ge and 2N2907 versions. The 2N5087 version was picking up too much noise from nearby radio and TV stations for a decent measurement at full gain. I'm using a Tektronix curve tracer to measure the beta.
It's interesting that you can use a silicon transistor for Q2 and it doesn't change the bias very much. What can change the bias is the drop across the 100K resistor so the beta of Q1 changes the operating point. I measured an input impedance of about 8K on the Ge and 2N2907 versions. The 2N5087 version was picking up too much noise from nearby radio and TV stations for a decent measurement at full gain. I'm using a Tektronix curve tracer to measure the beta.
More good info Loudthud.
Sorry to hear about your RF pickup problems. Might be alleviated with a 10nF cap, socket sleeve to chassis ground, if you're in a box, maybe plus some pFs b-c on Q1. Nothing you don't know already I should think, more as a general point of info for others. I've got a mast about 4 miles from me, in site, terrible reception, rrrrr!
Tektronix curve tracer, nice! Sigh....
Sorry to hear about your RF pickup problems. Might be alleviated with a 10nF cap, socket sleeve to chassis ground, if you're in a box, maybe plus some pFs b-c on Q1. Nothing you don't know already I should think, more as a general point of info for others. I've got a mast about 4 miles from me, in site, terrible reception, rrrrr!
Tektronix curve tracer, nice! Sigh....
I've been reading about the Fuzz Face all day. There are about 2000 threads on the diyStompbox forum. Thinking about the beta selection issue, I'm leaning towards the opinion that the low beta of Q1 gives slightly lower distortion before clipping. If both transistors had high beta, Q1 would be very close to one diode drop between Base and Collector because the drop across the 100K would be low. Any drop across the 100K increases the B-C voltage of Q1. But using a silicon device for Q2 also increases the B-C voltage of Q1. Not many people seem to have tried that. Additionally, increasing the current through Q1 by decreasing the 33K would increase base current and thus the B-C voltage of Q1. An interactive mess to get Q1 to generate the right amount of 2nd harmonic distortion.
Apparently JH would leave the FF on all the time and just backoff on the guitar volume control for rhythm parts. The FF would cleanup at low volume yet give bite and increased drive for lead parts.
Apparently JH would leave the FF on all the time and just backoff on the guitar volume control for rhythm parts. The FF would cleanup at low volume yet give bite and increased drive for lead parts.
Interactive mess, yes! Fantastically economic and elegant also. And used by the man. The perfect little puzzler really.
In case this hasn't yet been mentioned, there's a fuzzface bias calculator at
Fuzz Face Bias Resistors Calculator
Runs from a small javascript within the html of the page (7kB), not server-side stuff, so you can save and use locally. Possibly interesting to play with, either for prediction or comparison, and the javascript for the maths is quite readable for those so inclined - it's only a few lines, the equations are in the middle.
Anyone who does end up testing Ge devices, please take note: Handling directly with fingers alters the readings you get quite markedly!
In case this hasn't yet been mentioned, there's a fuzzface bias calculator at
Fuzz Face Bias Resistors Calculator
Runs from a small javascript within the html of the page (7kB), not server-side stuff, so you can save and use locally. Possibly interesting to play with, either for prediction or comparison, and the javascript for the maths is quite readable for those so inclined - it's only a few lines, the equations are in the middle.
Anyone who does end up testing Ge devices, please take note: Handling directly with fingers alters the readings you get quite markedly!
So... got the 33k resistor in place. Didn't help the collector voltage on Q2 much, measured it at 2,something. So i removed the 8K2 resistor and tried a few values in there. I ended up putting a 5K6 resistor in there as it came closest to the 4,5 mark with 4,7. Now it's working perfectly
So turn off the computer, turn on yer amp and give it some!
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