Single Transistor Boost Questions

[blackcorvo]

All I have on hand is a new battery, but here are the voltages (if I measured correctly and have the correct pins labled):

Batt: 9.53V
E: 28.2mV
C: 6.55V
B: .643V

Does this explain anything to anyone??
-Joe Hart

I see you biased the collector a couple volts higher than the usual.
When biasing bipolar transistors, people usually go for the rule of thumb "collector at 1/2 supply voltage = good biasing", and when they want distortion, they usually "underbias" it, reducing the collector voltage by increasing the resistor connected at the collector (the 10k one on your circuit).

In this circuit, the use of a bypass capacitor makes little to no difference in the sound because of how small the emitter resistor is.

I'm no expert in theory myself, but from what I know it just seems like a normal PNP booster, but biased in an unusual point of operation.

[MikeH]

I am curious too. Joe's schematic looks like a negative supply, while the scheme in the simulation appears to to be a positive supply.
(Did I get the new nomenclature right?)

Atta boy!

[alanlan]

Actually, this looks pretty close to what Alanlan estimated. I don't know enough about electronic theory to understand what it means, though!
-Joe Hart

Not quite, the base voltage is only just enough to turn the transistor on.  3V across the collector resistor = 300uA, so it's actually not far off ideal biasing but it would be quite variable across temp, battery voltage, actual transistor being used etc.  As said before, it's more of a happy outcome.

[LucifersTrip]

All I have on hand is a new battery, but here are the voltages (if I measured correctly and have the correct pins labled):

Batt: 9.53V
E: 28.2mV
C: 6.55V
B: .643V

Does this explain anything to anyone??
-Joe Hart

I see you biased the collector a couple volts higher than the usual.
When biasing bipolar transistors, people usually go for the rule of thumb "collector at 1/2 supply voltage = good biasing", and when they want distortion, they usually "underbias" it, reducing the collector voltage by increasing the resistor connected at the collector (the 10k one on your circuit).

I have found that there's no "rule of thumb" with many of the vintage fuzzes. The 4.5v holds true for the Fuzz Face and many of it's variants, but I get great fuzzes with Q2C voltages all over the place..

...and remember, the most famous 1 transistor treble boost, The Rangemaster, which is very similar to this circuit sounds best at around 7v

http://www.geofex.com/Article_Folders/Rangemaster/atboost.pdf

[Joe Hart]

...and what transistor # did you use?

It's a scavenged transistor. It's a "plain old" black plastic semi-circle shape, and it's PNP. Here's what's printed on the flat side:

Top line:  a triangle shaped logo (reminds me of the Alcoa logo)
Middle line:  "A666"
Bottom line:  a circle logo with a vertical line through it, then a "Q" or a "g" little logo thing, then "2F"

No idea what type it is. I don't even remember where it came from. Anyone have any ideas??

-Joe Hart

[Quackzed]

if you extrapolate what the sine wave output would be without the asymmetrical clipping, it looks like your clipping the bottom peaks before the 'zero crossing' of the sign wave... that is, if you had a sine wav with zero crossing at 0v, and clipped the bottom peaks at 0v you still wouldnt be clipping as much as you are in the analysis picture...
so it might just be that the signal is so assymetrically biased via the 1m and 100k resistors... you might get more or less of the effect if you lowered or raised that 100k... you could controll how much dc shift there is...

[Joe Hart]

so it might just be that the signal is so assymetrically biased via the 1m and 100k resistors... you might get more or less of the effect if you lowered or raised that 100k... you could controll how much dc shift there is...

So a higher value for the 100K to ground would create less asymmetry? Would a 1M there be "no" asymmetry?
-Joe Hart

[Joe Hart]

In this circuit, the use of a bypass capacitor makes little to no difference in the sound because of how small the emitter resistor is.

I had noticed very little change in the sound (if any), but didn't know if it was just my ears playing tricks on me. It seemed to sound better (maybe?!?) with it in, but I wasn't at all sure. Can anyone with electronic theory knowledge tell me what's happening here and if it would make any difference? If it makes no difference, I may as well take it out (the cap is pretty big -- almost an inch long), but if it makes even a little difference, I'd rather leave it in. But I don't have the knowledge to understand what it does (it was just in other circuits, so I put it in -- but I didn't just blindly throw random parts in, I did test it soundwise). Thanks!
-Joe Hart

[Gus]

Joe

I modeled your gain stage with stock LT spice transistor models. It is a standard textbook gain stage LPB, Rangemaster etc types.

2N4403 C to ground -6.60VDC
2N2907 C to ground -4.87VDC
2N5087 C to ground -4.87VDC
2N2905A C to ground -6.49VDC

[Quackzed]

so it might just be that the signal is so assymetrically biased via the 1m and 100k resistors... you might get more or less of the effect if you lowered or raised that 100k... you could controll how much dc shift there is...


So a higher value for the 100K to ground would create less asymmetry? Would a 1M there be "no" asymmetry?
-Joe Hart

yeah, i think so , more or less, so a 10k or so instead of the 100k should give you an idea.. also that 100ohm r will have an effect as well, 500ohms would be less assymetric and a bit less distorted i'm thinking,that also might be a good spot to try some different r values and see what you get...

[EATyourGuitar]

I did the thing with the transistor biased right at the turn off/turn on. it does make a good one transistors fuzz but I threw a booster before it to get more sustain and more fuzz. I'm not exactly sure if my filter is redundant but it sounds cool. I used a germanium diode. you can put the diode on a switch to flip it around. one way clips the side thats already square. the other way makes it square on both sides. both usable tones. my russian pnp's have a forward voltage (BE) of 0.34V with a Hfe in the 50 to 70 range. Q1 base is 1.0V and Q2 is right at 0.34V. Q2 is the fuzz. the rest is optional. good information in this thread

[EATyourGuitar]

all the components carry the same numbering as the schematic above. germanium PNP transistors with a germanium diode.

[Gus]

I left a hint in my other post.,  Note the collector voltages and the transistors number.
Look up the transistor numbers and note the specs.

The first schematic posted(transistor gain stage part) is a circuit you can find in book like "The Art of Electronics"  

The 4 resistors around the transistor are all part of the gain and biasing of the transistor.  The resistor base to ground and resistor emitter to ground along with the base to power supply can be adjusted for how sensitive the circuit is to different transistors used.

However it is all about tradeoffs things like input resistance, number of parts needed, temp stability and more.

NPN circuit with original 2 resistor and 1 cap values in blue  IMO you need the guitar sim to understand the input loading of the guitar part of the circuit

It might not sound the same










Note the tone control curves  I did not adjust the tone control potentiometer maybe try a 500K the green is about 0  blue 500k and red is about 1meg, not much change from blue to red.  Also note how close it is the RM values.  One could make the emitter resistor 4.7k and adjust one or both R4, R3 OR...

[Gus]

Did you try the PNP circuit with the 1meg changed to 470k and the 100 ohm to a 3.9K?

Did the sim help? note the collector voltages reading with different hfe transistors, they are close to 6.55VDC

[Gus]

Did you try the PNP circuit with the 1meg changed to 470k and the 100 ohm to a 3.9K?

[Joe Hart]

I apologize, Gus, I have had a VERY crazy couple of weeks! Hopefully, I will try it out today. If not, my parents are taking my two kids for a couple of days so I will definitely get to it in the next couple of days. It's been driving me crazy because it's just sitting on my breadboard mocking me!!
-Joe Hart

[Joe Hart]

I have some disappointing findings. I first tried messing with the 100K resistor and the 100R resistor to get more or less asymmetry -- I think it worked, but it was extremely subtle at best. It sounded like it smoothed the fuzz a bit, but if anything it robbed some character from the circuit, I feel. To me, it pretty much sounded the same either way.

Then I changed the 1M resistor to 470K and the 100R to 3.9K (well, I didn't have a 3.9K, so I added a couple resistors together and got 3.7K, and I figured that was close enough). Again, pretty much no change. But I did notice that if I took the bypass cap out, the output was a bit lower and MUCH less fuzz and none of the "ratty" growling tone I was getting. In the original circuit, the bypass cap seems to make little or no difference, but I think it smooths the fuzz a tiny bit -- at any rate, I'm leaving it in because that's what I had in there to start with and I can't be sure that it doesn't change the sound.

Maybe I'm just missing something? It sounds really cool and even if I do what people have said should make it "correctly" biased (the 470K and the 3.9K resistors), it still sounds the same as the original circuit. Hmmm...
-Joe Hart

[Quackzed]

The 100k 100r thing was a bit of a shot in the dark, as the signal already had a big dc shift... still worth a shot just to find out...

[Brossman]

Any hopes for a sound clip? 

[Gus]

Joe

There are many things to know, to design a simple gain stage like this one.

Yes if the emitter bypass cap is removed the gain will go down and the tone will change(degenerative feedback).  I am not sure I understood you last post, did the sound stay the same with the 100k, 3.9k and emitter bypass cap your first circuit ?

Next try this get different transistors of different Hfe
Install the transistors in the original circuit and the 100k, 3.9k resistor changed circuit, note the collector voltages. The collector voltages with the 3.9k and 100k circuit should stay closer with different Hfe transistors compared to the first circuit.

The following is a sim based on a circuit I built years ago using a TIP29 as the transistor.  R11 is an 500k external control of the transistor bias for different tones.