Tone Bender Mk II - Impedance sanity check question

[bushidov]

So, using this as my schematic for a Sola Sound Tone Bender Mk II:


At the input stage, after the 5uF coupling capacitor, is that 100K resistor getting pulled to ground before going into the Q1 PNP transistor.

If I am understanding this correctly, this resistor is responsible for a majority of the input impedance. Not all of it, obviously, but a good chunk of it, I am guessing?

If I am correct, and I increase that 100K to 1M that should also increase the input impedance quite a bit as well. Not anywhere in the range of a proper 1M impedance or anything, but quite a bit more, none-the-less, correct?

And if those two things are correct, what would I lose, sound or tone-wise, by increasing this resistors resistance?

[mozz]

Are you using those transistors or something closer to OC75? The later uses a 10k resistor there.

[bushidov]

Are you using those transistors or something closer to OC75? The later uses a 10k resistor there.

Using those transistors there. Yeah, I noted that the Marshall Supa Fuzz, which uses OC75s has a 10K pull to ground instead. I'm not in front of my circuit, so I can't physically test it yet, but I am curious to how that effects the sound, and if I am even correct on my input impedance assumptions.

I am guessing that it probably will "sound" close at the full volume on my guitar, but as the resistor goes up in value, it stops "cleaning up well" with the volume knob on my guitar. However, this is just an uneducated guess, and has a lot of assumptions.

[iainpunk]

If I am understanding this correctly, this resistor is responsible for a majority of the input impedance. Not all of it, obviously, but a good chunk of it, I am guessing?

no, the emitter of the transistor is straight to ground, that is way less impedance than whatever resistor you'd put in there.

If I am correct, and I increase that 100K to 1M that should also increase the input impedance quite a bit as well. Not anywhere in the range of a proper 1M impedance or anything, but quite a bit more, none-the-less, correct?

changing that resistor, you change the bias of the transistor, this might work depending on the transistor, but it would have negligible influence on input impedance

And if those two things are correct, what would I lose, sound or tone-wise, by increasing this resistors resistance?

it might gate out, or even not work at all.

cheers, Iain

[antonis]

If I am understanding this correctly, this resistor is responsible for a majority of the input impedance.

Unfortunately no..!! 

Emitter intrinsic resistor (r_e = 0.026/I_Collector) times (h_FE + 1) is responsible for the majority of input impedance..
It should be considered in parallel with any bias configuration..
(the above stands for all grounded Emitter CE amps ..)

[bushidov]

Thanks Antonis and Iain for setting me straight on this. So this leads me to another question.

Hypothetically, what would one need to do to increase the input impedance of this circuit significantly, without changing the topology too drastically?

[antonis]

Either place a buffer in front of Q1 or replace Q1 with a Darlington configuration raising 100k value simultaneously..

[rankot]

You may add a buffer, but it will also change the loading of your pickups, so probably change the sound.

[bushidov]

Either place a buffer in front of Q1 or replace Q1 with a Darlington configuration raising 100k value simultaneously..

I think Ranko beat me to it, but I'd be leery of placing a buffer before a Tone Bender. The idea of the Darlington transistor does sound like a winning idea. I don't think the first transistor stage is a buffer, but rather a full on collector-follower PNP amplifier, which if driven at full-bore, the Darlington will just be driving a full-bore too, but I get the added impedance bonus after adjusting the 100K resistor. Am I understanding this correctly?

[Big Monk]

Is this a solution in search of a problem? What is driving you to do this?

[Rob Strand]

Something to realize about many of those old germanium designs is the base to ground resistors don't *provide* the base current to bias the transistor, they *divert* excessive base-current which is caused by leakage from the base to the collector.

If you built a silicon version of the circuit it would not work.   The transistor would not bias at all, you need to add a base to +V or base collector resistor to bias the transistor.  The added resistor provides the base current that was coming from leakage in the germanium case.

[antonis]

What Rob said..

You can consider 100k resistor as a leakage DC path to GND..

[amptramp]

Some people would put a resistor in series with the input capacitor just to prevent the treble from disappearing when the guitar volume control is near the maximum.  Due to the biasing from leakage current, this is a design that is only suitable for room temperature or a narrow temperature range, even more so than the Fuzz Face from which it is derived.

Of course, the series resistor would add noise.

[antonis]

Of course, the series resistor would add noise.

Out of bounds for a noiseless pedal like Tonebender...

[Big Monk]

In my limited experience, cleanup, like that of a Fuzz Face, is not really possible in this circuit. What you get is varying levels of dirt but never the "cleans" of a Fuzz Face rolled down.

One thing I like in this circuit is a Pre-Gain control like in the Gagan Easy Face. It helps to tame some of the extreme gain and gets you some more crunch tones without sacrificing the full whack fuzz tone as well. I've found this control interacts in a different way than just rolling the guitar volume off.

[Big Monk]

...even more so than the Fuzz Face from which it is derived.

Not to be a stickler, but the Tonebender Professional Mk. II was a refinement of the Tonebender Mk. I and not the Fuzz Face. Most of the history would indicate that the Fuzz Face is a derivative of the Tonebender Mk. 1.5, with changes to the biasing of Q2.

[bushidov]

Is this a solution in search of a problem? What is driving you to do this?

Curiousity, as well as trying to wrap my head around LTSpice doing something vs not doing something to what I expected. I know, LTSpice is just a simulator, and always use your ears and your meter first, but at the moment, I just had shoulder surgery, so I'm down one arm for a couple weeks and can't exactly work on my circuits as I'd like to.

[antonis]

Get well soon, Erik..!!! 
(and don't bother too much about impedance calculations mismatch..)

[bushidov]

Aside from a simulator, what else is a good way to solidly measure impedance, as in, I have the circuit board in front of me and I have a soldering iron and breadboard?

I won't be able to use the soldering iron for a couple weeks (and this is killing me. Me and my dang hobbies), but I still can breadboard (clumsily, until I get my right arm back)

Thanks for the well-wishes!

[Big Monk]

Aside from a simulator, what else is a good way to solidly measure impedance, as in, I have the circuit board in front of me and I have a soldering iron and breadboard?

I won't be able to use the soldering iron for a couple weeks (and this is killing me. Me and my dang hobbies), but I still can breadboard (clumsily, until I get my right arm back)

Thanks for the well-wishes!

I guess the question is still this: other than curiosity, what are you looking to achieve? You mentioned cleanup originally but I was not 100% sure what your goal is here.