Tonebender mk2 transistor problem

[dorukakkus]

Hey I built myself a npn tonebender mk2. But something is horribly wrong. It might be funny but it only works with a pnp transistor as q1. Every connection is ok the schematic is correct I have tried a lot of transistors but I cant solve this issue. Please help I am going nuts.

[antonis]

Hi & Welcome..

What particular BJTs you use..??

[anotherjim]

This?

If you don't have germanium NPN, it probably can't work - there is no positive bias voltage for Q1. Silicon needs more base drive voltage and won't have any leakage current to help either. A PNP could sort of work, but not properly.

[Rob Strand]

If you need a silicon version, search the forum archives.  I believe there's a silicon bender corresponding to the MK2.
I think one thread was started by member mac.

[antonis]

What Jim above mentioned..

[dorukakkus]

As for bjts q1=2n3906 q2=2n2222 q3=bc108. So as far as I am concerned to solve this issue I need to add that 470k resistor or find a npn germanium transistor. For now I will add the resistor.I will keep you guys updated. And are q2 q3 transistors ok for this purpose or should I change them ?

[Steben]

As for bjts q1=2n3906 q2=2n2222 q3=bc108. So as far as I am concerned to solve this issue I need to add that 470k resistor or find a npn germanium transistor. For now I will add the resistor.I will keep you guys updated. And are q2 q3 transistors ok for this purpose or should I change them ?

Nothing wrong in your thinking.
The resistor is not needed for germanium because they leak current, giving the base some juice to open up. You know, they are technically lower quality. The resistor leaks the current instead for the silicon one.
A silicon bender is one of those to do's in my list. Soundwise they are more than ok. They can sound brighter than germanium for several reasons.

1 point perhaps. The 2n2222 and bc108 may have a lot of difference in their gain (hFE) value.
It might be completely rubbish, but I'ld rather combine 3906 with the 108 in the fuzz part. But that is - you know - taste. these circuits are very finicky when it comes to the gain values. They bias different with each transistor. Hence the trimpots. Even the 47k resistor at Q2 can be a trimpot to fine tune.

[dorukakkus]

Ok I have done everything you guys said. But I couldn't find any 2n3904(I think Mr. Steven meant 2n3904 because 2n3906 is a pnp transistor)I used BC237s. They are both around 330 hfe. Q3 is biased around 4.5v. And this is the aftermath: https://drive.google.com/file/d/1IAGEEUAqvlXNGF26x3hdYy5D7s5Ztr41/view?usp=sharing
It sounds farty ,noisy, bassy and  I don't know why, it makes weird noises when I roll the volume of my guitar. And it hisses when it is completely off. My guitar has CTS pots in them so they are not the issue. And it is very loud. If the transistors are the problem I have BC546s and BC239s lying around. I can swap them easily.

[dorukakkus]

I think I have found the problem. The schematic Mr. Antonis kindly submitted has resistors connected to the emmiter legs. The schematic I used didnt have them so  I will add them now.

[antonis]

Resistor on Q1 Emitter leg sets Q1 stage gain, which is - approximately- the ratio of Collector to Emitter resistor values..
In case of Emitter resistor absence (grounded Emitter), is the ratio of Collector resistor times Collector current to B-E junction thermal voltage..
(theoretically maximum stage gain..)

You're free to add (or not) Emitter resistor but you definatelly have to add Collector-Base resistor for proper Si n-p-n BJT bias ..
(you can even add that particular value resistor with its right leg connected to power supply, instead of Collector, with similar result..)

P.S.
I champ at the bit for "rasionalist" guys (other than myself) to come and argue about maintaining same resistor value, both for PS & Collector connection..

[dorukakkus]

Thank you for the information you have shared with me Mr. Antonis. But adding the emitter resistor did solve the issue now the pedal is working flawlessly. I now understand why this pedal is very popular. Thanks to everyone who has helped me.