Tone Bender Mkii with bias mod, oscillation?

[Arcane Analog]

All interesting stuff to learn here!

So I breadboarded my first one and it  appears to work fine, but it sounds, well.......not too good yet.

I'm guessing this is because my tranny's are WAY above the recommended HFE (which is why I asked).  I've got a Q1 at 110-ish,  and Q2 & Q3 over 200.  I will now try swapping them out for some lower gain trannys and see how if I fare any better.

You should be able to get a decent sounding Bender from the gains posted if the transistors sound good. Having said that, rarely do I use the first few transistors I audition. There is interplay in the circuit so I would recomend auditioning several transistors for each position and find a set that work well together. In my opinion there is alot more to transistor selection than HFE.

I have also seen variations on 3 other parts of the circuit where I have questions:

1). the first resistor before the base of Q1, I have seen as 100K and 10K.  I have my breadboard using the 10K.  I will try to swap that for a 100K before I change the tranny's.

3).  I have seen the resistor on the collector of Q2 as either 100K or 47K.  I have my breadboard using the 47K, but I will try swapping that out too.

The MKII and the Supa Fuzz have very much in common and differ only in a few values. The difference might be attributed to the type of transistors used. For instance, the OC75 versions tend to have the 10K/47K arrangement while the OC81D units appear to have the 100K/100K pairing. Socket them and see what happens when you change transistor types. If you do not have OC75s try AC125 as they are a 'direct' substitute.

2). I have seen it mentioned to change the 470 ohm resistor to a 1K to boost output.  Not sure why this would boost output. I would assume increasing resistance would decrease voltage.  Just curious.

Yes it does!

[Arcane Analog]

To be more specific on my answers to questions 1 & 3 above, I would suggest that the difference with the OC75s at 10K/47K and the OC81Ds at 100K/100K would revolve around the typical gain range of each transistor and especially getting them biased properly in the circuit.

[chromesphere]

Hey Fuzzy645,
Are you getting that oscillation / farty noise when you crank up the attack like my build?
Paul

[Arcane Analog]

Hey Fuzzy645,
Are you getting that oscillation / farty noise when you crank up the attack like my build?
Paul

You could probably eliminate that issue with a trimpot on Q2 and/or the 100R resistor on Q3 as previously mentioned. It might tighten up your build by removing the bass flab.

[chromesphere]

Im considering rebuilding the mkii again.  I think i will have to put a trimpot in the q2!  Which resistor would i replace for the q2 bias trimpot?
Cheers!
Paul

[fuzzy645]

Thank you ALL for your helpful response.

OK, let me start by saying it is now sounding AWESOME.  I am using OC76 tranny's BTW.

As an experiment on the breadboard, I changed the 10K resistor to a weak 100K pot (measuring 90K) in series with the 10K resistor,  so the resulting sweep was 10K to 100K.

For the 47K, I changed that to a 47K resistor in series with a 50K pot, so the sweep was from 47K up to 97K.

Guess what sounded the best (drum roll please):  Hands down, no doubt about it, moving the 10K up to the 100K was night and day a beautiful amazing tone.  Maybe a bit "thicker" than early Zep, but a great tone none the less.  Now as far as the 47K, what happened was the diff was not huge, but it seemed to have less noise (I guess less "farty") up around 97K.

BTW, for the 8.2K fixed resistors, I do have a 10K trimpot, and that does seem to sound the best at close to 10K.

All I can say is I'm psyched and I want play my guitar all night hooked up to the darned bread board!!!!

[Arcane Analog]

Im considering rebuilding the mkii again.  I think i will have to put a trimpot in the q2!  Which resistor would i replace for the q2 bias trimpot?
Cheers!
Paul

Q2's collector needs a 100K trimmer to dial it in. You could even use this as a feature and turn it into an external bias. Really though, Q2 can be a set and forget. The external bias would have more use on Q3.

[Arcane Analog]

All I can say is I'm psyched and I want play my guitar all night hooked up to the darned bread board!!!!

Which is the end game! Sometimes I think we all spend too much time mucking about with mods, changes and tricks when we should be playing the damn thing instead. Get it sounding good and play!

[chromesphere]

Which is the end game! Sometimes I think we all spend too much time mucking about with mods, changes and tricks when we should be playing the damn thing instead. Get it sounding good and play!

My collection of 70+ pedals gathering dust can testify to this

Paul