Tone Bender Mkii with bias mod, oscillation?

[chromesphere]

Hi everyone,

As i mentioned in an early post, i built a tonebender mkii and it sounds great. (following the top schematic in the list):

http://fuzzcentral.ssguitar.com/mkII.php

I used a 20k pot for the bias of q3 as suggested in the notes

one thing i notcied on certain settings, in particular when the attack is up full, i got a low bassy, sort of farty sound.  If i roll the attack back about 1/8th, it goes away.  

At half turn of the bias, its like a bassy drone (fast frequency).  As i turn the bias up, the frequency slows down until at full turn it turns into a fast bassy pulse and sounds more like a farty sound. It sort of sounds like the 'stability' dial on the fuzz factory, a little.  Is that what it is or have i messed something up?

Thanks in advance for your time!
Paul

[Megatron]

In my opinion, you will get much better results with the MKII circuit using a fixed value for Q3 and biasing Q2. I suppose once the bias of Q2 is where you want it a trimmer on Q3 might help with fine tuning the build to taste. Getting Q2 solid should be the paramount concern with regards to biasing and making Q3 hit ~7-8V.

[chromesphere]

Hey megatron,
Thanks for the info i
might try this.
Would u say though that the low frequency hum im getting as i mentioned before is normal though?
Paul

[Megatron]

I just checked out the link you provided. I am not sure why you would aim for 4.5V on Q3 - that seems much too low. Perhaps the author is trying to superimpose the Fuzz Face Q2 biasing on Q3 of a MKII. I do recall reading opinions on looking at the MKII as a Rangemaster feeding a Fuzz Face. I personally do not find that to be a useful comparison.

Ages ago I built a MKII test bed with sockets on pretty much everything in efforts of learning how the circuit ticked. Biasing Q2s collector and thereby bringing Q3s collector over 7V at minimum has the most effect on dialing in the MKII sound. I would also look at lowering the base resistor for Q1 all the way down to 10-20K.

[Megatron]

Hey megatron,
Thanks for the info i might try this.
Would u say though that the low frequency hum im getting as i mentioned before is normal though?
Paul

No - I have never had any low fequency hum nor any other kind of hum outside the normal sounds you would get from using old germanium. I think there is definitely something wrong there.

[Megatron]

I should have added that 'farty' sounds in a MKII means the circuit is misbiased or the transistors are not suited to the build.

[chromesphere]

Thanks again for the info megatron! I think u just confirmed the circuit is working because the farty noise only happens wen im playing round with the bias pot (and as u said misbias the circuit). I thought that cood b the reason for it.
I also noticed 4.5v on q3 collector wasnt the best sound for this circuit. As u said higher sounds better.
Thanks again all useful info!
Paul

[Megatron]

Yes - the misbias sound would be the 'farty' sound but I am not sure the bass hum is bias related - it could be though.

Glad I could help.

[LucifersTrip]

Yes - the misbias sound would be the 'farty' sound but I am not sure the bass hum is bias related - it could be though.

you'll get a farty/blatty/muddy sound if the voltage is too high...at lower voltages 5, 4, 3V, etc it'll be thinner, lower vol and more saturated. if you drop to a really low voltage, it'll get farty right before cutoff.

the hum could also be caused by using a wall wart rather than a battery. I've build numerous MKII's and most had hiss (shhh) and/or degraded tones with a wall wart. If you want to use a 9V wall wart, bring the power in via a 50-100 ohm resistor. That solved the problem in all cases.

you can also get  oscillation when the fuzz is maxed, which is probably the reason some units had that 100 ohm resistor

[chromesphere]

Thanks for the reply Lucifer!

I just plugged in my fuzz factory to see how similar the 'stability' knob is to this mkii bias behaviour.  The stability knob starts out really high pitched, but if i mess around with all the knobs, i can get the frequency / pitch low enough to get almost the same farty sound.  It MUST be the sound of a misbiased germanium...I know you guys already said "yes it is", i just really need to confirm it 100% before i can move on.  You know how it is

Ill try that 100ohm resistor, thanks lucifer!

Ok ill leave it at that unless i discover something else.
(Thanks again)
Paul

[Megatron]

You will lose a lot of gain with that 100R limiting resistor.

[fuzzy645]

All good info!

What about transistor gain and leakage recommendations?  I have a bunch of various PNP's I picked up relatively chieap on auctions over the past few months. A few have gains in the 200 - 300 range, and others with gains in the 100 range as measured with a  Peak Atlas DCA tester.   

Also wondering what is the purpose of the 100 ohm resistor on the emitter of Q3 that is on the posted schematic, but not on the schemtaic at fuzz central.

Thanks!

[Megatron]

The 100R resistor serves a limiting function and tames the amonut of fuzz available. With that in place you can only access 90% of the pedal's fuzz. My comment above about "losing gain" should have read along the lines of a loss of fuzz.

[Megatron]

...which I would assume is why Lucifer recomended adding that to chromesphere's build. The 100R resistor mught cure the oscilation.

HFE ranges are ourely a matter of taste. You can make low gain trios work as well as high gain. They will obviously sound different. Just keep the ratio similar to the Q1=70-80, Q2=70-90 and Q3=100-130 or thereabouts. Or not! Experiment to see what you prefer.

[chromesphere]

I was going to respond with that megatron!  The oscillation only occurs at 90-100% on the attack dial so if the 100r resistor reduces the gain by about that much, it might get rid of the last section of the attack dial thats oscillating.  I think i'll just leave it the way it is though.  You can still use that last 10% of the dial turn,  you just get an annoying hum / farty noise in the background when you stop playing.

I ended up going something like, 60 / 75 / 90 which might be realitively low for q3, but i still think it sounds just as good as q3 120 (which i also tried). I tried something like 60 / 60 / 90 but found it a touch low on fuzz and also something like 40/40/40 just for fun, and it sounded fine for a low gain fuzz sound, but yeah, you dont get the hairy, blow your pants off settings that high hfe's can produce.  Increasing q2 gain from 60-75 also seemed to add a bit more fuzziness.

Anyway im happy with it now.  Just wanted to confirm that oscillation was normal and im happy to conclude from what everyones said that, yes it is.
Thanks again!
Paul

[Kesh]

100R on something that draws maybe 2mA is not going to do a lot to the power. certainly not kill 10% of the fuzz. . just knock 0.2 volts off. 50R will do even less.

I pretty much always have 47R on my power rail.

[Kesh]

whoops

[Megatron]

100R on something that draws maybe 2mA is not going to do a lot to the power. certainly not kill 10% of the fuzz. . just knock 0.2 volts off. 50R will do even less.

I pretty much always have 47R on my power rail.

Please, enlighten us as to what the 100R resistor really does.

[chromesphere]

im confused... 

[Kesh]

makes an RC low pass with the power input cap to smooth the power supply a little