MkII bias and general pointers (calling JakeFuzz, Timbo, and mgwhit!)

[Bret608]

Hi all, 

I just dug up and re-read the Tonebender thread of awesomeness:

http://www.madbeanpedals.com/forum/index.php?topic=5623.0

My reason for doing so is that I just built up the following MkII variant, a Supafuzz to be exact:

http://tagboardeffects.blogspot.com/2012/02/marshall-supa-fuzz.html

So, my first impression is that it rocks--total Ron Asheton tones. I went with the Supafuzz specs because I thought the additional bass would complement my Mustang well. I just have a couple of questions:

1) Is there an ideal bias range for the Q3 collector? Are there any other bias points to check? I'm using 2n1305s of 74, 83, and 103 hfe for Q1, 2 and 3 respectively.

2) This one is a bit harder. The circuit initially didn't fire up--turned out the 10uf input cap I grabbed at work was bad; you could even see some residue at the base of the negative lead! I thought I had fried it on my test rig, even though I hooked it up with the test leads reversed for positive ground, etc. After changing that, it did fire up, but gradually, like no sound at first, then a clean-ish sound coming through, then the full fuzz. I hooked it up for the second time last night--kind of the same deal but it came to life much quicker.

Now, I know it's an unruly circuit, but is this normal? It was funny--the level of gain (or the bias?) almost seems to change on its own. Could the 47uf cap, which I believe is the power supply, be iffy? I initially had it in the wrong spot and had to move it, and it got pretty hot when I was soldering it to the place it's actually supposed to be. Then again, I was also testing it out of the box, during summer, in a room with a ceiling fan going. Too many variables, I know.

3) And lastly, how essential is it bump the 470r resistor up to 1k? I think I may be getting sub-unity output, but it's hard to tell without it hooked up to a switch to a/b it with the clean sound.

Thanks for all your help and for the research you undertook on the circuit last year!

Cheers,

Bret

[JakeFuzz]

Hmm its been a while since ive played around with the biasing in my MKII's but I could check the collector voltage on Q3. That last gain stage looks exactly like a fuzz face though and it has a coupling capacitor before Q2 so I would think Q3 is going to float around 4.5v like a FF. I have heard some people bumping it up though but I think I tried it once and it didn't help much.

That is really weird about the sound fading in. To me it almost sounds like a capacitor is charging up or something. It definitely doesn't sound right though. I would maybe go through and swap some of the electrolytic caps around the power supply and maybe that emitter bypass cap as well.

It is all up to your setup really whether or not you would like to change out that 470R. Note that it also adds some low midrange when you change that value. I swap that value out to beef up FF's some times. I would say play with it and see if you need more volume. With all the MKII's I've built though I never had to bump it up.

Also to get a more balanced bass sound (the MKII can be a treble monster!) I would slowly bump up the 10nF output cap and increase the value of the volume pot to 500K.

[Bret608]

Thanks Paul, that is helpful. I think I'll at least change out the 47uf cap where the -9v comes in and see what that does. This is what I get for just digging for parts at work! I have no idea how old the electros are.

Also, I'll check out the bias tonight and just see where it is at the moment.

[Scruffie]

The MkII doesn't follow the Fuzzface biasing convention, while you may like it there in general vintage units measure around 7-8v on Q3s collector.

But yes I agree does sound like a bad or wrongly orientated electro charging up.

[Bret608]

Thanks Scruffie--now I am wondering about an incorrectly oriented electro. I know I've got the big axial one right. But on the two radials, I had a hard time telling what was the negative side so I tried to look at the schematic to see. I think negative is the lighter-colored area on the ring that represents the electros' shells.

Would you mind looking at the layout I linked above and let me know if the negative side on those two radials should indeed be facing "north" i.e. the top of the board?

[Scruffie]

Thanks Scruffie--now I am wondering about an incorrectly oriented electro. I know I've got the big axial one right. But on the two radials, I had a hard time telling what was the negative side so I tried to look at the schematic to see. I think negative is the lighter-colored area on the ring that represents the electros' shells.

Would you mind looking at the layout I linked above and let me know if the negative side on those two radials should indeed be facing "north" i.e. the top of the board?

Yup that's right 

[snz728]

I tuned my mkii by ear then measured it@ 6.6 v sounded good.

[Cortexturizer]

It just so happens I am building a MK II too Just finished it last night - no sound
damnit. I measured almost perfect voltages, btw, when we are at it, what should be the voltages here? I am asking because voltages that I found on freestompboxes, home-wrecker, and general guitar gadgets are totally different. Although those on FSB and home-wrecker are in a similar ballpark. Pretty close.

I wonder why I get no sound...hm, gotta be a soldering error. Been on an error streak lately, I gotta take a vacation from this guys haha. Cheers

P.S. I found some excellent info about mk II tunning and a lot of other stuff thanks to you guys and the epic thread that Bret mentioned, great stuff!

P.S.2. How come that EVERYONE is frightened of doing an MK I? I mean, anywhere I go there's a person saying that doing a MK I is incredibly hard. And I am like, how harder that dialing a good sound of a fuzz face can it be, for example... What do you guys say?

P.S.3. And a question for you guys that made MK I and MK II, which one has more gain, which one has more bass, which one has a more focused sound, and which one has less spitty less gated-y sound? Thanks!

[Scruffie]

It just so happens I am building a MK II too Just finished it last night - no sound
damnit. I measured almost perfect voltages, btw, when we are at it, what should be the voltages here? I am asking because voltages that I found on freestompboxes, home-wrecker, and general guitar gadgets are totally different. Although those on FSB and home-wrecker are in a similar ballpark. Pretty close.

I wonder why I get no sound...hm, gotta be a soldering error. Been on an error streak lately, I gotta take a vacation from this guys haha. Cheers

There were some vintage unit measurements on the D*A*M forum, seems I saved them;

OC75 MkII (10K on Q1 base, 47K on Q2 collector)
Battery -9.01
Q1 c -8.50 b -0.038 e 0
Q2 c -0.13 b -0.076 e 0
Q3 c -8.29 b -0.13 e -0.08

OC81D MkII #1 (100K on Q1 base, 100K on Q2 collector)
Battery -9.55
Q1 c -8.51 b -0.06 e 0
Q2 c -0.13 b -0.068 e 0
Q3 c -8.72 b -0.13 e -0.07

OC81D MkII #2 (100K on Q1 base, 100K on Q2 collector)
Battery -9.50
Q1 c -8.76 b -0.062 e 0
Q2 c -0.22 b -0.075 e 0
Q3 c -8.13 b -0.22 e -0.14

[Cortexturizer]

Fantastic, I'm gonna measure mine and post them asap.
Thanks Scruffie!

Please address my other questions as well, if you don't mind
And I have another one - what about leakage?
I read somewhere that the circuit needs leakage to even work properly. Some wrote it is for Q1, and others that you need leakage on Q3. I suppose the latter is right..

[Bret608]

Glad to see more folks are joining the conversation and feeling the pain!    Well, mine does more or less work at least, just not consistently.

For what it's worth, I measured the bias on the collector of Q3 last night. I'm getting 6.3 volts pretty consistently--it only varied from 6.31 to 6.34. I've got 10k on the Q1 base and 47k on the Q2 collector as in Scruffie's first example.

Let me know if this points in any particular direction!

[snz728]

I used a 2kb pot for fuzz like the mindbender specs with good results.fwiw

[Bret608]

Sorry for the unceremonious bump! 

So for you folks with TB MkII/Supafuzz experience, does that 6.3v on the collector of Q3 sound suspect? If that is off, I most likely would suspect the 47uf power supply cap.

Cortexturizer, how is yours coming? To try and touch on your question about the difference between MkI and MkII sounds, I think the MkI tends to be more gated and temperamental, whereas the MkII wants to sustain forever. I think people get scared off of the MkI because the circuit is picky about transistors in a big way!

[Cortexturizer]

Cortexturizer, how is yours coming? To try and touch on your question about the difference between MkI and MkII sounds, I think the MkI tends to be more gated and temperamental, whereas the MkII wants to sustain forever. I think people get scared off of the MkI because the circuit is picky about transistors in a big way!

Well, mine is behaving like a little biotch   Here I made a clip of it, it can have wonderfull sustain, but is either gated or absolutely oscillating like crazy!
Here's the clip
[soundcloud]https://soundcloud.com/kuato-design-stompboxes/problematic-bender-behavior[/soundcloud]
FIrst I play with the bias trimpot and the Attack pot, and you will hear some really crazy sounds and oscillations. Then I bias it somewhere [hadn't looked] and play with the Attack pot, it sounds great but has all sorts of oscillating sounds underneath.
Then I put the Attack on zero, and things become better. It can have crazy sustain and is pretty gainy, but as soon I even touch the Attack pot it becomes absolutely unpredictable and really starts to struggle with me playing [a great example of this is at the end of the clip, I am having the attack on zero and then suddenly start turning it from zero to max in equal increments and it sounds like I am playing a synth]
It's crazy that I like it the way it sounds now  
but of course, I wanna make it sound as it should.
This on the clip is just like 15 different sounds concatenated, after every sound I changed something.

[Bret608]

Hey Cortexurizer,

Thanks for sharing--I had a chance to play with mine a bit more this weekend. I don't have an adjustable bias, so I wasn't getting as crazy of a sound as you, but the "normal" sounds are very similar. I have a quick question:

How do you find the volume knob interacts with the circuit? Mine cleans up very nicely, but there are some scratchy sounds as I'm actually turning the pot. I'm not sure if this is normal for the circuit or not.

I'm happy to report I'm not having the "charging up" issue any longer. The circuit is behaving a bit more stable and as expected. I wonder if, after I changed out the bad input cap, the new one was maybe a little old and had to "burn in" a bit at first?

Also, for what it's worth, I changed Q3 to one with the same hfe but slightly more leakage. After doing so, the collector on Q3 gets much closer to 7v (6.8 or so). To my ears, this did give a slightly improved sound.

Cheers and good luck,

Bret