Phatt (he has given me some ideas to try), recommended that I post the schematic here as there are many SS experts here that could help resolve my pedal problems. First I need to give credit to Torben and kusi at FSB for the schematic and Sabrotone for the layout. This is a dumble style pedal similar to the ROG umble. The schematic is in a pdf form so I added the link to it, http://www.freestompboxes.org/download/file.php?id=1579. I have built the pedal following the schematic and layout. It works but not as I had hoped. My main problem is I'm good at reading schmatics but need help with modding them. The pedal has what I believe is called "ice pick" overtones when the drive or gain is turned up or when heavy picking, (far from the smooth overdrive I was expecting). I was expecting a bit more midrange control. Others compare it to "chewing tin foil". This sound compares to my MIM strat with Tex Mex style pickups which I changed because of the ice pick sound. If some one with experience could look over the schematic and offer suggestions I would greatly appreciate it!
Gotta be a member to download.
Could you post it here please?
Yeah Gets confusing try to cross reference between sites xP
I've added it here.
my first thought was slope resistor might be too large (R22)
Phil
Thanks Phatt! I was not sure how to set the link.
Thanks to phatt for posting the file here!
Some questions about your build:
1) Did you use J201s or some other JFET?
2) What is your Supply voltage? Have you tried any other?
3) How did you adjust all the trim pots? Have you tried tweeking the pots slightly, maybe a volt high or low from the "ideal"?
4) The slope resistor (R22) seems high, Can you install a trim pot there?
5) Regarding R32 (location F3), did you use 470 ohm or some other value?
Thanks for asking!
1) I used J201's.
2) I use a Dunlop "Brick" 9vdc/8vdc to power all my pedals.
3) All trim pots were adjusted to 4.5V. I have not tried any adjusting the input voltage. I believe that these are "cascaded" gain stages?, and I actually thought of cascading the voltage but I have no idea what that might achieve.
4) Installing a trim pot would be difficult but possible. I believe I might have a 50k or 100k I could try.
5) R32 = 470 ohm
6) Thanks very much!
100K Trim pot installed! Had a bit of time to tweak and settled in the area around 85K until I have more time. 68K seems to be a bit muffled. Certainly you both were correct, 150K is to high for my tastes. What I really need is someone to turn the pot while I play at different intensity to get a real feel. What effect would reducing the bias voltage's have on the gain stages? When the drive or gain knobs are maxed out the gain is more than I would ever use. I do have the trim pot for the overdrive input VR2 set very low? Could this maybe be to low for the overdrive section? In my unknowing mind, could this mean the overdrive stages are trying to gain up a signal that is too weak?
Quote from: sjturbo on February 20, 2014, 04:43:40 PM
What effect would reducing the bias voltage's have on the gain stages?
This thing has lots of stages and most have some tone shaping between stages. There are three sections. The first section, Q2, Q3 should preserve a fairly clean signal and just add low amounts of distortion. 4.5V or 5V would be good for those stages. In the OD section, anything goes. This section is where the Ice Pick tone probably comes from. Try tweeking VR3 high and VR4 low, or vice versa. You can't hurt anything, just try to get a pleasing tone without the Ice Pick treble. The third section around Q7 and Q4 will have to be a compromise to get good tone in both modes. You should probably stay close to 4.5V but try anything. VR7 gives a treble rolloff, use it in moderation to get rid of fizz. I suggest you just adjust the pots by ear, take notes and then measure and write down the voltages. It may take some time to find what you like.
Thanks much Loudthud! Sounds like plan. Tweaking is good when you can put all the pieces back! I think I may play with the bias voltage in the tone stack section also, I believe it has more gain than required. Interesting you mention the tone shaping on each stage.
The tone controls (T,B,M) all seem to interact. I still don't hear much difference in the 80's/90's switch. I would like to increase mids on one position of the switch. Would that just require a change in the rc network?
Again thanks for everyone's help!
Just simulated the first 2 stages and it's ho hum average :-X
but I'll tell you now that dumble tone stack is just way overrated.
I should know I've tested most of the famous tone stacks in the last 20 years 8)
My advice;
R22 try around 50k
Remove C2
C4 and R17 does nothing except a slight 2dB loss below 1khZ. up to your ears?
This screen shot shows the difference between the given schematic "Yellow trace"
and above changes "Green trace"
This tone stack can only deliver about 12dB of tone change.
There are better ways likely delivering more than twice that control with less parts,,if you are interested?
As loud thud notes the next part could be anywhere but I'd get the front working first.
Phil.
{willya lookit all those furshlugger trim pots! :duh }
Quote from: Roly on February 22, 2014, 12:15:48 AM
{willya lookit all those furshlugger trim pots! :duh }
It probably cost more for the trim pots than the JFETs. But the upside is the endless possibilities for tweeking. Nice to have one of those little plastic screwdrivers made for tweeking pots. :)
It was hard to part with but I traded one furshlugger for all the trim pots;). And yes I can tweak all nite long:0 ?
Next- Back to reality, Phatt, taking your advice I really don't need a 2db loss. So, I used another trim pot to dial in R22 to 50K, and removed C2, C4, and R17. Spousal unit is napping currently so I cannot try it out.
What's next?, stage 3 and 4 or 5,6? Twice the control would be nice on my next pedal which will likely be as soon as I can wrap this one up. Thanks again for the help and humor!
One step forward, two steps back. Modded to Phatt's suggestions and biased overdrive stage fets to 5v (?) based on tone, sounded good and turned pedal volume up and poof! something went south. No sound unless turned all the way up and then just faint in the background sound. Will have to troubleshoot tomorrow.
Update! Found the problem. One of the fets had a slight disconnection when measuring. I also returned the bias to ~4.5v. Played for about an hour just tweaking knobs. I will resume bias tweaks on the overdrive stage tomorrow. Oh, everything sounds very good except that when the Level or Drive are increased on the OD channel it sounds like I get noise like water flowing?
Thanks again!!
Better minds here can explain the details of this issue meantime here is my observations.
The dreaded hiss issue is likely due to every stage having hi Z input. :-X
Take a look at the classic tube screamer circuit.
Note the input has a buffered input (a BJT on early units ,,later just an opamp stage)
the next stage has the gain but with a much lower input Z. :dbtu:
What may not be obvious to the casual observer is that makes for MUCH less hizz and buzz and helps keeps the thing from wild oscillations. 8|
Some Boo-weak pedals actually remove that buffer claiming a much purer sonic result. :lmao: :lmao: :lmao:
What is sad is that the masses buy/build that crap thinking they are getting something better but in truth all you get is noise and a whole lot of problems that just makes life harder. Oh yes you may get x2 more gain but at the expense of x10 more problems.
I don't have the exact answer for you but it seems you have this on a test board of sorts so experiment with other options.
I know a lot of those starting out assume some circuits on the net that get rave reviews must be good as so many build them but until you have a bit of experience under your belt you have no real way to do valid comparisons so you don't really know how good they are.
As others here with heaps more experience than myself will tell you this is not a good design.
But keep experimenting it takes a lot to get a real picture of how tone is won and lost.
Phil.
Quote from: phattI know a lot of those starting out assume some circuits on the net that get rave reviews must be good as so many build them but until you have a bit of experience under your belt you have no real way to do valid comparisons so you don't really know how good they are.
As I have been heard to say "Compared to WHAT?". Mainly in the context of computer stuff, operating systems, software, languages and so on, because so often the raves come from people who are basically newbies who
don't actually have any experience of anything else to compare.
A very telling question is to ask "so, what is
wrong/not good about the thing you are raving about?" and if you can't get a sensible answer then you can safely dismiss the rave as uninformed and one-sided because whatever we are talking about has an area of application, and outside that area it's not so great - it may be a wonderful spanner, but it ain't a screwdriver. There is no such thing as "one size fits all".
Thanks Phatt, for the buffer/high Z thoughts! I will mull over some possible changes. I am actually quite pleased at the present with the tone, other than a bit to much bass and the noise issue, I am getting out of the pedal but please don't mistake my enthusiasm for raving. I realize I am a novice at tweaking build pedals but I have been listening to them since I bought my first Maestro Fuzztone in New York in 1966 after hearing "Satisfaction". I try not to base my decision to build a pedal on hype but there is always the first time. Again thank very much for the time you have spent on my build.