Stratoblasters, crazy pots, and hamsters being raped by whales... need help!

[aka_basse]

Allright, i've built a stratoblaster pedal!

I dont know shit about electronics, and this little fella doenst quite work as I want it to:

First of all the gain/volume pot isn't of much use. Only the very last turning of the pot gives a boost?! From something like 70% and up you can change the amount of boost. But anything below that sounds the same. If I have the knob up halfway, I cant hear any significant difference between having the pedal on and having it bypassed.

The other thing, is that the pedal goes crazy when I use it to boost the full gain (dist channel) of my amp. It gives a high pitched feedback, quite like the sound of a hamster being raped by a whale.  With gain turned up on the clean channel of the amp it actually sounds good, which is quite amazing since the clean channel usually sounds crap (gives a crunchy distortion).
When I tried the pedal without case, it didnt give that hamsterwhale feedback, only a shitload of hum. By looking at the inside of this puppy kinda gives you the idea that its the cables which are the problem (The sight would make any electrician cry instantly).

Another relevant fact may be that my amp is a piece of shit marshall valvestate which perhaps doesnt react the same way with a booster as a tube amp?

Whoa, think thats it. I would really appeciate some help with this mess if anyone of you had the time

Edit:
http://www.generalguitargadgets.com/index.php?option=content&task=view&id=96&Itemid=124
where you find all the schematics and stuff

[brett]

Hi.

It gives a high pitched feedback

That happens to a lot of us when we use a high gain pedal into a high gain amp.  There are a few things you can do.  A popular approach is to do some "chopsticking".  This means opening the box and getting a plastic or wooden stick and touching various wires and components.  If you get weird results when you touch one, resolder it or anchor it.

Also, check your earth connections:  input and output sockets, battery and circuit board.

cheers

[Somicide]

Basic stuff, more than likely.  sounds like you have the wrong type of pot.  Make sure your wires are as short as they can be, only as long as absolutely necessary.  Check all your solder joints for cold joints.  Hope that helps!

[dr]

.....I could really use a pedal that sounds like that....mine sound like whales being raped by hamsters.....

[petemoore]

  I like to check drain voltages after determining that the Jfet Boost does in fact boost...so to say if it's boosting it's probably at least working.

[aka_basse]

thanks for the help, ill try to do some of the things you guys mentioned!

I tried the pedal today, (and this time there wasnt anyone sleeping in the house so I could crank the amp up) and realised that I had put the pedal right in front of the amp yesterday. So moving it away reduced the feedback to a controllable level.

The strange thing is when you knock or move the pedal casing, that sound is amplified. I cant find anything that touches the case from inside which would make it a part of the curcuit so to speak.

Basic stuff, more than likely.  sounds like you have the wrong type of pot.  

Its a 100k linear pot, just like the one the GGG site says its supposed to be.... strange isnt it?

[Somicide]

that is odd; i checked the parts list, and indeed it does call for a lin pot.  What you describe though, is what happens (or very similar to) when you put a linear where a log pot should go.  Strange.

[Mark Hammer]

The "taper" of pots matters when:
a) you need to use the position of the pot to visually convey information to the user (e.g., if the "balance" control is set to midpoint then left and right are at the same volume)
b) the control covers a wide enough range of settings and the user needs to have them evenly distributed across the full rotation of the pot.

In this case, the first aspect is not all that relevant.  The second one clearly is, though.

In a great many instances, exotic pot tapers are recommended for circuits largely to make it possible to cover an overly broad range of settings while still permitting somewhat even distribution of settings across the rotation of the pot.  The case that instantly comes to mind is the MXR Distortion+.  The stock version uses a 1M gain pot with a reverse-log taper.  People drive themselves crazy finding one, but here's the thing: the first 900k of that pot's resistance doesn't really DO anything.  From the point where it is at full resistance to the point where it is down to 100k, the gain changes only minimally, and certainly not enough to introduce audible changes in distortion.  That's right, the reverse-log aspect is only necessary to get you through that 900k in a hurry so you can get to the settngs that actually DO something.  Only the last 100k really matters and if you replace the 1M reverse-log with a 100k linear, you probably won't notice any loss of diability and certainly no change in functioning.  Brilliant planning, huh?

So, notwithstanding the possibility that your pot is not in the greatest of shapes and is only functioning properly for part of its rotation (a very REAL possibility; happens to me all the time), what you may want to do is figure out what portion of the pot's range is of most interest to you and replace it with something that only covers that range.  The GGG Stratoblaster schem shows a 100k pot.  Happily, the 0-100k range has a number of standard-value pots falling within that range: 50k, 25k, 10k, 5k, etc.  What you may want to do is measure the 100k pot setting and see what the resistance is when you get to settings that, to your ears, "matter".  E.g., if you find that you don't hit meaningful changes in gain until that pot is down to 30k, then you may find that a 25k or 50k pot is actually going to provide you with more "dialability". 

Note as well, that you can easily modify the taper of pots as well as alter their overall value by means of parallel resistors.  See the "Secret Life of Pots" article over at www.geofex.com to find out how its done.