Sparkle Drive Problems

[silentmike]

Hi everybody!

I've just started getting into building my own stompboxes and am building a Valvecaster with a switchable Sparkle Drive inside. After lots of work, I finally got the Valvcaster working yesterday after having put it on vero, deciding that was stupid and then stripping the vero building on the back of the valve holder and messing about. But now I'm having problems with the Sparkle Drive...

I'm using this layout for the Sparkle Drive:
http://www.aronnelson.com/gallery/main.php/v/diyuser/Sparkleboost.gif.html

When I turn the trimpot up, the top 4 lines all seem to be receiving 9V (or close to), but I can't see any connections that shouldn't be there. The ground lines are all in place and the transistor is an MPF102 which seems to be the right way round as far as the DSG bits go. I'm sure that the polarized capacitors are in the right way round and I haven't subbed any of the component values at all. At the moment I've just got it hooked up to the pots and the jacks, so no switching at all. When I plug in I'm only getting mains hum when connected to a power supply and with a battery it's much quieter, but still no guitar coming through. I've double and triple checked the wiring, but to no avail, I'm stuck! 

Can anybody please help me?

[silentmike]

Here is a picture of it currently, if that helps...



Ta!

[Ripthorn]

This may seem like semantics to you, but the sparkledrive and sparkleboost are totally different effects.  That being said, this is a great build.  Did you make sure to jumper two of your trimpot pins together?  Also, did you bias your FET to ~4.5V?  If you could post a solder side shot of the board, that would help as well.

Edit: did you make the appropriate cuts on the right traces?

[ampman50]

I'm not seeing the 2K7 resistor from source to ground on your board.

[silentmike]

Sorry! I did mean Sparkle Boost. Must have been confusing myself...

Anyway, here is the back of it. It's rather untidy, but I've circled the cuts and marked the 9V and ground lines for bearing.



I did jumper the trimpot pins. I only saw it on the schematic this morning. I don't know how to bias the FET though - so I'll shall look for a guide to help me!

Also, Bill, I didn't see that I needed a 2K7 resistor from source to ground. I've got the one going from ground to S on the transistor. I thought that was all I needed?

[Ripthorn]

The trim pot is what biases the FET.  You adjust the trimmer until you get ~4.5V on the drain (D) of the FET.  S is for source on your diagram.  If the trimmer is turned all the way to one side or the other, you won't hear anything.

[silentmike]

OK, right. I've got it to 4.32V (it's so touchy that's as close as I could get). So, the voltages I've got across the transistor are:
D : 4.32V
S : 4.17V
G : 4.32V

I'm still getting no signal into my amp though. In order to get a couple of parts to fit on this bit of vero (I tried my own layout first. Didn't go too well) I had to re-solder on bits of old legs that had been chopped off. Do you think that this might be affecting it?

[Ripthorn]

Judging by those voltages, my guess is you fried the transistor.  Having approximately the same voltage on each pin is usually a sign that something got roasted.  Try swapping it out (I would put in a socket) and try a new one.  I would say build all your first projects with transistor sockets to make sure you don't fry any transistors (a very frustrating thing, frying a transistor is).

[silentmike]

Aha! Well then it is all good as i have a spare!   Though a transistor socket would be a very good idea. Anyone know of anywhere in the UK that sells them?

Thank you very much for your help so far Brian! It has been most useful! Hopefully I won't have to bother you with too many more questions.

[Ripthorn]

Don't look for transistor sockets, look for SIP sockets (they are a long line of pins that you can snap off).  If you were bothering me, I wouldn't be here, so no worries .  Besides, you will have plenty of questions as you move forward.  That's why we're all here.

[silentmike]

Hahahaha! It works now!   Now just to put in the switches and LEDs and I've got a fully functioning pedal! Thank you very much again for your help! Next time I will try not to fry the transistor!

[silentmike]

Right. I've got another small problem now... When I had the valvecaster on the breadboard and fed a sparkle boost into it, the whole thing went to proper overdrive (I'm using it with bass so it broke up less with the standard circuit anyway, hence the need for the sparkle boost). But now that it's all working and wired in, the SB isn't effecting the VC very much at all. Volume stays the same and if anything, it sounds a little cleaner. I'm gonna take it over to my brother's tonight, and see what it does to a guitar, but I got a much more distorted signal before. Anyone know why this might be?

[Ripthorn]

Sounds like something is happening to make the output of your sparkle boost be less than unity.  This could be a short with offboard wiring or on the board.  I would stick an audio probe in at the end of the sparkleboost and see if it is louder than just the guitar straight in.  Try both the volume and tone pots up a ways.

[silentmike]

Yeah, for all the good that the sparkle boost is doing me, it may as well not be there. It seems to lose a little bit of volume as it leaves the board, so I'd think it was something on there, but I can't find anything. Also, I checked the transistor again (now with a J201) and I've got:

D : 4.51V
S : 0.44V
G : 0.05 when on the 200mV setting

I would have thought that G would want to be higher than that? Perhaps it'd be worth taking it all off the vero again and trying it on the breaboard? Or perhaps re-doing the vero a bit neater?

[Ripthorn]

I would simply disconnect it from whatever follows it and test just the sparkleboost between your guitar and amp.  Then use an audio probe to see what the level is like just after the transistor.  I will see if I can get some voltages from my sparkleboost and post them here for you.

[silentmike]

Ok, I will do. Kinda wish I bought a lucky bag of capacitors the other day though. This means another trip to the shops!

[silentmike]

Right, I'm not really sure what to expect from probing. I can hear a signal from everywhere on the circuit except the ground and 9V lines, all at about the same level. The only place that I'm getting a dip in signal is at S on the transistor.