Crazy Resistor Problem!

[soupbone]

I did some modding to my Dunlop Cry Baby Wah.3pdt switch with led.I changed all the transistors to BC109B's(NTE 123A's).1 of them is a MPSA13,and 2 MPSA18's.I made sure the pinout's were correct.I put in a 100k resistor next to the inductor.(most have 33k's there.)I put my meter on the resistor to make sure it was reading close to 100k.It read 97.7.After i soldered it in it read 15k!I don't understnad why this would be happening!Anyone have any clues?

[jasperoosthoek]

You cannot measure a resistor in a circuit. You just measured the resistor and everything connected to it. If it is in parallel with the inductor you will also measure the DC resistance of the inductor. I would expect a lower value though...

[soupbone]

Interesting.Thanks for the info!I'm still learning about circuits and how they work.

[jasperoosthoek]

Don't worry, I remember a PhD student in physics ask me the same question: "Why don't you just measure that resistor with a multimeter?" I think I replied to him that you can't ask electrons to only go through the resistor. They go anywhere they can go .

[starekase502]

it took me along time to learn this...also you cant measure a resistor by useing both your hands to clap it to your dmm leads. your body becomes a parallel resistor.  I changed the battery in my dmm 3 times before reliseing this

[Johan]

MPSA13 is a darlington and replacing it with a BC109 most likely wont work. if for nothing else, the bias will be off..put the MPSA13 back in and your probably good to go.
J

[ayayay!]

MPSA13 is a darlington and replacing it with a BC109 most likely wont work. if for nothing else, the bias will be off..put the MPSA13 back in and your probably good to go.
J

Mmm, you'd be surprised.  I've run that input buffer with a tranny w/ hFE as low as 150 and it was fine.  A little anemic sounding, but fine.  (I can't remember but I think it was a 4401???) 

it took me along time to learn this...also you cant measure a resistor by useing both your hands to clap it to your dmm leads. your body becomes a parallel resistor.  I changed the battery in my dmm 3 times before reliseing this

True, but you can clamp it on one side only with your thumb & forefinger.  As long as one end is free as you measure, it'll be accurate enough.    Same with your board soupbone.  Lift one side of that resistor and it'll be almost spot on, if not completely. 

[soupbone]

Cool.Thanks folks!I'm learning new stuff everyday!

[zombiwoof]

The MPSA13 is part of the buffer circuit, don't change it.  The other two MPSA18's are the high gain trannies to change.  And I would advise against the 100k in parallel to the inductor myself, I found it was too intense an effect with that, try something between 33k-68k if you find the same thing.

Al

[soupbone]

The MPSA13 is part of the buffer circuit, don't change it.  The other two MPSA18's are the high gain trannies to change.  And I would advise against the 100k in parallel to the inductor myself, I found it was too intense an effect with that, try something between 33k-68k if you find the same thing.

Al

Cool Al!I didn't know not to change the MPSA13!Can i put BC109B's,or 2n5172's for the other two?I like a 83k resistor for the inductor.

[zombiwoof]

The MPSA13 is part of the buffer circuit, don't change it.  The other two MPSA18's are the high gain trannies to change.  And I would advise against the 100k in parallel to the inductor myself, I found it was too intense an effect with that, try something between 33k-68k if you find the same thing.

Al

Cool Al!I didn't know not to change the MPSA13!Can i put BC109B's,or 2n5172's for the other two?I like a 83k resistor for the inductor.

BC109B's or any NPN trannies in the 350-400 hfe range are specified by some for vintage sound.  If you have two different gains, put the higher gain tranny in Q1.

Al

[soupbone]

So,I can use 2 BC109's and that would be fine?

[zombiwoof]

So,I can use 2 BC109's and that would be fine?

Unsorted BC109's might be higher or lower than the ideal 350-400 hfe range, if you can get some in that range it would be fine.  Even if they are higher than the ideal range they will work, but the idea to get the vintage sound is to have the lower gain transistors.  BC109B's are best because they are already sorted into a medium-gain range.  If you have a bunch of BC109's measure them to find ones in the target range or close to it, if you are going to buy transistors for the pedal just buy BC109B's to make it easier.  There are guys on Ebay that sell pairs of BC109B's for pedals, supposedly they will be in the good range, and BC109B's are also available from electronics suppliers.  Then again, if you aren't concerned with making your wah sound vintage, you don't have to be that picky about it.

Al

[soupbone]

I got a couple of NTE123A's(the bc109b equivalent)at my local electronics parts store.It says,hfe 200 typ on the package.Do you think that would be ok?Pedal Parts Plus has them too,but i don't know what the hfe is on them.

[zombiwoof]

I got a couple of NTE123A's(the bc109b equivalent)at my local electronics parts store.It says,hfe 200 typ on the package.Do you think that would be ok?Pedal Parts Plus has them too,but i don't know what the hfe is on them.

Personally, I stay away from NTE transistors, they are sorted and relabeled parts from other manufacturers and it's a crapshoot getting good ones IMO.  But if they are cheap, you could try them.  I'd buy more than you need and measure them, and get a pair in the 350-400 hfe range.  I think you're better off just buying a pair of tested BC109B's, there's some on Ebay.  Or maybe someone here has a pair in that range that they'd sell you.

Al

[soupbone]

How do you test the HFE?

[zombiwoof]

How do you test the HFE?

For silicon trannies like those, I use my multimeter which has a transistor checker.  For germanium transistors it's more tricky, you have to set up to test for leakage, as they are more prone to leakage.

Al

[soupbone]

Cool!Yeah,I need to get a good multimeter.I just have one of the pocket size meter's from radio shack.