NPN rangemaster biasing debug help

[tonefiend]

Hello Everyone,

Im trying to breadboard an NPN version of the Dallas Rangemaster...  first build of this particular pedal..   im using a 2n3904 silicon tranny
for right now while im learning...   ultimately I will be using a germanium 2n1306 that I got from SmallBear.  

my problem is that when I take measurements at the collector to ground,  my reading is always 9.3 volts, which is the voltage of my new battery
that I am using.   My base voltage is .933 volts.   Im using the nominal 470k and trimpot set up to dial in the bias, but no matter if I use
all the trimpot settings same value always appears at the collector....    Also,  If I measure from collector to where the emitter is grounded
I get a different value much lower,   1.68v reading?    hmmm what to do?   Ive looked up and down at my breadboard and cant find where
I went wrong...      I get the same result using the small bear germanium npn with supplied resistors as well...  







here is a link to the schematic  http://www.generalguitargadgets.com/pdf/ggg_geb_npn_sc.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

any help would be greatly appreciated..    

please know I did try an extensive search and poured through lots of biasing threads before posting

[smallbearelec]

Hi--

Something isn't wired right, and nobody can help you without actually seeing what's there. Pls find a way to get a Clear pic and post it.

[mth5044]

A picture is key. Also please post a schematic when asking for help. Use the layout of this thread to post so we can help: http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

[tonefiend]

Hi--

Something isn't wired right, and nobody can help you without actually seeing what's there. Pls find a way to get a Clear pic and post it.

i cant figure out how to insert a pic...   

[tonefiend]

ok got the pics up..   pic 1 is the input section of my breadboard showing C1 .0047 capacitor R1 470k going to positive voltage and R2 68k going to ground, with green
jumper wire going to base of tranny 2n3904 which is in pic 3.  I left the R8 1.5 meg pulldown resistor off so you could see pic better.

pic 2 shows output section with purple wire going to the wiper of 10k pot C2 .01uf output cap, 1.5meg pulldown resistor to ground

pic 3 shows tranny,  white wire going from collector to Lug 3 of 10k pot,  green wire going from base up to junction of r1 and r2
emitter going to R4 3.9k to 47uf to ground

Lug 1 red wire of 10k pot goes to positive, lug 2 wiper purple wire to output, lug 3 white wire to collector

DMM readings:

C =collector to ground 9.61 volts (new battery) or 1.39 if i probe from collector to emitter ground?
B = 1.19 volts - this should be about right based on (supply voltage * (r2/r1+r2))
e = 0

[smallbearelec]

Hi--

I'm looking at the third pic. On many of these breadboards, the power bus is split in half. If you are using the whole length, the halves must be joined with jumpers. It looks like this is not the case, so check that and measure again.

BTW: Once it's wired right, you may still not be able to get it to bias. Both the 2N3904 And the 2N1306 are higher gain than this circuit is looking for. Doesn't mean it won't work, but the result may not have the subtlety of the true RM. Once you have a working circuit on the breadboard, I can give you a few suggestions for going forward.

SD

[tonefiend]

Hi--

I'm looking at the third pic. On many of these breadboards, the power bus is split in half. If you are using the whole length, the halves must be joined with jumpers. It looks like this is not the case, so check that and measure again.

BTW: Once it's wired right, you may still not be able to get it to bias. Both the 2N3904 And the 2N1306 are higher gain than this circuit. Doesn't mean it won't work, but the result may not have the subtlety of the true RM. Once you have a working circuit on the breadboard, I can give you a few suggestions for going forward.

SD

Ha!   that was it,  the power bus has a gap on the breadboard...   another lesson learned,  thanks very much for your help,  will try and bias up the germanium and let
you know how it goes..     thanks again

[smallbearelec]

another lesson learned,  thanks very much for your help

Good job, and you're welcome! That's a very common issue that drives many beginners nuts. My suggestions now:

The 2N3904 typically has a gain of about 200, which is out of the correct buckets for the RM. The 2N1306 Can get below 100 where you really want to be, but I know this lot, and most parts run much higher. This is not a bad thing for other purposes.

If you find that you can't properly bias the devices you have and want to know what a "real" RM sounds like, save them for building something else...maybe an Ursa Minor:

https://www.smallbearelec.com/HowTos/BreadboardUrsaMinor/BreadboardUrsaMinor.htm

and do a little more shopping. For the RM, Low- to medium-gain silicon devices are not expensive. Try the 2N697s that I just posted, or a 2N2369A. If you would like an assortment of reasonably-priced germanium NPN that will perform Well in the RM, look here:

http://www.smallbearelec.com/servlet/Categories?category=Germanium+Transistors%3ALow+Gain+and+High+Gain

These are "remainders" from our sorting process. They aren't quite "hot" enough to sell for top dollar, so they can be super values. Also note that low-gain parts can be cascaded to create "perfect" RM devices for not much money:

https://www.smallbearelec.com/HowTos/BreadboardGeDarlingtons/BreadboardGeDarlingtons.htm

Happy experimenting!

SD

[duck_arse]

that big green electro, and the lowest resistor: are they connected to anything, or is it a trick of the light?

damn light, always playing tricks .....

[digi2t]

If ever you decide to try a PNP version, try a 2N2904A. They come in around 70 to 85 hfe, and work wonderfully well. Got one in my Skyripper, never looked back.