newbie help with rangemaster bias question

Started by noco, April 22, 2014, 05:22:06 PM

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noco

I am new to pedal building. I've started with a breadboard to help me get my head around this stuff.

First thing I did was follow the tutorial for breadboarding a PNP treble boost. I'm not sure if it's ok to mention that site or schematic so I'll hold off for now. It seemed like a pretty easy build and it helped me get acclimated to the breadboard. Everything worked as I thought it should and the circuit sounded pretty good through humbuckers.

THEN, I read the Keen article on dialing in a rangemaster.

When I measure the collector voltage to ground I get the battery voltage - 9.3VDC. I tried adding pots in place of the biasing resistors and I can move the voltage, but not much. I know I am doing something wrong but I'm not sure what.

I should say that my first Ge transistor was a NTE something or other that was labeled with a "typical" hfe around 90. The second one I used I bought in spec for this build from the store on this site. They actually both sound pretty good in the circuit. The first one gets pretty harsh and brittle when I max out the gain, but the second stays nice and smooth. Which is what I would expect after buying the first one blind at a local electronics store, then trying to get something specifically for this type of pedal.

So, just placing the the DMM black on ground and red on the collector bus of the breadboard in a PNP circuit should show negative VDC?

I'm inclined to believe that I'm measuring incorrectly as it seems these circuits are working and I get the same readings on both. But, I may be laying things out wrong too. Thanks for any help offered. I've done a lot of searching and reading and my head is spinning a little!

mth5044

Posting the schematic and layout would usually help a lot.

For PNP circuits, you are going to have a postive ground, so reverse the leads (red to gnd and black to the supply). If you can show us how you put the pots in, it would help. Sorry if you did, my work blocks a lot of images, but you didn't reference it so I'm assuming not.

LucifersTrip

Quote from: noco on April 22, 2014, 05:22:06 PM
I'm not sure if it's ok to mention that site or schematic so I'll hold off for now

you can post almost any of those vintage circuit schematics. you mainly have to worry about current in-production stuff.

Quote
So, just placing the the DMM black on ground and red on the collector bus of the breadboard in a PNP circuit should show negative VDC?

yes, when reporting voltages from a pnp circuit it's customary to give (-) voltages. I'm not sure what you mean by "collector bus", but
regardless, give us all the voltages for the transistor and we might see where you're going wrong with the - 9.3
always think outside the box

noco

I may have found my mistake. I reversed the emitter and collector of the transistor and also swapped the positive and negative leads. I am now able to bias via the 68k resistor.

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/tb_schem.jpg.html

Here's the board before I made the change:

http://www.aronnelson.com/gallery/main.php/v/diyuser/noco/noco_bb.JPG.html

Am I on the right track?