Biasing a rangemaster type circuit

kgemenis@mail.com · Post by **kgemenis@mail.com** » 13 Mar 2023, 01:06

I have a rangemaster circuit on my breadboard and I am trying to get it to sound right. I tried several PNP germanium transistors which I measured for gain (selected those with 70-160) and leakage (selected those with <150ma). I am not sure I am biasing the transistor right though, so any help will be appreciated. Here is what I tried so far:

I start with a R1/R2 voltage divider where R1 is fixed at 470K and R2 is a 100K trimpot. I adjust R2 to get -1V at the base and then measure the voltages at the collector and emitter.

For transistors with a gain around 100 (which is supposed to be ideal) adjusting R2 to -1V usually gets me the emitter at -0.8V, but there is no way I can get the collector at -7V. No matter how I adjust R2, I always seem to end up at around -4.5V at the collector.

Higher gain transistors get me closer to the -7V at the collector (at the expense of a little deviation at the emitter) but I get a lot of hiss/noise when I crank up the volume in the circuit (and the gain of the preamp that I am feeding it to).

Lower gain transistors get me the desired voltages all around and the noise is gone, but when I measure resistance across the potentiometer lugs I get 7K, which I find surprisingly low.

What is more, I am not sure that I am getting the rangemaster tone with either of the transistors. The distortion that the circuit is producing decays rather fast, leaving cleaner sustaining notes.

I was wondering whether I am doing something wrong or whether it's just a case of the setup and whatever I have on the breadboard.

mauman · Post by **mauman** » 13 Mar 2023, 03:59

Welcome!

Could you post the schematic you're working with?

Your transistors have a beta in the right range (75 to 100.) To get a correct bias, you may need a trim pot between the emitter of Q1 and ground (stock in this position is a 1K resistor paralleled with a bypass cap.) Yes, the Q1 collector should bias between -6.8v and -7.1v based on the ones I've built.

kgemenis@mail.com · Post by **kgemenis@mail.com** » 13 Mar 2023, 04:13

Thanks for the quick response. I am working with the schematic that has a 3.9K resistor from the emitter to ground (e.g. https://fuzzcentral.ssguitar.com/rangemaster.php or https://www.electrosmash.com/images/tec ... rcuits.png)

mauman · Post by **mauman** » 13 Mar 2023, 18:09

Yeah, try using all your stock parts, except reduce that 3.9k between emitter and ground. As you decrease it toward 1k or so, your collector bias voltage should move from your original -4.5v closer toward -7v.

tonyharker · Post by **tonyharker** » 13 Mar 2023, 19:58

A true Rangemaster is not a distortion device but a treble booster. The output should not be distorted.

CheapPedalCollector · Post by **CheapPedalCollector** » 14 Mar 2023, 03:04

It should compress a bit and yes, even have a bit of distortion when you slam it with a humbucker, or a high output pickup.

The circuit is not configured for max gain, although some units were (with a 20K pot). 18K is about the ideal collector load for maximum output, and you can test a bunch of 20K pots to find one if you want. You may need to adjust the base bias up a bit to avoid cutoff. 600k (a 270k and 330k in series gives that) and 100k divider works nicely for this. I tend to use a 5k trim pot on the emitter to dial it in perfect.