biasing a Germanium NPN (Rangemaster)

[Britt Termaine]

New to the forum, but I have been lurking for a while. I have done a few mods which are below:

1) I started off slow and did a simple mod to my Boss SD-1. I changed the diodes to LEDs and I changed a resistor and cap to increase the gain. It is amazing what changing only 4 parts can do.
2) The next mod I did was the Keeley Seeing Eye mod for my Boss DS-1. This worked like a charm and right now, its my favorite pedal.

After the first two mods, I decided to build an LPB booster from scratch. I was able to get that working with essentially no problems (I had an intermittent short and once I tracked that down, I was good to go).

As it turns out, the LPB was not the type of sound I was looking for so I decided to go for a more vintage flavor of boost and decided to build a Rangemaster.

I ordered a couple of Germanium NPN's from SmallBear and I was assured that the leakage for these transistors was low enough so that I would not have an issue.

However, I am having a lot of trouble trying to get it working. I have two transistors and I have not been able to get either to work correctly.

I have done a bunch of searches but I have not been able to find a thread that shows the easiest/best way to set the resistor values to bias the transistor correctly.

I did read RG Keen's PDF file on the "Austin Treble Blaster" but he describes how to bias a Germanium PNP. I tried applying the same methods as outlined in the PDF file but obviously I am doing something wrong.

Does anyone have any hints/tips on how to bias a Germanium NPN for a Rangemaster?

Any help would be greatly appreciated.

Many thanks.

Britt

[frank_p]

Post removed: did not read well.

[PRR]

> have not been able to get either to work correctly.

"Doesn't work" is not much to go on.

What DOES it do? Silence? Garbage? Smoke? Sparks? Zero volts everywhere?

Do you understand the difference between PNP and NPN? (Apologies if you know this; it is hard to gauge someone else's understanding from a single post.)

Are you trying to follow some specific instructions?

I found that GEOfex has R.G.'s advice. What value of Re (originally 3.9K) are you using? What collector voltage can you get?

As I see it: reverse all polarities for NPN instead of PNP. Keep collector load at 10K pot, stick with 3.9K Re and 470K for upper base resistor. The lower base resistor is adjusted for the specific gain and leakage of your part. Leave it open, a very-good transistor should pull emitter and collector together, both near 2.5V. If the emitter won't come up to say 1.3V, gain is awful low.  

Now lift the Base end of the 470K. An ideal transistor should cut-off, the collector rise to full 9V. Germanium leaks. If the collector won't come above about 8V this way, your part is too leaky for this low-current circuit.

-OR- you are mis-wired. While it is comforting to blame the parts, 99 times in 100 the parts are good, you are overlooking your own mistake. Trust me, I'm an expert mistake-maker, and my bad-parts pile is essentially nil (not counting the ones I killed with my mistakes). Yes, buying ancient parts is much more of a gamble than current production. But Small Bear knows what you are doing and has probably given you good parts.

If you can get <2.5V and >8V at collector, good. But either way it won't pass audio: we just proved the transistor can be jammed into saturation and cutoff.

Now put back the 470K and add a lower base resistor. From remarks on GEOfex the emitter voltage should be in the 0.65V to 0.86V range. For a perfect transistor, the lower base resistor should be about 58K. Some assumptions about gain and leakage suggest that a real Ge device might need a 200K kick or a 15K suck to get to the desired bias. Try 47K fixed in series with 100K trimmer. If you can't get near 7V at collector that way, something is wrong.



BTW: have you checked the E-B-C pinout? They are not all the same. It is critical to know which pin is Base. At 9V, some transistor circuits can work with Emitter and Collector reversed, but a low-gain or hi-leakage Ge transistor may not get happy in inverse connection.

[jdub]

I bought a couple of the Rangemaster germaniums from Small Bear and they worked well.  On the ones I received, Steve had given resistor values specifically tailored for each trannie to be subbed into the stock Rangemaster circuit.  Have you tried these values?  Apart from that, PRR is right- more info is needed...

[Britt Termaine]

Merry Christmas!!! 

Do you understand the difference between PNP and NPN? (Apologies if you know this; it is hard to gauge someone else's understanding from a single post.)

Well, I am not an engineer so I do not know all of the technical differences between an NPN and a PNP.

I do know they are different and that you cannot substitute one for the other.

By the way, no offence taken since I am a newbie.

BTW: have you checked the E-B-C pinout?

Yes, that was the first thing I checked so I am 100% sure I have the pinout correct.

> have not been able to get either to work correctly.

"Doesn't work" is not much to go on.

You are right, sorry about that.

I started off with:

1) A 3.9k resistor (Re) from the emitter to ground
2) A 68k resistor (Rb1) from the base to ground
3) A 470k resistor (Rb2) from the base to +9V
4) A 10k pot wired at the collector (leg1 going to collector, leg2 being output, and leg 3 going to +9V)

I did get a signal but it was about half compared to just plugging straight into the amp and rotating the pot did not vary the output that much. It also sounded bassy and muddy (not sure if that helps any).

I then tried using RG's technique where I wired a 100k pot for Rb1 and a 10k pot for Re and did the adjustments as outlined in this PDF file from RGs website (please see page 4 where RG says "I recommend you do the actual tuning this way:")

By tweaking the above, I was able to find a spot where the output was louder but it was still less then compared to just plugging into the amp. It was also brighter sounding.

I essentially get the same results with both transistors. Maybe I should also ad that I am doing this on a breadboard. I did this on a bread board because I knew that tweaking Germainium transistors can be difficult so I wanted to find the right resistor values before soldering them into the board.

I don't have a meter right now but my friend says that I can borrow his (I will get it after tomorrow) for a few days so I will give your suggestion try?

Any other hints/tips that I can try in the meantime?

Thanks for the help.

[PRR]

> ...differences between an NPN and a PNP.

Did you reverse the battery and the electrolytic capacitors?

Current flows one way in PNP, other way in NPN. If you have a plan for PNP devices, and you only find NPN devices, you "simply" reverse all the "+" and "-" signs.

A meter would be good to have. You can't "see" electricity. The $3.99 DMMs you can find on eBay work OK (for a while). Small Bear's $50 meter is surely more rugged and has more features; if I didn't already have 6 or 7 meters, I'd get something like that.