Picking transistors for FF Clones

Copyright 2000 R.G. Keen. All rights reserved. No permission for local copies or serving from pages other than http://www.geofex.com.

You've determined to build the best Fuzz Face clone in the world. You've collected parts, including some dyn-o-mite PNP germanium transistors and are just itching to get soldering. But- out of that batch of germanium transistors, how do you tell which are going to sound good and which will not? To a first order, you can just gain select them in a DMM that has a transistor checking range. However, all modern DMM's assume that the transistor being testing has no leakage at all. They just put a metered amount of base current in and look for how much collector current comes out. With germanium's inherently higher leakage, this just makes a leaky device look like a higher gain device. Here's how to separate the wheat from the chaff.

This is one way to sift the leakage from true gain. You hook up a couple of resistors and a DMM to the device, and the resistors set up conditions you can control to see what is what. If you really want to do this, get a 2.2M resistor and a 2.4K ; better, get one each 2.2M and 2.49K metal film 1% resistors. This will set you back about US$0.30 if you get them from Mouser, and slightly more or less than that from other sources. If you're going to do much of this, get a transistor socket to, so you can easily test a large number of devices. 

If you are satisfied with an indication of gain but are willing to settle for lower accuracy, you can carbon film at 5%, but recognize that the accuracy will be less. If you can, get several 2.4K resistors and measure them. You may find one that's closer to 2.472 ohms, which would be ideal. I'm being picky about the ohms because if you get exactly 2.2M and 2472 ohms, and use a 9.0V battery,  you'll find that the voltage across the resistor will be numerically equal to the indicated gain! That's why the somewhat odd resistor values, and the discussion on the values. It makes the final numbers on your DMM come out about right.

To do the test, stick the transistor in the socket, and read the DC voltage across the 2.4K resistor. The resistor will convert any leakage current from the transistor into a voltage that you can then read on your meter. A 2472 ohm resistor is 2.472 volts per milliamp, so a milliamp of leakage will cause 2.472 volts to display. That is incredibly too much leakage, so any transistor that does that is not going to be useful for a FF. In fact, although it will differ a bit, any transistor that shows more than a few micro amps of leakage is suspect. Because of the resistor scaling, the indicated value on your meter is "false leakage gain" and will have to be subtracted from the total reading that you do next.

To test the total gain, press the switch that connects the 2.2M resistor to the base. This causes a touch more than 4 microamps of base current to flow in the base. The transistor multiplies this by its internal gain, and the sum of the leakage (which doesn't change with base current) and the amplified base current. If the transistor has a gain of 100 and no leakage, the voltage across the 2.4K resistor is then (4uA)*(100)*(2472) =  0.9888V - which is almost exactly 1/ 100 of the actual gain. Pretty neat, huh? 

But we know that germanium really does have leakage - that's why were doing this little dance in the first place. So, let's say that the device leaks 100uA to start with. We stick the device into the socket, and read the voltage before we press the switch. It reads (100E-6)*(2472) = 247mV. So the leakage is making the meter believe that there's a "gain" of almost 25 with no current into the base at all.

How much leakage is too much? 100uA is common, 200 happens pretty often. More than 300uA means the device is suspicious, and more than 500uA I would say is bad.

Let's say the device really leaks 93uA, and has a gain of 110 - a prime specimen. What happens when we test? We chuck the thing in the socket, and read (93uA)*(2472) = .229V. Then we press the switch, and read 1.330V. To get the real gain, we subtract 0.229V from 1.330V and get 1.101V. The true gain is just 100 times the reading. 

Hey! How come it's 110.1, and not 110? Well, that's from this being an imperfect world, and from this tester being built with some approximations. The exact base current is 4.046...uA, assuming that the transistor's base conducts that much with a forward voltage of 0.1V (reasonable with germanium at these currents) and that the battery is *exactly* 9.0000V, and that the resistors are 2.20000M, and...   well, you get the picture. 0.5% accuracy is doggone fine for work with such blunt tools, and much better than you actually need to make a fine sounding FF. Besides - if you're clever, you'll flip the switch and watch the voltage while you  put your finger on the transistor. Simple finger heat will make the gain rise rapidly. What's the real gain? All of them are - at the temperature and conditions of the moment.

Don't get too hung up on the exact numbers - they'll change in a second anyway. Look for low leakage, and approximately the right gains.

The right *real* gains are from 70 to about 130. Within that range, people report the best sounds. Some people prefer equal gains, others prefer having a lower gain of 70-100 for the first transistor and 90 to 130 for the second. Fortunately, you now have the info to figure out what gains YOU prefer.

You may have heard that there actually exist NPN germaniums. While most germanium transistors are PNP, there are some NPNs, and they do make good Fuzz Face clones. To test them, just reverse the battery polarity and the meter leads in the tester diagram. The really nice thing about NPN germaniums is that you get the germanium mojo, and you can still use negative-grounded power supplies, which lets you run NPN versions from the same AC adapter that runs your other pedals without shorting out the power supply or needing a second one.