Ok-I built a GEO gain checker-Hows it work?-Smallbear doing something different?

[jimbob]

So I decided to learn how to test germanium transistors from the GEO website.

Heres the problem- I thought a good way of understanding would be to use/check a transistor I bought from Smallbear with a gain listed as 153 on the package. So if I came up with that same number then I learned something new. but what actually happened was this:

I got this number with the switch off and using a battery (9.3v) = 25.6

and w the switch on=77.8

From my understanding these/this number should be multiplied by 100 to equal gain.
So this would then be =2560
and... 7780


How does any of this equal 153?
This transistor I bought was actually from the "All American" 2 transistors for $15.00.
Steve at Smallbear did send a list of resistor values r3=27k, r4=680 ohms, r5=11k, r6=110k

The other resistor it came with was a lower number, maybe 120?
Does smallbear do something different to get a gain value?

GEO states:
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 - multiply the voltage by 100, and that's the gain.

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? 

[brett]

Hi
to get the true gain, you calculate the *difference* between the two numbers.  ie.  78 - 26 = 52, which is a fairly good number for Q1

The leakage in microamps (uA) requires some algebra.  1000 uA causes 2.5 V to be displayed.  You can work out how much leakage you have with 25mV being displayed.

Of course, you don't actually need the leakage number in uA.  Any device that reads less than 200mV of leakage on this tester should be ok.  Less than 100mV is even better.
cheers

[jimbob]

Ok - thats starting to make more sense except that you have its 153 which is what was on the packing for the transistor and not 52. Is there 100 thats supposed to be added somewhere in there?

[JOHNO]

if you check the smallbear sight,click on "how to's" then click on "the faqs for the fuzz face" and scroll down a bit you'll see there how smallbear check there germs.they dont seem to use the 2.4k resistor. i have one of the geo ones that i made and it worked ok for me ,though now i dont recall how to do the sums.anyway check out the smallbear sight . i hope this helps cheers

[R.G.]

You've run into one of the issues with germanium - it's very difficult to say what the gain is. Or rather, it's like Bill Clinton talking: he tells the absolute truth, which he admits varies depending on what he decides the words mean at the moment. Germanium has a gain, which is all over the map.

Small Bear's setup measures currents directly. The GEO setup converts them to a voltage and measures that. Both setups use the approximate voltage of a battery and the conversion of a voltage to a current in a resistor to set up the gain measurement. Neither of the tests are laboratory-grade measurements. They're a rough-and-ready field test which deliberately sacrifices accuracy for simplicity. Even the same test of a Ge device is going to produce different numbers for gain if done repeatedly, if only because the device self-heats because you're flowing electricity through it while you're testing it.

It is not even slightly unusual for the real gain of a Ge device to vary by 3:1 over temperature. It is unusual that you'd hit the exact opposite ends of the spectrum by accident. So I'm not sure what to tell you.

Probably the best thing to do is to note that both tests produce a value that is usable, and the leakage is sufficiently low. If it were mine, I'd pop it into a socket and listen to it. Let your ears be your guide.

[jimbob]

As R.G. suggests - Ill give it the ear test now. Thanks for all the responses.

James

[jimbob]

Ok. I finally figured it out! AWSOME!! My fuzz face sounds amazing. In fact I thought about experimenting w it more for different sounds but decided against it since its perfect to me.

Now its time to experiment w my NPNs. Im on a roll- sort of.