Geo tester for germs HELP????????

[Stephen]

On the test to see how much leakage in the pic of the 2.49K resistor you should have half of 2.4 volts, or if you had half .. would be 500uv ..in other words 1.2volts is 500uv and less is less uv RIGHT?....RG says 500 is no good so you should have a reading of less than 1.2 volts which is 500uv...this is confusing now.... a good reading would be what than??? for lets say 300u   1.0 volts??....He is a little deducing the theory more than a clear explanation....If you had a trans with 1.0 volts that is around 300uv  ( RIGHT) 
and than switch the switch and you get 1.300 volts you have a gain of 100 ..... 1300 -300= 100 hfe
and leakage in range..I thought I knew this but I am confused.


I guess it needs to be like this is leakage, than this is the switch, than this is the gain

i would really like to grasp this

I am sorry for the stupidity but I never did this I used another way but had trouble too!!

[Stephen]

WHo can explain the useage???

[captntasty]

I'd love to hear from some of the other knowledgable guys on this... I won't pretend I really know what's right, but here's how I understand it.  If it's 1.2 or less it's useable as far as leakage goes (edit: wrong!).  Flip the switch and subtract your first reading from that number.

For example here's a reading from a 2N1408 - first reading leakage = .26  second reading = 1.07
I remove the decimal 107 - 26 = 81.  My understanding of the article is that all the "math" is done by the components and it's a matter of simple subtraction.  If I'm doing this incorrectly, please somebody correct me!  I don't want to be spreading bogus info.

I doubted how I was doing this test for a long time but got some trannys from Smallbear and compared his gain measurements to what I was getting and they were pretty close - I figured my battery wasn't exactly 9V (a little higher) and resistors weren't exact.  So I started to trust these readings as fairly rough but overall useable.

Hope that helps or at least sparks some interest in this thread. 

[R.G.]

The circuit is designed to show gain as a voltage. The indicated voltage is 1/100 of the gain. For instance, a measured voltage of 1.32 is a gain of 132 times.

On the test for leakage, you put the device into the socket and measure the voltage across the 2.49K resstor. You do this with the base open, not connected to the bias resistor. That means that no current at all should be flowing, so any current that flows is leakage. This leakage shows up as a false "gain". So you have to do the leakage test first, then subtract that "gain" from the actual gain measurement.

In operation, you measure the voltage across the 2.49K resistor and divide that voltage by 2490 per Ohm's law, and that number is the current. The leakage current I is equal to V/2.49K. Just to help you not have to do the division, you can use the following chart:

Measured	Leakage
Voltage	Current
0.249	100uA
0.498	200uA
0.747	300uA
0.996	400uA
1.245	500uA

So you should really have a leakage reading under 0.747V.

Once you measure the leakage "gain", you switch in the bias resistor and that then gives you a total gain which is composed of the real gain plus the leakage "gain" which you measured first. Here's a chart of gains:

Measured	Measured
Voltage	Gain
0.249	24.9
0.498	49.8
0.747	74.7
0.996	99.6
1.245	124.5
1.500	150
2.000	200
3.000	300
4.000	400

Once you have measured the false "leakage gain" and the total gain, you subtract the "leakage gain" to get the true gain. Notice that the total gain can never be less than the false leakage gain, so the true gain you get will always be zero or greater.

In your example, if you measure 1.00V with the swtich off, then the false leakage gain is 100. That corresponds to a leakage current of about 400uA, so this transistor is of questionable goodness anyway because the leakage is so high. Then you connect the switch and the reading is 1.3V. This is a total gain of 130, of which 100 is false leakage gain. So you subtract the two and get a real gain of 130-100=30. This transistor is high leakage and low gain, and not likely to be a good sounding device.

Did that help?

[Stephen]

RG thanks your examples make it so much easier now to understand

[Stephen]

here is stupidity..

What if you measure leakage and get 1.245 v...500u leakage... than switch the switch and get 3.000v subtract 300-500= -200hfe
a negative hfe of 200........
but yet will sound good with gain and all in the circuit??

[captntasty]

My math comes out 300 (real gain + leakage gain) - 125 (leakage gain=500uA so high leakage) = 175.  Not a great specimen due to leakage.  The voltage reading of 1.245V*100 (125 rounded off) is what you subtract from the 3.00V*100 (300) not the current(500uA) that voltage represents.  Apples (voltage) need to be compared to other apples (voltage) even if one of those apples(voltage) is taking the place of an orange (current). 

[Stephen]

Hey Thanks that makes it all clear now!!

[R.G.]

One last item - leaky germaniums tend to be noisy. Some of the leakage mechanisms are from surface contamination, and that makes a lot of noise. If I had a transistor of the right gain, but leaky, I'd use it but I would replace it with a low leakage device the first chance I got.

[Stephen]

I have a lot of good sounding Ac128 but teating them they are quite high on leakage........Will leaky ones drift a lot maybe sometimes not work until you warm up the pedal?

[mac]

Stephen,
I wanted to trace a curve hfe = hfe(ic), ie, how hfe varies with collector current so I wrote this post:
http://www.diystompboxes.com/smfforum/index.php?topic=45481.0
Maybe it hepls.


mac