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Old 14th January 2013, 05:09 AM   #1
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Default Germanium transistor madness

A small question,

I recently build a small fully Germanium transistor amp.
But yesterday the strangest thing happened to the Left mono block.
It started to make a deep brown noise and sputter.
So i started to wiggle all the transistors (they are socketed) but no result.
Finaly i touched the VAS transistor with my finger and the speaker started
to hum. (this should be possible since it's fully isolated from the can)
So i tested this on the other amp -- but it stayed quiet.
I measured the quiescent current and the voltage at the output but remarkably
this stayed fairly stable, No sound was coming from the speaker though.
finally when i warmed the transistor up or cooled it down with my finger it slowly started to make brown noise again and some music.
Off course i changed to faulty device and the amp works fine now.
But I want to ask everybody:

What are the typical types/kinds of failures you guy's have seen in germanium transistors.
I find this behavior strange.
I am used to silicon transistors either shorting out or staying open at one or more junctions.
but never this almost faulty tube like behavior.

I could literally steer this behavior by tapping or touching the transistor.

Thanks in advance, Pim
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Old 14th January 2013, 05:48 AM   #2
Root2 is offline Root2  United Kingdom
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Hi Pim,
In the past, I found that some Germanium transistor would produce random noise also.
The OC series was very bad for this and if the Black paint was remove they do become sensitive to light. The case you describe make me think you are using AD series, in power supplies I found these particle bad for 5-10KHz parasitic oscillation, and often took weeks to become active.

Mullard books circa 1975 have many good article about this in them try and find a copy.
I have a few so I will dig them out and copy and paste info.

Best Col
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Old 14th January 2013, 07:59 AM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Captain archer View Post
A small question,

I recently build a small fully Germanium transistor amp.
But yesterday the strangest thing happened to the Left mono block.
It started to make a deep brown noise and sputter.
So i started to wiggle all the transistors (they are socketed) but no result.
Finaly i touched the VAS transistor with my finger and the speaker started
to hum. (this should be possible since it's fully isolated from the can)
So i tested this on the other amp -- but it stayed quiet.
I measured the quiescent current and the voltage at the output but remarkably
this stayed fairly stable, No sound was coming from the speaker though.
finally when i warmed the transistor up or cooled it down with my finger it slowly started to make brown noise again and some music.
Off course i changed to faulty device and the amp works fine now.
But I want to ask everybody:

What are the typical types/kinds of failures you guy's have seen in germanium transistors.
I find this behavior strange.
I am used to silicon transistors either shorting out or staying open at one or more junctions.
but never this almost faulty tube like behavior.

I could literally steer this behavior by tapping or touching the transistor.

Thanks in advance, Pim
Some type of Ge transistors in metal case were prone to case/electrode shorts due to whiskers growth. That could be the case here
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Old 14th January 2013, 08:29 AM   #4
blu_glo is offline blu_glo  England
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I agree with Elvee and was going to post exactly the same - namely old GE transistors are at risk of growing crystals which can lead to all sorts of odd behavior.
I have rennovated a Germaniun transistor based Leak Stereo 30 (not the 30 plus) and discovered a similar problem during doing so. The Stereo 30 works fine, now, and credits modern sources such as CD wonderfully. Touch wood - all the original electrolytic capacitors are somehow still doing what they say on the tin!!
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Old 14th January 2013, 08:43 AM   #5
Mooly is offline Mooly  United Kingdom
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Quite a good page on this issue here,
Anecdote: Tin Whiskers Inside of AF114 Transistors
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Old 14th January 2013, 01:05 PM   #6
blu_glo is offline blu_glo  England
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That's quite an insight, I originally thought it was the germanium itself growing more crystals!
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Old 14th January 2013, 04:50 PM   #7
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Thanks for the replies guys.

i knew about the whisker problem in germanium transistors, but the fact is that the transistor i was talking about is a AC187 bullet type filled with some resin or epoxy with the leads sticking out. (see the picture)
So the chance of growing whiskers seems small (not impossible).
I can also imagine that the complete setup of germanium and leads is completely covered with resin so contact with the metal shell should actually be impossible and the case should be completely isolated.

I would certainly like to learn more about those parasitic oscillations occurring in some transistors.

So ROOT2 / Col--> if you ever come across that article then i would love to read it.
but i wont dismiss the whisker problem so easily (maybe i'll open it up)
I have a few very strong Leica microscopes at work.
they also have a camera so if i can take pictures i will, and post them!

Thanks again
Attached Images
File Type: png AC187.png (68.5 KB, 260 views)
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Old 14th January 2013, 05:07 PM   #8
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Captain archer View Post
Thanks for the replies guys.

i knew about the whisker problem in germanium transistors, but the fact is that the transistor i was talking about is a AC187 bullet type filled with some resin or epoxy with the leads sticking out. (see the picture)
So the chance of growing whiskers seems small (not impossible).
The case is filled with a silicone compound (silicone oil mixed with a mineral load), not resin. I don't think it would be an obstacle to the growth of whiskers
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Old 14th January 2013, 05:30 PM   #9
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Quote:
Originally Posted by Elvee View Post
The case is filled with a silicone compound (silicone oil mixed with a mineral load), not resin. I don't think it would be an obstacle to the growth of whiskers
I've read the article on the growth of whiskers and i must say it's a scary thought that many transistors from this era have these tendencies.
I better start searching for replacements or it will be more like very soon.
Maybe i am lucky and it stays working with the replacement or it is just slowly aging to its doom even when the amp is brand new.

Well again the transistors are socketed so replacing (even with silicon types) will be very easy -- just a little adjustment and its ready to go again.

I just hope this was just a random faulty transistor and not one that was plagued by this whisker phenomenon. They tend to short out from the whiskers and this one only partially lost its function. So there is hope!

Included some pictures and the circuit (AC128 should be AC187)

Thanks
Attached Images
File Type: png Post PNG.png (827.7 KB, 251 views)
File Type: png Amp I.png (631.3 KB, 244 views)
File Type: png Amp II.png (544.7 KB, 231 views)
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Old 14th January 2013, 07:25 PM   #10
Bone is online now Bone  United Kingdom
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The trouble with tin whiskers, is that the new lead free solder which is mostly tin exhibits the same property, which is why the military are reluctant to use it.
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