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Old 6th November 2006, 09:44 PM   #1
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Default fighting germanium noise?

i got 100pcs of mullard germanium transistors (mostly oc76 but some others) they came NOS with army packing. after making some audio tests they are all hissy. i have another oc44 germanium transistor that has almost no hiss so i know how good germanium transistors can sound. is there anything i can do to minimize the hiss noise?
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Old 6th November 2006, 11:07 PM   #2
Duo is offline Duo  Canada
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Design a better circuit?
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Old 7th November 2006, 06:41 AM   #3
jerishi is offline jerishi  United States
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that's a tough query... If I remember right, reason for those germanium transistors ... at least in "guitar world" .. was for unique distortion they produced. This is years later. As we all know, there are other ways to get/obtain ...that same distortion. However, (personally) I still think there's some value in the 1 out of a hundred that test right and work well in a 'fuzz' circuit....
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Old 10th November 2006, 09:06 AM   #4
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Default Germanium Transistors

Hi,

if I remember right - the OC76 was a medium power device ( all the packages in the OC series looked the same so there are no clues ) It's probably intended to go in a later stage in an amplifier circuit, where the noise figure doesn't matter. The OC71 was the input device.

Germanium processes were much less repeatable than even the earliest Silicon, so there will probably be a 4:1 spread in the gain and other specs between the devices you have. The noise figure would probably never have been tested - or specified.

A lot of these devices were defined by the manufacturing process and NEVER graded by voltage or gain - i.e. they sold every device they made...

The only remaining advantage of Germanium is the lower saturation voltage - which might explain some of the mythology re distortion - nowadays all you can get are high-power devices for low-dropout applications where the saturation voltage helps...

Graham
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Old 10th November 2006, 11:11 AM   #5
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Just checked my old Mullard data books, in 1970 the OC76 ws no longer listed (but the OC71 still was), and in the 1973 book none are listed anymore.

Incidently, you could convert the OC71 to an OCP71 (photo-transistor) by scraping the black paint off the casing - although in later production they started filling them with opaque grease to prevent this!.
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Old 16th November 2006, 01:43 PM   #6
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Default Germanium noise

Hi,
In my experience, not even the germanium tr. gets noise, the silicon tr. also get noise. As long as the transistor been made for more than 2 decades, these element would became " decayed " and unstable. Today if any product which been produced more than 20 yrs. You will find they become unstable, and with " Hiss " noise, in any appliance, especially in tuner or amplifiers, It's obviously been noticed noise come from the speakers. After replaced the tr. with newly manufactured, the noise disappeared.

If the tr. used in ocl circuitry, the output will appear a few DC volts. after replaced the differential two tr. the offset voltage becomes zero. no more DC voltage.

Above case may some member has experienced. If so, let us hear about your case!




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Old 16th November 2006, 05:34 PM   #7
Albertb is offline Albertb  United Kingdom
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Nigel, I remember the OC71 ands OCP71 trick. There's a wrinkle too. They rarely filled up the case sompletely with the grease. If you nip those long legs as in a drill and carefully centrifuge them the grease gets thrown to the top and exposes the germanium. Voila, servicable OCP71s.
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Old 29th November 2006, 02:27 PM   #8
prorms is offline prorms  United States
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Default hiss

it way of germanium transistor !!!
you might wana try ac187 ac188 you dint alot hiss out of them
also try using a multimeter with transistor socket for testing help you detrmined if they good or not i noticed that ones with higher gain
are almost ready to go bad this transistor are old think they been sitting around for 20 years plus
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Old 20th December 2006, 09:54 PM   #9
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Did you try a 22pf across base-collector? I use this on jfets and it helps.
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