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Old 11th September 2011, 03:33 PM   #1
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Default Sub a silicon trans for a germanium

Hi You'll,
I'm kinda new at this. I'm trying to restore an old transistor amp. It has germanium transistors in it and I dont have any on hand. Is it practical
to sub with silicon transistors, I have a bunch laying around? The transistor
in question is in the pre-amp so its low power.

Thanks
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Old 11th September 2011, 04:05 PM   #2
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Apparently the 22k and 15k resistor sets up the bias voltage on X20 base. (assuming that's the bad transistor) You'll need to come up with another ~.6 volts for the base-emitter junction which you might do by increasing the 15k resistor and decreasing the 22k resistor, ideally so that the total resistance of the divider is the same. You can fine tune it so that you get the same emitter voltage on X20 indicated in the schematic and then subsequent direct coupled stages will have original bias. Then there's gain, transition frequency, power, and capacitances which would ideally be pretty close to the old transistor. If you handle all that it should work fine. Maybe you can look up data on the old transistor number.

Last edited by Andrew Eckhardt; 11th September 2011 at 04:07 PM.
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Old 11th September 2011, 04:06 PM   #3
jez is offline jez  United Kingdom
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It can be done but is not that easy.... Lots of resistor values would need to be changed to set suitable bias conditions.
Are you sure the original part is dead?
A very limited range of germanium transistors is still available if it comes down to that.
Mouser Electronics seems to have a decent range in your country.
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Old 11th September 2011, 04:25 PM   #4
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Funny , I searched all over Mouser Texas and found only RF germanium transistors, not generic variety. We Hammond owners substitute 2n3907 and even 2n2222 for the germanium input transistor in AO44 reverbs all the time without changing anything. Doing the resistor changes mentioned above have even better chance of results.
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Old 11th September 2011, 04:44 PM   #5
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What about AC128?
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Old 11th September 2011, 04:47 PM   #6
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Mouser shows 519 NTE176 in stock!

see Germaniun Transistor Selector Guide for the rest.
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Old 12th September 2011, 01:44 AM   #7
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Thanks to all who replied. I just did not want to leave the house
today, kinda lazy. I did go to Frey's and picked up a NTE 158. That
and a cap replacement did the trick. All is good now.
Thanks again.
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