# Building Aft Preamp, question on transistor substitute



## rsvaughnua (Dec 17, 2020)

Hi all, I am working on building an Aft Preamp and when I was going over all my parts I had a transistor set aside for this build I thought was a npn ge but its actually a pnp ge.  I looked again and I did not have any npn ge transistors on hand.  My question is can I replace that npn ge with a similar silicon npn transistor?  I am assuming the answer is no but I did want to ask here incase my understanding of this is incorrect in that there would have to be more modifications to the resistors in the circuit to go with a silicon npn.  I do have some 2N3904 's, 2N5088 's, 2SC381 's and some 2N2222A 's on hand but all the ge stuff I have is pnp.

The npn ge listed in the build doc is an OC139


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## Cybercow (Dec 18, 2020)

The OC139 is a low gain (~20) Ge NPN BJT and as such may require different biasing to accommodate a silicon replacement.


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## Chuck D. Bones (Dec 19, 2020)

The answer is "Probably yes, but the tone may change and you will have to tweak the biasing."  These days people put Ge transistors in guitar equipment for a couple of reasons:
a) Marketing.
b) Ge has a different tone than Si. Depending on the circuit, that difference can be subtle or pronounced.

I have a question of my own: How about sharing the schematic?


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