Breadboarding Aft Pre Amp

[steviejr92]

i was thinking about breadboarding the aft boost. the build docs ask for an OC139 NPN transistor. I dont have an OC139 but i do have other NPN transistors. My question is will any other work in place of the original?

 

[BuddytheReow]

The way the circuit is set up you could use any NPN, but germanium would be the most “stock” version. You may have to adjust R5/C3. If you’ve only got silicon I would try a 2n2222a or pn2222a

 

[steviejr92]

I have 2n5088, 2n5089, mpsa18, I also have bc238 and bc108b. In terms of having to adjust r5/c3 how would I go about doing that? Should I go higher/lower in resistance/capacitance?

 

[BuddytheReow]

Build the stock circuit first to make sure it works. Then start subbing components. Any of the BJTs you have should work fine, but with different results. As you raise the resistance value the gain will be lower, but you'll need to take out c3 entirely. You'll also notice a volume drop when you take the cap out. If you keep the cap in then changing r5 won't do much. Don't just take my word for it. Try it out yourself!

 

[steviejr92]

Build the stock circuit first to make sure it works. Then start subbing components. Any of the BJTs you have should work fine, but with different results. As you raise the resistance value the gain will be lower, but you'll need to take out c3 entirely. You'll also notice a volume drop when you take the cap out. If you keep the cap in then changing r5 won't do much. Don't just take my word for it. Try it out yourself!

Going to be experimenting!