Aft Preamt transistor sub

[allsmoke]

Hi all, just finished the Aft preamp and I used a 2N2222A since I didn't have an Oc139.

Hfe was 275. It's super noisy, almost microphonic and seems like there's almost a delay before the sound kicks in. Just a weird vibe.

The OC datasheet I looked at had the Hfe about 33. So my dumb fault for not doing my due diligence first.

Fortunately I have a solder station with a rework gun so I can desolder it.

Are there any suitable silicone replacements that may work instead, or should I flip it, and the electrolytic capacitors and use a PNP. If so, any recommendations?

Thanks everyone

 

[jwin615]

Have any npn gems on hand?
From CDB

Get something low noise, like BC549C, MPSA18, 2N5088, etc. HFE is not critical in the Range Finder. HFE only affects the biasing in that circuit and you can always tweak the biasing if req'd.

From this thread

Another day, another question! Recommended silicon transistor for a Range Finder build?

Hello again team! I'm gearing up to build a range finder. I currently am borrowing a friend's "Strange Master" pedal which allows you to switch between Silicon and Ge diodes. I expected to prefer the Ge but to my surprise, the Silicon is far supreme! Im wondering what Silicon transistor you all...

forum.pedalpcb.com

 

[swelchy]

You could use the 2n2222..... but looks like that particular circuit... " If I'm seeing the schematic correctly" has a fixed bias resistor... put a trim pot in place of that fixed resistor to bias the board... or you could wire in a pot... find the sweet spot.. measure the resistance of the pots setting and solder in a resistor of that value? Am I right here guys?