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Old 13th May 2022, 5:01 pm   #8
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 1,645
Default Re: Germanium transistor replacement

I recently replaced a noisy OC71 (PNP) with a high gain BC558 (PNP) and changed one of the bias resistors. This is in a Decca TP85 (1963).

It works but needs adjusting. This first audio stage seems to have been originally designed for minimum current consumption. The emitter current is about 30 microAmps. The collector voltage is near Vcc and so there is not much room for voltage swing.

Leakage through the bypass capacitor which is across the emitter resistor, and leakage through stage coupling capacitors (electrolytic) can have a significant effect on the bias.
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