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#6708
Billy
Moderator

Q4 the original was a low gain 2N5133 which didn’t require resistance at R25 having a low minimum hFE of around 60

Because you are using a higher hFE BC549C you will need some resistance at R25 to stop the transistor being amplified too much causing it to oscillate and make a squealing sound when all the controls are at maximum

The hFE value is the factor by which the base current is amplified so for example if your base current is 1mA and you are using a 2N5133 with a hFE of 100 your current at the collector (output) would be 1 x 100 or 100mA

If you use a higher gain BC549C with an hFE of 470 for example then it would be 470mA over 4 times as much and this may cause oscillation causing it to squeal so you add resistance to stop that happening

Put in SIL sockets and experiment for learning, start with a jumper and see if it does indeed squeal then add resistance 10K at a time until it stops