[vtech]

As you may know the DTC series is aimed at providing the bias resistors internally. The value of RB has a marked effect on what the transistor is able to operate as a package. Depending on your application, since you do not have access to the junction of RB & RBE, you can not change values by external resistor;
Looking at the datasheet, you'll see that the same transistor can be tailored to handle different input voltages/currents especially when daeling with saturation (ON/Off applications).
At least in this scenario, you'd have to modify your input voltage level for proper switching. However notice how it affects your current capability and if it falls within your application, it should be ok. Otherwise, it's a lot easier to start over from scratch with something like a 2N3904 and make your own biasing.

[Guest]

I have one question: Can I use a transistor DTC143X (S) instead of transistor DTC143Z (SA)? It is in the audio amplifier, and if I understand the purpose of this transistor, it is connected to power on / off function.
It seems to me that the difference between the two transistors is only in built resistance: the 143X comes with Rb = 4.7K, RBE = 10K, while in the 143Z are Rb = 4.7K, RBE = 47K. Can I damage the amplifier if I use DTC143X instead of DTC143Z?
Would it be possible to hook resistor of 39k onto the emitter, so I get a similar effect as the original transistor?

Sorry for my bad English! Many thanks in advance and greetings!