This *should* be easy ... simple transistor amp stage

[ronsimpson]

But when I change R9 to 33Ω, the output waveform degenerates:

View attachment 89702

The circuit before this amp, biases the triangle waveform around 1.6 volts. By just changing any resistor you will probably upset the bias point. Then the waveform will be too close to ground or +12V. The gain has to do with the ratio of the emitter and collector resistors.

The pullup is done by a transistor so it has low impedance.
The pull down is done only by a 10k resistor. You could use a 1k resistor here.

 

[audioguru]

When you increase the supply voltage for the amplifier then the base current increases so the transistor saturates a little and clips the bottom of the waveform.
So I increased the value of R7 and removed the emitter follower completely. I reduced the value of R9 to 30 ohms then the output level is as high as it can go.
You can see that the collector of the amplifier almost reaches the 12V supply voltage and drops down almost to the emitter voltage.