I am working on your standard SE 300B With DC heating and Potentiometer controlled input. It is very quiet at lower volume. As you turn it up it has a hum that gets louder and louder as you increase volume. Is this normal? It is quiet in actual use with 96db speakers. Using it with a pre amp means turning the 300b up just enough to get a good volume and using the pre for the rest of the amplification.
If you short the inputs to ground, and your hum level still goes up and down with volume it generally means you have an issue in your ground or wire layout.
No, I have several different implementations of 300b circuits here (by others) and none of them show this.