Tube Replacement & Bias
Pre amp tubes
Changing the pre amp tubes is a popular hobby and sometimes also a kind of religious activity. As long as you are using tubes of the same type, the differences are rather low and because of the serial dispersion poorly predictable. But I have favorite tubes, too, for whatever reason.
Selecting another type of tubes, you mostly want to change the amplification for reducing gain. There is an overview of the current tubes with 9 pins, which usually are easily replaceable.
However, a tube of another type not only changes the amplification factor. If it has a different internal resistance, the operating point and the effect of a tone control changes. Please keep that in mind.
Rules and general statements are almost impossible to mention, the opportunities are versatile. A harp amp tinkerer should always have a 5751 and a AY at his fingertips.
Bias is a word that is often used, but not always comprehended. Bias is the electrical "bias voltage" of the grid of a tube.
On cathode resistor a certain voltage drops and this is passed on as a potential via the high resistance grid resistor to the grid. Inside the tube, the cathode is the zero potential, so the grid is even lower by the voltage drop. Thus, the negative potential to the oscillating amplitude of the signal arise ..
At a pre amp tube and at power amps in SE mode, this voltage is usually determined by the cathode resistor. If you want linear transfer, select the bias voltage centered to the linear portion of the characteristic curve of the tube. Going up or down, you might get asymmetries that may have a positive effect for the sound of the harp player.
In a PP circuit it works a little different, because bias voltage controls when the amp goes from A to AB operation. The AB operation is more controllable for harp, although its sound does not change much. That is also the reason why the 5E3 sounds more like a Champ running extremely long in A mode. But more on that later with the power amps.
Power amp tubes
To change power amp tubes of the same type because of sound reasons is hopeless, there might be exceptions. If you can not set the output tubes in the "Fixed bias" because a control option is not available, there are changes. But this is not a good example. Normally you compensate the differences between the tubes and set the desired closed current.
To change the type of a tube usually requires an adjustment of the circuit, but may be interesting sound-wise. This should only be done if you know what you are doing and not just because someone on the net made this and thought it's cool, because here we have power on the wires.