One of the most often asked questions about tube amps I get is: 'Which operating point do you recommend for tube xyz?' There is also a lot of discussions on forums about the 'optimal' operating point for certain tubes. I think it is time to present my view on this.
I'm puzzled when I see such a question posted somewhere, and straight answers are given down to the decimal point of a milliampere, without asking how the tube is loaded. I've also seen reports from people claiming that they tweaked their amp to the best possible operating point and that miniscule changes in that cause the sound to change significantly. Quite often people even think that their amp is of exceptional quality because of that. They think it must be a really good amp because it makes small changes in the operating point audible. Well, I think your design is marginal, if it is sensitive to small changes in operating conditions.
First let me point out, that the choice of a good operating point is largly dependent on how the tube is operated. Does it use a resistive load, inductive or a constant current source? What is the function of the tube under question, what constraints are there, what is the B+ voltage and in which range can it be adapted if necessary. And of course it is important to also consider the capability of the components around the tube. For example if it is transformer coupled, the current rating of the transformer is important.
If all that information is known, then the ideal operating point is chosen such that the tube's grid can be driven with about equal amplitude in both positive and negative direction before significant distortion kicks in. This point can be easily found on the loadline. But if the tube stage under question is designed with a lot of headroom as I described in an earlier post, then it will not be critical to hit the exact spot in the middle of that load line. Because of the headroom the operating point can be changed by some margin without significant changes in sound character. And this is important if you want a stable design. Tubes and also passive components age and shift their parameters with use. This results in shifts of operating points. I want my amps to have the same sound over the widest possible range of the tubes and components life time. Also in amps with passively filtered power supplies (which I prefer) the B+ voltage will change with mains voltage fluctuations. These should not impact the sound of an amp or preamp.
Let's clafrify this a bit further using a 801A/10Y linestage as an example. I chose this because in the recent days I got many questions about the ideal operating point of the 801A in a transformercoupled line stage. Let's assume the design is using a line output transformer which can be run with 20mA DC on the primary. This means it is capable to be run with up to 40mA before significant core saturation kicks in, so we have a range around the 20mA point which is about symmetrical from 0mA to 40mA. Before the transformer winders among you get excited: This is just for simplicity for the purpose here, actually things are a bit more complicated. We want to be able to use both 801A and 10Y, so we stay within the voltage limits of the 10.