I have built a 3-stage amp that's using KT66 output tubes running in ultralinear mode. It also has some feedback from the speaker output to the cathode of the driver stage (6SN7GT). A couple of additional details: I have a 150R resistor in line with the UL tap back to the screen grid on the output tube (to help prevent instability/oscillation). I also have a 1k resistor in line with the feedback to the cathode of the driver tube. FWIW, the output transformers have 5k impedance.
Question: if I wanted to make this circuit switchable between Triode and UL mode, what's the best way to do that?
My thinking: (ultimately thinking that I would execute this using a switch)
1) I completely disconnect the UL tap(s) coming from the output transformer(s) and,...
2) I strap the screen grid to the plate on the output tube(s); should I use a ~150R resistor here too?
Question about the feedback... my sense is that the feedback resistor is usually around 1.5 to 3 times higher than the cathode resistor on the driver stage. The higher end of this range provides less feedback - lower range is more. In my case, I have a 1k feedback resistor and the cathode resistor is also 1k. In hindsight, this is a LOT of feedback. I have plenty of power to drive my Klipsch Fortes in spite of the heavy feedback, so that's not really a concern. But, some purists would prefer having less or even no feedback at all. Question is: is this feedback a separate issue from the Triode-UL switching idea? Can I treat these "features" independently? What is likely to happen to the sound (subjective, I know) if I reduce the feedback by increasing the feedback resistor value? What if I take it to infinity by simply disconnecting the feedback? Could I put a potentiometer in line with a 1k resistor so that feedback could be "tuned" up or down? If I am successful switching to Triode mode, the gain/output power will be reduced significantly - maybe less feedback would compensate for that?