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?
Triode mode normally has less gain and also requires less NFB to get to the same levels of distortion. WHen you factor that in, you can reduce some of the gain from using triode mode by reducing the amount of NFB.