I'm building a B15 clone amp , and although I have an output transformer that's useable I'd like to wind a better one. I've wound them before so the making bit doesn't worry me , but looking at the 6L6 data I have a question. I'm wanting to put a Cathode Bias / Fixed Bias switch in , and looking at the Plate to Plate resistance of the 6L6 in class AB mode , the Fiixed Bias resistance is given as 6600 ohms down to 3800 ohms , which gives the standard sort of Primary/Secondary ratio you find on most commercial OPT for B15's. But the Cathode Bias p-to-p resistance is 9000 ohms ,twice as much . So the calculator I'm using for designing my transformer gives different ratios for each of the various p-to-p values , from a ratio of 21-ish for 3800 ohms to 33-ish for 9000 ohms. This suggests that you really need to have a tap on your OPT to suit , and I'm wondering if this is right - if so , I'll do that. It also seems to suggest that if you are using an OPT that is somewhere between the 3800 and 6600 ohms , around 5000 ohms is a number I've seen quite often which gives 25:1 ratio using 8ohms , then if you switch to Cathode bias you'd be better off with an OPT with a 16ohm winding , that would reflect as 10000 ohms....a bit much , but closer to 9000 ohms than your 5k transformers 8 ohm winding. But I've been reading about the B15 Bias Switch in Basschat amps / technical and don't see any mention of that, so maybe I'm barking up the wrong tree.
