I've been reading and drawing out bias circuits recently, and wondered if folks can suggest other good sources for design and values as to pots, resistors, caps, and 'features.' 1) A nice resource for *calculating* values and understanding design features is, not surprisingly, Merlin. He favors a 10kl pot: The total resistance of P1+R2 forms part of the power valves’ grid leak resistance, and there will be a maximum allowable value given in the data sheet. Since we will usually want to make the grid leak resistors as large as possible to avoid loading down the driver stage, the resistances used in the bias supply need to be small. A 10k bias pot is ideal. But he notes there are a zillion variants out there. One obvious difference arises if we have to borrow HT voltage from the rectifier socket (eg pin 4 or 6) or OTOH if our PT has a bias tap (often 50V). He has an elegant and simple method to calculate the maximum (negative) bias voltage you'll need, for each power tube type, based on screen voltage. He discusses the logic (with some trial and error) for sizing the 'tail' resistor (or as he says, 'idiot resistor') that prevents the pot from zeroing out the bias, and he shows a simple and elegant 'emergency' resistor that prevents meltdown if the pot should fail. He goes into helpful detail about how the bias scheme can be modified for different rectification schemes. 2) Another great resource is of course @robrob -- his generic mods page shows nice examples of several generalized and specialized bias circuits, with his usual elegant diagrams. He includes *two* options for balancing the bias and an elegant 'universal' bias circuit for amps without a bias-tapped PT. 3) Another good source is Doug Hoffman, whose bias page I've used a lot and seen used by other builders. The drawings there were clearly done in an earlier era of computer graphics, but they're especially helpful for the popular Fender bias-wiggle circuits, and include options for 10kl and 50kl pots. Doug also carries both 10kl and 50kl mini trim pots you can mount on any bias board, and the nice Fender-style CTS 10kl screwdriver pot that allows external access, plus those great test-point sockets. You'll never know what 'easy' is until you adjust bias without dropping the chassis. I've tried reading Aiken on bias, but although he explains in detail what it does in the tube, I couldn't find much info about designing bias circuits. There's still a lot I want to know. How much filtering is enough? One cap or two? What are the pros and cons of 10kl v. 50? What are the constraints on resistor value? Are there advantages of making the pot a variable resistor or a voltage divider? How does bias-wiggle trem change the design? So your thoughts and suggestions will be welcome.