I was going to opine something similar. One thing I like to do now and then is to have the capability of really shutting down the output section---really choking it off. To do that on a 6L6-fired output pair we're talking into the -70 volt range. This is useful for times when I am fiddling with the preamp stages and have no need to cook my 6L6 tubes. At the same time, for normal operation, I'd like to have the bias pot sit near the center of its nominal range, or at least not all the way to one side. Of course there is also the idea that I don't want to be able to dial it down (actually "up") to -30 volts, which would be near redplating, I suspect. So, if "normal" bias is let's say 50 volts, I'd like to have a range of say -42 to -70. (I am aiming for a cleaner sound than most tweeders and like the idea of longer-life for my outputs)
Normally, I am building these things from salvaged PTs and stealing bias from one leg of the HV winding. If you are using a "proper" PT with an actual bias winding, -75 to -80 volts may be as much as the real bias tap can deliver through a half-wave single diode rectifier. So, you need to have the range closer to the "top" of that range if you need -50. In my case, starting from the full-out HV winding, I have waaaaaay too much negative bias volts and have to position the range of my bias much closer to ground, 0 volts. Because I might be starting out with -250 volts (!) I usually develop the bias divider from junkbox resistors tacked together and floating up in the air and when I get the range where I like it, then I pull out the nice new resistors and solder them in clean and neat.
Topic: tip request! handy size bias resistors (Read 1952 times)