I see a bunch of postings about getting audio out of old comm. sets, proper bias, and so forth. Many of these sets use a 6V6, single-ended. You can get into a lot of folderol setting up a 6V6, and reading the literature doesn't simplify things. I'll cut through some of this with my super-simple approach. This assumes that B+ in the set is 250 volts (285 on the 6V6 is essentially the same), and that the output transformer is around 3000-4000 ohms. For good hard core class A operation, set the bias around -12.5 volts. Remember that this voltage is measured grid-to-cathode. This will give right on 50 ma. cathode current, of which 45 is plate current, 5 screen current. This "10% Ik" for the screen is standard for beam tetrodes. Pentodes like 6F6 and 6K6 have a higher percentage in the screen. Audio drive should be 20 volts peak-peak, maximum. With a plain vanilla 8-inch speaker in an open box, that is a ton of audio power. That gives a range -2.5 volts to -22.5 volts on the grid, which is about all you can put on a 6V6 without approaching the near-cutoff distortion point too hard. Note that a 6V6 is essentially cut off at -25 volts on the grid, but is still conducting current at -30 volts. For a self-bias arrangement, 240 ohms is the correct value to use for this current level. A 25 mike bypass cap across the resistor will give you low degeneration down to reasonable frequencies for speech Now, if you want to reduce drain on the power supply, reduce heating, and so forth, you can bias the tube down as far as about -16 volts and still run class A at modest power output levels. This will still be genuine class A operation at audio drive up to 10 volts peak-peak. Anything between is a compromise, and generally gives quite acceptable performance for the type of listening a communications set is used for. Push-pull class AB1 is generally set up with around -17 volts of bias, giving Ik per tube around 34 ma., zero signal. Class B operation of a pair of 6V6's is with about -21 volts bias. If you run a single-ended tube that way, you are going to get wall-to-wall distortion. Remember that a tube does not cut off suddenly, like a transistor, and as I mentioned, a 6V6 still has transconductance between -25 and -30 volts, but it falls off so badly that you are going to see tremendous lopsidedness in the output current waveform between positive and negative current peaks. One way to linearize the output is to remove the cathode circuit bypass cap, which essentially puts the transconductance of the tube in series with the conductance of the resistor. This is a standard AA5 gimmick, howver, you will want to remember that the transconductance of a 50L6 is much higher than a 6V6, and the cathode resistor used is typically 150-180 ohms. The unbypassed resitance gives degeneration, so you need enough additional audio drive to compensate. The typical 6SQ7 drive circuit only has about 30 volts of headroom in the usual zero-bias detector/avc/audio amp circuit, so if you need more audio drive, you need to rig up something else. So far as voltage charts in Sams and Rider manuals go, I sometimes wonder if these measurements were taken with a roulette wheel instead of a voltmeter. On a single-ended beam tetrode circuit, cathode current should be between 40 and 50 ma. zero signal, whether the tube is a 50L6 or a 6L6. Things don't change until you get into the 807/6146/813 class. If it is very far different than that, you are going to get distortion and/or red-glowing plates. -- =================================================================== Hank van Cleef E-mail vancleef@netcom.com or vancleef@tmn.com ===================================================================