Increasing distortioin in PP 6L6

[bluefoxicy]

RC-30 lists various configurations for a Class A1 SE amplifier running 6L6 tubes. For the plate, screen, distortion, and output values, it gives 250/250/10%/6.5W, 300/200/11%/6.5W, and 350/250/15%/10.8W. Of course the first 2 are for 6L6, and the last is 6L6GC... a 6L6 maximum plate voltage is 360, 6L6GC 500.

For a Class AB1 PP amp, it gives 450/400/1.8/55W. I guess you can get 20W from a 10W tube in AB1 bias, but that 10.8W might get up to 42 ... where does 13W come from <_<;

Anyway that's not important. What's important is the 1.8W of THD. In a guitar amp I want more (jazz excluded). The given load is 5600 ohms; raising this is likely unwise, lowering it will probably increase THD but I can't find a reference graph. It may also help to lower the screen grid voltage, impairing the tube's operation at higher volumes (i.e. bigger input signal). Using somewhat mismatched tubes instead of a matched pair should help increase second-order harmonics.

Any thoughts?

[Steve Conner]

Yeah, to increase distortion just crank it up until the output stage starts to clip grossly. That's how rock'n'roll was invented.

[Old Tele man]

...with Class-B push-pull operation, each tube operates one-half of the time, upon one-half of the whole signal. So, each tube is working at 1/4th of its capacity, which means each tube can theoretically produce 4-times its rated "avearge" wattage; but, that number must tempered by the push-pull "PI/4" factor, so the actual "maximum" limit becomes about 3.14-times its "average" wattage.

...and a further reduction occurs with Class-AB1 operation because there's some "overlap" (idle wattage greater than 50%) which causes each tube to operate about 200-210 degrees (instead of exactly 180-degrees). This slight overlap in conduction is used to mitigate the "crossover distortion" that occurs with Class-B operation at 50% idle wattage.

...thus, the "rule-of-thumb" is you can get about 3X the tube's average plate wattage when operating Class-AB1 push-pull.

[Enzo]

Remember that in the RC30, the 6L6 - plain 6L6 - they refer to is the original metal tube. You rarely see them, and they suck for amplifiers anyway. 6L6G is the modern glass tube, with 6L6GC being the latest version of it. The metal tube had much lower ratings, and that is what those spec columns represent.

In a guitar amp you want more than 1.8% distortion or whatever, but keep in mind most of that distortion usually comes from the preamp anyway. The only way to overdrive a power tube is to push it past the specs.

[stokes]

Increasing the current draw of the power tubes will make them break up sooner.Like Enzo said,push it past its specs.Just be aware that if you increase the plate volts it will give you a "harsher" tone,increasing the current will give it more of a warmer break-up.Or you could tweak the PI to distort a little more,as well as hitting the power tubes harder.