Hi Pat, The 20K resistor idea was published in an old article on getting 120 watts from a pair of 807s, but I have been told it really didn't produce that much power, it did some really odd things at low audio levels, and it really didn't sound good.
The idea about tying the control grid and screen of a 4-400 together and using it as a triode might be better, but I suspect that the 3-500 will be a better triode than a kludged 4-400. Tying the screen of a 4-400 to the plate will *drastically* reduce plate voltage limits, and plate current will be much lower at acceptable grid potentials (instantaneous E-grid *below* instantaneous E-plate) - so it would be impractical, because the power output capability would be massively reduced. To do ultra-linear right would require a separate, center-tapped screen winding on the mod transformer. I am in favor of the 4-400, but with active screen drive to improve linearity. Some kind of circuit could take instantaneous control-grid drive and produce a corrective pre-distortion to the screen voltage, aimed at making the grid-plate transfer as linear as possible, and making overload graceful. With lower voltage tubes such as the 6L6, 807 and 8417, it is actually possible to accomplish this with simple resistors in series with the screens. But the big, higher voltage tubes have so much secondary screen emission that such resistance can not be tolerated. So with higher voltage tubes, a more complex active screen control is required. But when the smoke clears... you will have the output and gain of a tetrode, but with the linearity of a triode (or better). With decently designed feedback, you can have the source resistance of a triode as well. I've done it with 6L6s and 8417s. Some day maybe I'll try it on 4-400s or something. Bacon, WA3WDR
