[Alan Douglas]

If you know someone with a high-end Hickok, a 539, 580, I think the 750 and 752, or a Cardmatic, a model that actually measures grid current by placing the 100µA main meter in series with the grid, you can settle the output-tube question for certain. I imagine you could also make a socket extender and put an external meter in series with the control grid. I think your tubes have grid emission, which only shows up when the grid gets heated from the adjacent cathode. A good one will show less than 1 microamp, but currents up to, say, 5µA are within spec for power tubes. If you do the math with the grid resistor value you can see how much that shifts the bias.