If it were my amp I would upgrade it a bit with directly grounded grids. Then float the B- and use a diode to ground from the B- to clamp the B-. Tom W8JI has this at his webpage. Then once you do this you can monitor the grid current between B- and ground. You can add the "vox relay" which can be used to bias the tube off and on therough the self bias resistor. The cut off is 99% complete and perhaps only a couple of millamperes gets through. It barely shows on a 0-1 amp meter.