NewYorkDave
Well-known member
Just wondering, what's the voltage at the screen grid (G2) with the tube plugged in? You definitely have the screen grid tied to the plate? What's the voltage at the input grid (G1)?
Another thought: with that humongous 1G grid resistor and the coupling cap, if the tube is gassy, the grid leak current could actually be biasing the tube off.
To test the tube off the circuit board, just grab a socket and wire up a little replica of the same circuit, point-to-point style. It should take about two minutes. Don't replicate the WHOLE circuit, just the DC portion, the plate, cathode and grid resistors. Use any convenient value of grid resistor, it doesn't have to be 1G. Lay it down carefully on a nonconductive surface. Connect your heater supply. Connect B+ to the plate resistor, and B- to the low side of the 220-ohm cathode resistor and the low side of the grid resistor. Check your voltages at the various electrodes with regard to B-. If everything looks good, just for laughs, try it again with a 1G grid resistor. See if anything funny happens. Measure amount of funny.
There are easier ways to connect up a tube just to check for emission, but doing it this way will give a direct correspondence to the voltages you should be seeing in your circuit.
For convenience, use clipleads for the power connections, if you have 'em. Be sure to disconnect power when changing things, keep one hand in your pocket, clip the negative lead of your voltmeter to the circuit and only use one hand to probe. Whipping up little circuits "rat's nest" style for testing is fun and easy, but you must exercise great caution when doing it with higher-voltage circuits.
Another thought: with that humongous 1G grid resistor and the coupling cap, if the tube is gassy, the grid leak current could actually be biasing the tube off.
To test the tube off the circuit board, just grab a socket and wire up a little replica of the same circuit, point-to-point style. It should take about two minutes. Don't replicate the WHOLE circuit, just the DC portion, the plate, cathode and grid resistors. Use any convenient value of grid resistor, it doesn't have to be 1G. Lay it down carefully on a nonconductive surface. Connect your heater supply. Connect B+ to the plate resistor, and B- to the low side of the 220-ohm cathode resistor and the low side of the grid resistor. Check your voltages at the various electrodes with regard to B-. If everything looks good, just for laughs, try it again with a 1G grid resistor. See if anything funny happens. Measure amount of funny.
There are easier ways to connect up a tube just to check for emission, but doing it this way will give a direct correspondence to the voltages you should be seeing in your circuit.
For convenience, use clipleads for the power connections, if you have 'em. Be sure to disconnect power when changing things, keep one hand in your pocket, clip the negative lead of your voltmeter to the circuit and only use one hand to probe. Whipping up little circuits "rat's nest" style for testing is fun and easy, but you must exercise great caution when doing it with higher-voltage circuits.
