|[AMRadio] Tube Tester Question (TV7D/U)|
k4kyv at charter.net
Sat Feb 13 16:55:07 EST 2010
My mutual conductance tester TV7D/U has a gas test function. The way the manual explains its function is that the tube is set up with a fixed plate voltage, and when you press the Gas 1 button, a negative bias is applied to the grid, and you manually use the bias pot to adjust this voltage until the plate current deflects the meter to the first large division (10 on the 0-120 scale). Then, while holding down the Gas 1 button, you simultaneously press the Gas 2 button. This inserts 80,000Ω in series with the grid. If the tube is gassy, it will draw a small amount of grid current which will cause the bias voltage to decrease, due to voltage drop in the series resistor. This reduction in negative bias voltage causes the plate current to increase. The manual recommends that a tube be discarded as gassy if the plate current increases by more than 10%, that is to say, if the meter increases more than one small division.
My question is, what if the pointer deflects downwards when the Gas 2 button is pressed? I see it deflect downwards far more often than upwards, and with some tubes the decrease is several small divisions. If the tube is free of gas, shouldn't it remain still? Nothing in the manual mentions the possibility that the plate current might decrease when the resistor is inserted in the grid circuit.
There is no such thing as "negative gas" in a tube, but it would seem that if inserting resistance into the grid circuit causes the plate current to change in either direction, this could be a problem, since it is very common to have 80k or more resistance in the grid circuit of a tube type amplifier, particularly with R-C coupling. What could be the cause of a decrease in plate current, other than a small contact potential bias, which should be thoroughly swamped out by the external fixed bias in this test circuit?
I am aware that sometimes a triode amplifier that is designed to handle a very low signal level, such as a microphone pre-amp, may have no cathode resistor, and depends on contact potential to generate a fraction of a volt negative bias to operate the tube. But in these circuit, there is usually several megohms of grid resistance. I always thought of this as a rather flaky way to generate the operating bias for a tube, but I am not even sure this has anything to do with the above described phenomenon.
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