--- In ham_amplifiers@..., R L Measures <r@...> wrote:
It can't be Rauch because he knows that the grid has to be positive
with respect to the cathode in order for grid-current to flow.
Sounds like someone needs to review his basic tube theory.
Specifically, look up "contact potential bias".
Now, for the experiment.
Lay tube on bench. Any tube, as long as it works. Apply specified
filament/heater current. Place a high-Z voltmeter between the
cathode/fil and the control grid. Once the cathode warms up, what do
you read? Is the grid negative W.R.T. to cathode, or positive? Given
that the voltmeter has finite resistance, would you say that the grid
current is zero, or non-zero?
Connect a 100kohm resistor between grid and cathode. Measure the
potential difference between grid and cathode. Given that this
voltage is non-zero, is there grid current? What is the polarity of
the voltage?
The experiment is more dramatic with a transmitting tube. I don't
recall what the values will be for a 3-500Z, but I just ran through
some 4CX250Bs...typ voltage developed in the experiment is control
grid negative to cathode by 15-30 volts, with grid current flowing.
Let us know the results.
Z
