Nick (Surrey) A simple way to determine Ra without plotting curves would be to apply a fixed bias voltage to the grid and measure the voltages at the anode and the anode currents for two different load resistances, divide the difference of the former by the difference of the latter.
There's a set up in the RCA Home Study Course here (experiment lesson 22):
http://mikeyancey.com/files/RCA%20Home%2...iments.pdf
It shows a log pot across the -ve supply with a shift resistor in series for setting the grid bias, the log pot can be replaced by a lin pot, the shift resistor wouldn't be need then, calibrate the pot and you wouldn't need a voltmeter to measure bias voltage.
The B + supply used is 100 volts, that should put the reference points well into the most linear part of the Va Ia slope for most valves.
Here's the link about valve characteristics that's relevant to the above experiment, it's well written and clear and uses a known valve (6J5) and it's published curves and specs for the calculations, well worth a read.
http://km5z.us/files/RCA%20Home%20Study/...stants.pdf
The link to most of the rest of the course:
http://mikeyancey.com/RCA_Radio_TV_Course_1958.php
Lawrence.
There's a set up in the RCA Home Study Course here (experiment lesson 22):
http://mikeyancey.com/files/RCA%20Home%2...iments.pdf
It shows a log pot across the -ve supply with a shift resistor in series for setting the grid bias, the log pot can be replaced by a lin pot, the shift resistor wouldn't be need then, calibrate the pot and you wouldn't need a voltmeter to measure bias voltage.
The B + supply used is 100 volts, that should put the reference points well into the most linear part of the Va Ia slope for most valves.
Here's the link about valve characteristics that's relevant to the above experiment, it's well written and clear and uses a known valve (6J5) and it's published curves and specs for the calculations, well worth a read.
http://km5z.us/files/RCA%20Home%20Study/...stants.pdf
The link to most of the rest of the course:
http://mikeyancey.com/RCA_Radio_TV_Course_1958.php
Lawrence.
