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In Reply to: RE: confused again posted by vinnie2 on February 13, 2019 at 06:28:42
You have three variables: P/K voltage, grid bias voltage and plate current. Only two of those variables can be independent. That means that you can fix two of them but the third will vary from tube to tube. No two tubes are exactly the same and no tube will stay the same as it ages. If you want to get close to a published OP I have used a simple test jig. You would need a current source that can sink the desired plate current and a variable B+ supply. The current source does not need to be anything special. A single DN2540 MOSFET, 1K carbon comp. grid stopper and 50R or 100R trim pot would do fine. Ground the grid of the tube to be tested, the current source goes between the cathode and ground along with a 1R series current sense resistor. Connect the variable PS to the plate. Use the current sense resistor to measure plate current and adjust the current source as necessary. Vary the B+ supply until the P/K voltage is where you want it. Then measure the grid bis voltage. Or set any other two variables and measure the third.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Follow Ups:
"You have three variables:..."
No, there are at least five of consequence. Number four is the condition of the tube. The need to reduce grid bias in order to create predicted current levels implies the tube is worn out. Variable number five goes to the competence and experience of the project manager. Past threads on similar topics indicate a significant problem in this area. Don't take for granted that any aspect of the test is as described.
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" No two tubes are exactly the same and no tube will stay the same as it ages."
I think that I covered #4. #5? I was discussing physical variables.
This can be a useful test jig when setting up direct coupling of stages. Several tubes can be tested and ballpark values can be established so that appropriate cathode resistors etc. can be chosen.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Thanks for the explanation!
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