Hi Jim The idea that you have a triode connection is not exactly valid for this type of test because there is no series load resistor. The plate and screen voltage are held constant regardless of the plate current. It is however important that you measure the plate current separately from the screen current, if you want to measure the GM of a tetrode connection. Changes in screen current, is not what the test is about. The plate and screen voltage may come from the same source. The screen voltage is most critical and must be maintained constant as close as possible. Since plate and screen come from the same source then the source voltage should be monitored with a DVM and adjusted to be the constant of 250 Volts. This will require a little juggling each time you make a change in grid voltage. The actual plate current that you start with (90-110 ma) is not so important. But the change (delta) Ip with grid voltage change is what we are looking for. The only other thing that might be a problem is that the plate dissipation will be about 25 watts. Is this tube OK for that during the test? I would not use a delta Eg of more than 5 volts to make the test this would yield a change in plate current of more than 46.5 ma which is about 50 percent of the quiescent current of 100 ma. It is important that the increase in current when the Eg is made more negative be equal to the decrease in current when the Eg is made less negative by the same amount. In other words it should be linear. Keep in mind that when the plate current goes to 150 ma or more that the plate dissipation will be 50 percent more so make you measurement fast and don't for get to hold the EP constant with that DVM.
You could also place a 1 ohm resistor in the plate lead and capacitively couple to a sensitive AC voltmeter (scope) and measure the AC current produced as a change in Eg AC. A 100 ma change would be represented as 0.1VPTP on the scope. You would need a very large capacitor to Filter the source with. 73 John, WA5BXO -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim candela Sent: Sunday, October 12, 2003 11:08 PM To: amradio@mailman.qth.net Subject: RE: [AMRadio] Testing tubes Hi Group, I got a wild hair, and made a setup to compare a set of used Genalex KT77's to a new MP (matched pair) of Sovetek EL34's. I was following a note in the KT77 data sheet that stated when a KT77 is considered bad (< 9.3 ma/volt transconductance when EP, Eg2 = 250 volts held constant, and IP = 100 ma). Following this criteria, I made a test setup. I am not too sure of my technique as it is a work in progress. I am curious that the Gm can vary considerably when changing Vg1 +/- 1 volt from the value used to obtain 100 ma plate current. Maybe this is all wrong, but I averaged the two numbers assuming that is the final answer. Since Genalex stated to run the screen and plate at 250 volts, I connected them together (Fluke 8012a looks at plate current only), so this test is in triode mode (or is it not?). For a high voltage supply I used my Sherwood S8000 stereo amp / FM with a variac (get ~ 250 vdc at 65 volts AC input). Hey it was there (awaiting a re-cap job), convenient, and able to do the job! Is the data I've taken valid? If not, can someone enlighten me? Maybe I need to vary Vg1 less than +/- 1 volt, for better data? Regards, Jim Candela Test results: http://pages.prodigy.net/jcandela/KT77/genalex_kt77.htm KT77 Data Sheet: http://pages.prodigy.net/jcandela/KT77/kt77.pdf Test Setup: http://pages.prodigy.net/jcandela/KT77/PA110015.JPG _______________________________________________ AMRadio mailing list AMRadio@mailman.qth.net http://mailman.qth.net/mailman/listinfo/amradio