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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Here it is straight from a 752A roll chart... posted by Steve O on January 12, 2007 at 20:30:25:
I cut this out of a post at tubetesters.info:Testing the 300B----a good place to start would be to set-up the 539 or 752 for a 2A3 or 45, except you have to set the filament voltage to 5 volts (the 300B draws a fair bit of filament current, so remember to check and reset the line voltage after the tube warms up; for a 539, you are also supposed to reset the line voltage AND bias voltage while performing the GM test as well). Bias voltage is the key here-----the test data for checking 300B's on the WECO KS-15560 and KS-15750 show an actual bias of -15 volts, so that is a good place to start. It would be easiest to test the 300B in your 539 because it has a bias-volts meter (whereas the bias-dial scale on the 752 uses an arbitrary scale that does not directly corresomd to bias-voltage). Set the function switch to the "D" scale, 6000 mhos @ .5 volts of grid-signal (unfortunately, that's about as close as we can get to the WECO tester settings, which use a 5 volt grid-signal to test a 300B).
On a WECO tester, MINIMUM GM readout or "reject" point for a 300B should be 2900 mho's, vs. a book-spec for NOS 300B's of 5500 mhos; also, during the cathode-activity test (the reduced-heater-voltage "life test") the tubes' output should not drop by more than 25%..... but as the Alan Douglas tube-tester book notes, the test data is not truly interchangeable from the 539 series testers to the WECO units because the WECO testers have an independent screen supply, whereas in the 539 and other "standard" Hickok models (including the military units) the screen voltage actually drops as the bias voltage is increased. Also, you are testing the tube with .5 volts of grid signal on the 539 vs. the 5 volt signal specified on the WECO data, so the tubes' measured output will likely tend to be rather a bit lower when tested on the 539.......
If you are testing the 300B on the 539C, you can easily hook up a dc milliammeter to the testers' supplied "plate current" jacks (unhook the metal jumper-clip) and monitor the plate current as you change the bias voltage. The 150 volt (Hickok B+) plate curve in the manual shows the 300B as drawing about 80 milliamps with a grid bias of -20; or 50 milliamps at -25 volts (and the transconductance curves in the manual show 5500 mho's at 150 volts and -25 grid-bias); this should tell you if you are in the ballpark bias-wise. Remember to check/reset the bias and line-voltage when actually testing the tube on the 539, and remember also that if you change the bias voltage then you will also change the plate-current draw, and this will change the load on the power-supply, and the line voltage will fluctuate up or down accordingly.
If you want to test the 300B on your 752, see my earlier posts pertaining to making plate-current measurements on ANY Hickok (or other tube tester) with the use of a socket extender----a four pin socket extender will allow you to monitor both the tubes' plate current AND the actual negative-bias voltage as well. You'll probably have to compromise on the "best" setting for the Multiplier switch as well (which sets both the meter range AND the grid-signal voltage). Bear in mind that on a 752 you should reset the line voltage after the tube has warmed up but NOT while you are testing the tube----the 539 is the only Hickok tester where you are supposed to reset the line voltage(and bias voltage) while plate current is flowing through the tube, during the actual GM test.........
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- 300B on 752 - gortnipper 11:55:08 01/13/07 (0)
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