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Calibrating tube tester - one question
200, 200, -4.6, 50 according to page 4 of
http://www.retrovox.com.au/STC6BQ5.pdf Page 7 of the same report shows curves for Vs = 200, so you can check. There are tonnes of sites with EL84 data, check via http://www.duncanamps.com/ In general, you need to find the anode characteristic curves for whatever screen voltage you are using. Choose the Va you will use, and draw a vertical line on the curves at that voltage. It will intersect with several curves. Find one intersection that gives a convenient current: that gives you the required grid voltage. You may need to interpolate between two intersections. There are other presentations of the same data: grid and screen characteristics, that can also be used...you just need one that shows the relationship between Va, Vs, Vg and Ia. That's 4 variables though so no single graph can express it all. That's why each set of characteristics is a family of graphs, each with a family of curves. Perhaps screen characteristics for Va=200V would be better for you. Dunno if they are in that report, I don't have broadband. cheers, Ian "Richard J" wrote in message ... Hi guys - have got myself a tube tester that can do all sorts of tricks, itīs a German RPG70. It needs recalibration on the 50mA scale of the instrument. Long story, but getting there fast. Have documentation, but no calibration instructions. Short question: How much anode current will/does a new EL84 draw at the following parameters (stipulated by the RPG70 test card for EL84)? Ua=200V, Ug2=60V, Ug=0V An estimated guess would be somewhere in the region 17-20mA, but knowing is a lot better than guessing. From the tube data sheet, I can guess that it would draw abt 48mA at Ua=250, UG2=250, Ug=0V. But: the tube data sheets all begin at 250V....and not 200V - and my tube tester hasnīt got a 250V option. My math doesnīt cover this.... I have the following tester options: Ua=25, 50,100,150,200V Ug2=20,60,100,150,200V Ug= negative 0-18V Help, pse? /Richard (used to simple emission testers, this is a different ballgame) |
#2
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Ian,
thanks for the input. I found a chart that ties anode and G2 to the same voltage, all the way down to 20V or so, and shows plate current with respect to g1 voltage. From this chart, I can use the 50/50/0V, 100/100/0V points without having to use the negative g1 voltage (which I donīt trust yet) in order to get the 50mA and 100mA scales lined up reasonably well. WHen done, I might just try the 200/200/-4,6 trick you sent me :-) Cheers /Richard, tube tester wannabe "Ian Iveson" wrote in message ... 200, 200, -4.6, 50 according to page 4 of http://www.retrovox.com.au/STC6BQ5.pdf Page 7 of the same report shows curves for Vs = 200, so you can check. There are tonnes of sites with EL84 data, check via http://www.duncanamps.com/ In general, you need to find the anode characteristic curves for whatever screen voltage you are using. Choose the Va you will use, and draw a vertical line on the curves at that voltage. It will intersect with several curves. Find one intersection that gives a convenient current: that gives you the required grid voltage. You may need to interpolate between two intersections. There are other presentations of the same data: grid and screen characteristics, that can also be used...you just need one that shows the relationship between Va, Vs, Vg and Ia. That's 4 variables though so no single graph can express it all. That's why each set of characteristics is a family of graphs, each with a family of curves. Perhaps screen characteristics for Va=200V would be better for you. Dunno if they are in that report, I don't have broadband. cheers, Ian "Richard J" wrote in message ... Hi guys - have got myself a tube tester that can do all sorts of tricks, itīs a German RPG70. It needs recalibration on the 50mA scale of the instrument. Long story, but getting there fast. Have documentation, but no calibration instructions. Short question: How much anode current will/does a new EL84 draw at the following parameters (stipulated by the RPG70 test card for EL84)? Ua=200V, Ug2=60V, Ug=0V An estimated guess would be somewhere in the region 17-20mA, but knowing is a lot better than guessing. From the tube data sheet, I can guess that it would draw abt 48mA at Ua=250, UG2=250, Ug=0V. But: the tube data sheets all begin at 250V....and not 200V - and my tube tester hasnīt got a 250V option. My math doesnīt cover this.... I have the following tester options: Ua=25, 50,100,150,200V Ug2=20,60,100,150,200V Ug= negative 0-18V Help, pse? /Richard (used to simple emission testers, this is a different ballgame) |
#3
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The simplest way to fix this was actually to just load the circuit with a
2K/10W resistor and 100V=50mA fullscale reading. Turned out that one trimmer pot needed swapping. After fixing this, all measurements suddenly made sense. Thanks!/ Richard |
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