|5th April 2005, 01:47 PM||#11|
diyAudio Moderator Emeritus
Join Date: Jan 2003
Location: Near London. UK
All you really need to test an EL84 is a 300V HT supply, a heater supply, and a few components. Connect the +300V to the anode and screen grid (add a 100 Ohm 1/2W resistor in the path of the screen grid to prevent oscillation. Connect the cathode and g3 to 0V via a 270 Ohm 1W resistor. Connect the grid to 0V via a 10k resistor. Apply heater power and you should see about 11V on the cathode - indicating 41mA of current.
Shorts: The most dangerous short is between grid and cathode. But it may not show up hot. You could use a meter on its resistance range to test with heater power applied, but I wouldn't bother. I'd just pop it in the test circuit and switch off the 300V if there's a firework display in the valve or if the voltage across the 270 Ohm resistor rises beyond 15V.
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
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