Quote:
Originally Posted by earlyfilm
The .47MFD should be added across the meter leads to prevent the meter from loading the tube and giving a false reading. Any DC current meter adds some AC load to circut, and the added capacitor helps smooth it out.
Until you do this, you don't have an accurate measure of cathode current.
Usually this added load lowers the cathode current, but there are a very few exceptions.
James
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Actually, the cap nulls any added inductance (meter coil) and is necessary to prevent ringing which will destroy the HOT or other horizontal sweep components. Jack Darr commented on it, and Margolis' book details it better.
http://www.americanradiohistory.com/...-Page-0061.pdf
The meter's internal resistance acts like a larger value cathode resistor and has some limiting action, but the added inductance can bring about ringing that the tube and sweep components cant handle, so you bypass it with the cap.
I just bypass the milliammeter altogether, and add a 1 ohm resistor (
not a wirewound!) in the cathode of the HOT. Using the unity law, the measured voltage across the resistor = cathode current. An analog high-impedance meter works best (I use a WV-98C VTVM) so you can see the peaks and valleys while adjusting the efficiency coil. The Pomona 2712A test socket I have for the 6JS6s has a little booklet suggesting this.
I'm tempted to install a cheapo meter permanently, just to check cathode current often. RCA 113382 flys are not exactly common these days...
Cheers,