Look what the dog dragged home

[earlyfilm]

Originally Posted by Paul Knaack

Acorrding to Sams it says current should be about 280 ma. not to exceed 300 ma with a .47 mfd cap across meter. I'm not sure if my meter has a cap across it or how that would effect the reading. So do you think its ok were its at?

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

[Findm-Keepm]

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

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,