[R-390] R-390A B+ Current?

[2002tii]

John wrote:

>Does anyone know the approximate DC current on the R-390A B+ line during
>operation and standby?
>
>Has anyone looked at the DC / RMS voltage drop and/or waveform across the
>26Z5W?  (The measurement I'm wondering about would most likely be
>differential if you were using an oscilloscope.)

Here are four data points:

The schematic calls out the B+ secondary winding as 570 V, 200 mA CT.

The 26Z5 is rated for 55 mA DC per section, maximum (thus, 220 mA 
maximum as deployed in the 390A).

There is a 250 mA B+ fuse between the rectifiers and the power supply filter.

FC-6 replaced the 26Z5s with 1N561s, which is a 1 kV, 250 mA 
(average) part.

Typically, cathodic (indirectly heated) rectifiers drop from 10-25 V 
depending on how hard they are run.  The 390A runs the 26Z5s pretty 
hard, so 20-25 V is a good average.  In order to duplicate this drop 
if you use silicon diodes (these days, one would use 1 kV, 2.5 or 3 A 
rectifier diodes), you can put a resistor between the rectifiers and 
the first filter choke.  220 ohms is the traditional value for 
390As.  Note that the resistor will dissipate 3-5 watts, so it needs 
to be a 10 W power resistor.

Best regards,

Don


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