[AMRadio] N3RHT Shack Update

Donald Chester k4kyv at hotmail.com
Sun Mar 6 12:40:17 EST 2005

>I have the same phenomenon with the 872's or 575's (whichever happen to be 
>installed) in a homebrew rig. 3 light but one is dark, and the phenomenon 
>stays at that socket. No obvious other issues.. how curious.

Make sure the tube filament pins are making good contact.  I have seen it 
fail due to corrosion of the contacts in the sockets, or sometimes they get 
sprung to the point there is no tension on the pin.

Make sure you have the same filament voltage on each tube (check it with a 
voltmeter by actually touching the tube pin itself, not the socket 
terminals, with the meter probe).  Also observe the glow of the filaments to 
estimate if they are getting equal voltage.

If the filaments are getting proper voltage, try swapping the leads to the 
plate caps of the tubes.  Maybe the plate transformer has gone defective and 
is no longer putting out equal voltage on each half of the winding under 

Don k4kyv

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