I just got a batch of (6) IN-18 tubes and one of them displays this odd purple color only when the #1 segment is lit up. Is this something that will go away with time or should I have the person send me another IN-18 to replace this one?
This is the well known Blue Spot problem!
Here is the description I sent to Brian today:
The blue spots are caused by the following circumstances:
If low voltage drivers are used, as example the 74141 drivers, the cold cathode voltage (non glowing cathode voltage) drops down to the clamping voltage of the output transistors (70-100 Volts).
In this state the mercury slightly breaks down, slightly ionizes, and little current flows from the anode, through all 9 non glowing cathodes through the Z-Diodes (Driver protection diodes) to ground. And that
inonized mercury can be seen as blue spots in the tube.
How to avoid it?
Don't use any off cathode clamping voltage, but high voltage drivers.
And again, for the thousandth time: YOU CAN NOT DRIVE ALL IN-18 TUBES WITH 74141 DRIVERS!!!! SO DO NEVER USE THEM!
IF A BLUE SPOT APPEARS IN YOUR IN-18 TUBE THE CIRCUIT IS WRONG, NOT THE TUBE!
Bad circuit - Hmmmm my kit is one from tubehobby and the tube there that outgassed worked just fine. The replacement IN-18 has the blue dots for the #1 digit. So am I now faced with a bad circuit or a tube that is again out gassing?
Maybe I should switch kits ag go with a different setup.