Hatsune Mike
Champion
Just a heads up: I wouldn't run the MS-293x tube without a neck board without some serious caution. Pin 4 - often unused on tubes of this shape and age - presents the full ~28kV through a large resistor:
When I say large, I mean that it's so high resistance that it is immeasurable with my meter.
This resistor has two taps partway down that allow the tube to create a few extra grids using a division of the 28kV voltage, without requiring another voltage from the chassis. I expect these are part of why the Blast tube has such exceptional focus when it is in good condition.
Anyway, it's highly current limited, but the result of this is that you should still be weary that when left floating there exists the full HV potential at pin 4, which is not shrouded like the focus pin. Consequently, using an MS8 or MS9 chassis on this tube will result in some arcing between pin 4 and the grounded or negative pin 5, which is audible.
I also fear that this more complex and high-tech electron gun design may be responsible for HV arcing to the glass or outside of the neck (to the yoke?) which could help explain some damage underneath the yoke that we've seen.
When I say large, I mean that it's so high resistance that it is immeasurable with my meter.
This resistor has two taps partway down that allow the tube to create a few extra grids using a division of the 28kV voltage, without requiring another voltage from the chassis. I expect these are part of why the Blast tube has such exceptional focus when it is in good condition.
Anyway, it's highly current limited, but the result of this is that you should still be weary that when left floating there exists the full HV potential at pin 4, which is not shrouded like the focus pin. Consequently, using an MS8 or MS9 chassis on this tube will result in some arcing between pin 4 and the grounded or negative pin 5, which is audible.
I also fear that this more complex and high-tech electron gun design may be responsible for HV arcing to the glass or outside of the neck (to the yoke?) which could help explain some damage underneath the yoke that we've seen.