Just finished this unit and hooked up the cable to a "C" cell and the other to 10 9V series connected. The 1N5 filament opened, replaced the tube and the filament opened again. Checked the cable and chassis and both are at ground potential. Replaced tube again and connected the neg. lead to the chassis and the unit works fine. I'm going to do some digging into this one when I get some time.

:Just finished this unit and hooked up the cable to a "C" cell and the other to 10 9V series connected. The 1N5 filament opened, replaced the tube and the filament opened again. Checked the cable and chassis and both are at ground potential. Replaced tube again and connected the neg. lead to the chassis and the unit works fine. I'm going to do some digging into this one when I get some time.

Hi Joe,
I glanced at the schematic. This radio has parallel filament. Had you been overvoltaging the filament circuit, every tube would have blown!
Suspect that the original 1N5GT and first replacement 1N5GT were both gassey, and had not been turned since they became gassey. With enough gas the filament will just flash like a fuse.
When the replacement blows in the same way as the original it does give one a moment to wonder. Your instincts are right.

Best Regards,

Bill Grimm

Are you sure that both tubes blew out? Did you test them on a tube tester? Maybe the socket is at fault? Contrary to what everyone else says on here, tubes usually won't light with their normal voltage if air is introduced. It is easier to heat a wire in a vacuum than it is when gasses can carry the heat away by convection. You may ask, then, why a lightbulb burns out right away when exposed to air. A modern lightbulb contains an inert gas. Vacuum bulbs had become somewhat obsolete by about 1910 or a little later. The gas pressure allows the bulb to be lit brighter without the filament vaporizing, but the filament must be spaced close together because otherwise the air current would carry away the heat too quickly. Take any radio tube and break it and then try to light it at normal voltage.

It is quite possible that the tube did burn out, possibly even from gas. I have had battery tubes break before, though, and I always check to see if I can light them just because I am curious, and I never can unless I increase the voltage.

Thomas

:Are you sure that both tubes blew out? Did you test them on a tube tester? Maybe the socket is at fault? Contrary to what everyone else says on here, tubes usually won't light with their normal voltage if air is introduced. It is easier to heat a wire in a vacuum than it is when gasses can carry the heat away by convection. You may ask, then, why a lightbulb burns out right away when exposed to air. A modern lightbulb contains an inert gas. Vacuum bulbs had become somewhat obsolete by about 1910 or a little later. The gas pressure allows the bulb to be lit brighter without the filament vaporizing, but the filament must be spaced close together because otherwise the air current would carry away the heat too quickly. Take any radio tube and break it and then try to light it at normal voltage.
:
:It is quite possible that the tube did burn out, possibly even from gas. I have had battery tubes break before, though, and I always check to see if I can light them just because I am curious, and I never can unless I increase the voltage.
:
:Thomas

I checked both of them on the tube tester and the filaments were open on both.