Repaired and re capped a Zenith model MH table model and it was playing great for about an hour and then there was what looked like the grids were sparking inside the # 47 tube. Shut it down fast and ran the tube on the tube checker and it was ok. What would cause this tube of display inside the tube?

Hi Robert

Was the radio still operating when you saw grids sparking? If plate voltage is lost grid #2 can draw too much current, operate red and may spark.

Lack of grid #1 bias can also cause a tube to draw excessive current. Be sure grid #1 in negative around 15 volts in relation to filament.

http://www.nostalgiaair.org/PagesByModel/369/M0043369.pdf

Norm

:Repaired and re capped a Zenith model MH table model and it was playing great for about an hour and then there was what looked like the grids were sparking inside the # 47 tube. Shut it down fast and ran the tube on the tube checker and it was ok. What would cause this tube of display inside the tube?
:

:Hi Robert
:
:Was the radio still operating when you saw grids sparking? If plate voltage is lost grid #2 can draw too much current, operate red and may spark.
:
:Lack of grid #1 bias can also cause a tube to draw excessive current. Be sure grid #1 in negative around 15 volts in relation to filament.
:
:http://www.nostalgiaair.org/PagesByModel/369/M0043369.pdf
:
:Norm
:Hi Norm. The radio started thumping the speaker and the sound went down. I was watching i when it happened so it was only a few seconds of that.
::Repaired and re capped a Zenith model MH table model and it was playing great for about an hour and then there was what looked like the grids were sparking inside the # 47 tube. Shut it down fast and ran the tube on the tube checker and it was ok. What would cause this tube of display inside the tube?
::
:
:

::Hi Robert
::
::Was the radio still operating when you saw grids sparking? If plate voltage is lost grid #2 can draw too much current, operate red and may spark.
::
::Lack of grid #1 bias can also cause a tube to draw excessive current. Be sure grid #1 in negative around 15 volts in relation to filament.
::
::http://www.nostalgiaair.org/PagesByModel/369/M0043369.pdf
::
::Norm
::Hi Norm. The radio started thumping the speaker and the sound went down. I was watching i when it happened so it was only a few seconds of that.
:::Repaired and re capped a Zenith model MH table model and it was playing great for about an hour and then there was what looked like the grids were sparking inside the # 47 tube. Shut it down fast and ran the tube on the tube checker and it was ok. What would cause this tube of display inside the tube?
:::
::Norm. First thing I checked all of the voltages and all was fine. Installed my last 47 tube and the darn thing came alive again. I must of just had a bad tune in stock but we will see. I am going to add a dropping resistor the the circuit do to we are running 125 volts here in Montana.I would like to keep the old girl about 110v. Thanks for the heads up, forgot the - voltage. It was at -12 and that was running 110 volts on my varac.
::
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