11-13-2015, 04:07 AM
I suspect you're not quite understanding the flow of current through a tube... you can't just measure resistance from B+ to chassis because what you will probably end up measuring is a phantom... the charge in the capacitors will give you some kind of reading... which will change slowly since when you're doing a resistance measurement you are applying a small voltage to the circuit under test, which will build up the charge in the caps.
There are only two component types in the radio (this is best-guess since I can't find your schematic - I'm basing on the 49-500) that will pass DC from B+ to B- and one of those will only do that if its failed - the capacitors. Assuming they are good (because you've recapped the set) then the only path for electrons to travel is inside the tubes.
You can't just measure that with a resistance meter because (just as they are drawn in the schematic) each element in a tube is separated by an air-gap inside the glass envelope - there is no direct connection. The electrons will not flow without first being heated and attracted. Heating is done by applying a voltage to the filament, which excites the electrons on the cathode and creates a cloud of happy free electrons whizzing about. The second stage is applying a large positive voltage to the plate (and/or screen grid but ignore this for the moment) which draws the negatively charged electrons from the cathode to itself in a constant stream. Thus, electron flow occurs and the circuit completes from B+ to B-. No heater and/or no plate voltage = no current flow = no path from B+ to B-
As far as I am aware, there is no way to measure what I think you're trying to measure the way you're trying to measure it. That is, of course, if I understand what you were trying to do correctly?
Also, you may have also inadvertently shorted one or more of the filaments out of the circuit while taking voltage measurements - causing the other filaments to have to carry significantly more voltage - so that last problem you mentioned could be another dead heater (or two). Hopefully not, but thats my guess from your description.
Hope that helps
Steve
There are only two component types in the radio (this is best-guess since I can't find your schematic - I'm basing on the 49-500) that will pass DC from B+ to B- and one of those will only do that if its failed - the capacitors. Assuming they are good (because you've recapped the set) then the only path for electrons to travel is inside the tubes.
You can't just measure that with a resistance meter because (just as they are drawn in the schematic) each element in a tube is separated by an air-gap inside the glass envelope - there is no direct connection. The electrons will not flow without first being heated and attracted. Heating is done by applying a voltage to the filament, which excites the electrons on the cathode and creates a cloud of happy free electrons whizzing about. The second stage is applying a large positive voltage to the plate (and/or screen grid but ignore this for the moment) which draws the negatively charged electrons from the cathode to itself in a constant stream. Thus, electron flow occurs and the circuit completes from B+ to B-. No heater and/or no plate voltage = no current flow = no path from B+ to B-
As far as I am aware, there is no way to measure what I think you're trying to measure the way you're trying to measure it. That is, of course, if I understand what you were trying to do correctly?
Also, you may have also inadvertently shorted one or more of the filaments out of the circuit while taking voltage measurements - causing the other filaments to have to carry significantly more voltage - so that last problem you mentioned could be another dead heater (or two). Hopefully not, but thats my guess from your description.
Hope that helps
Steve
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