04-16-2016, 08:47 AM
I'm not sure that's close enough; it's 40% low, so it's got a short in there somewhere (assuming it is the right part--you might want to check the part number of the speaker to be sure.) For now, it's what you have to live with. But make sure you don't have any continuity between the field coil and any metal part of the speaker.
Your schematic might have voltage numbers at various points. Some do, some don't. If so, you can use them to calculate the expected B+ current and then use the voltage drop across the field coil, the choke, or the output transformer to calculate what it actually is.
The main thing now is to make sure you have B+ to the plates of the tubes, and to make sure it is within range of the published voltages, since that was the original problem you had. Once you get past that problem, you can move on to the next one.
You might have one or more resistors in series between the center tap of the power transformer and the negative end of the output filter cap, which is probably grounded to the chassis. The purpose of those is to drop the center tap below the chassis ground and generate negative voltages to bias the grids and maybe the cathodes of some of the tubes. If one or more of those resistors is open, you won't have a return path for B current, so check those resistors if they're shown on the schematic. Alternatively, the center tap might be connected directly to ground, in which case, ignore the above.
Once you've established continuity from the rectifier cathode through the field coil, choke, output transformer, and bias resistors (if present), if you don't get B+ at the output tube plate, or if it is very low, you've probably got a short somewhere, so turn the radio off and use your ohm meter and the schematic to find it. Otherwise something is liable to smoke. If you monitor the voltage drop across the field coil when you power up, you can see if it's in the right ball park very quickly.
Your schematic might have voltage numbers at various points. Some do, some don't. If so, you can use them to calculate the expected B+ current and then use the voltage drop across the field coil, the choke, or the output transformer to calculate what it actually is.
The main thing now is to make sure you have B+ to the plates of the tubes, and to make sure it is within range of the published voltages, since that was the original problem you had. Once you get past that problem, you can move on to the next one.
You might have one or more resistors in series between the center tap of the power transformer and the negative end of the output filter cap, which is probably grounded to the chassis. The purpose of those is to drop the center tap below the chassis ground and generate negative voltages to bias the grids and maybe the cathodes of some of the tubes. If one or more of those resistors is open, you won't have a return path for B current, so check those resistors if they're shown on the schematic. Alternatively, the center tap might be connected directly to ground, in which case, ignore the above.
Once you've established continuity from the rectifier cathode through the field coil, choke, output transformer, and bias resistors (if present), if you don't get B+ at the output tube plate, or if it is very low, you've probably got a short somewhere, so turn the radio off and use your ohm meter and the schematic to find it. Otherwise something is liable to smoke. If you monitor the voltage drop across the field coil when you power up, you can see if it's in the right ball park very quickly.
John Honeycutt
