05-26-2022, 11:54 AM
Hi John,
I didn't scroll down to see the voltage/current chart. In my head I did a rough calculation per tube. Rf, mixer, osc, IF, and pre audio equal about 3-5ma. Diodes and triodes configured as a diode 0ma. Audio output tube(s) 20+ma each. I guessed at the HV and used 250V. The only way the field coil would come in to play is if it was in parallel with the HV. Then it draws current.
As for math it all looks good. There is no harm in having the wattage higher than the actual dissipation. General rule of thumb is at least 2-3X. If you measure the physical size of the 20w vs the size of the bracket that's welded to the chassis It might be a good fit. A 20w power resistor isn't all that big. The other two could be wired in the circuit or attached to a terminal strip. I did see these https://www.ebay.com/itm/253830523764 I think they are 2 1/4" long but I'd check w/the seller if interested.
Replace the field coil with a resistor is doable situation with a 3k. In most sets the field coil is used to filter out the 60cy ripple from the dc HV. In this set the filtering is done by #'s 55,56, and 57. The coil is more or less as volt drop to supply the screen grid voltage to the rf and IF tubes. If there any instability in the circuits it can usually be quenched be adding a bypass cap to the screen line. And of course you will have to replace the speaker and add the an output transformer. As you've probably read already the Philco models 20B, 21B, 70B, and it's alliterations, and 90B's are the kinda the same well the coils are. The wiring and output transformers aren't.
There are some here on the Phorum in yrs past that have rewound K speaker coils. Early ones have a large nut on the back that makes it easier the remove the pole. The pole is a steel shaft that runs thru the center of the coil. Later K speakers the pole the pressed in. It can be removed with a large vise and a few sockets.
Here's a copy of the diagram w/the parts values. Makes it a bit easier to follow https://philcoradio.com/library/images/schem/90b.jpg
I didn't scroll down to see the voltage/current chart. In my head I did a rough calculation per tube. Rf, mixer, osc, IF, and pre audio equal about 3-5ma. Diodes and triodes configured as a diode 0ma. Audio output tube(s) 20+ma each. I guessed at the HV and used 250V. The only way the field coil would come in to play is if it was in parallel with the HV. Then it draws current.
As for math it all looks good. There is no harm in having the wattage higher than the actual dissipation. General rule of thumb is at least 2-3X. If you measure the physical size of the 20w vs the size of the bracket that's welded to the chassis It might be a good fit. A 20w power resistor isn't all that big. The other two could be wired in the circuit or attached to a terminal strip. I did see these https://www.ebay.com/itm/253830523764 I think they are 2 1/4" long but I'd check w/the seller if interested.
Replace the field coil with a resistor is doable situation with a 3k. In most sets the field coil is used to filter out the 60cy ripple from the dc HV. In this set the filtering is done by #'s 55,56, and 57. The coil is more or less as volt drop to supply the screen grid voltage to the rf and IF tubes. If there any instability in the circuits it can usually be quenched be adding a bypass cap to the screen line. And of course you will have to replace the speaker and add the an output transformer. As you've probably read already the Philco models 20B, 21B, 70B, and it's alliterations, and 90B's are the kinda the same well the coils are. The wiring and output transformers aren't.
There are some here on the Phorum in yrs past that have rewound K speaker coils. Early ones have a large nut on the back that makes it easier the remove the pole. The pole is a steel shaft that runs thru the center of the coil. Later K speakers the pole the pressed in. It can be removed with a large vise and a few sockets.
Here's a copy of the diagram w/the parts values. Makes it a bit easier to follow https://philcoradio.com/library/images/schem/90b.jpg
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got it!
Terry
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