Hello,I'm trying to replace a speaker for a pre-1942 Motorola console, has a 10 inch speaker that has a 800 ohms field coil thats bad, can this be replaced with a different 10 inch speaker say one with a 900 or 1000 ohms field coil speaker that has the higher resistance and would it work right? I've have'nt found out yet if the original can be re-wound since its field coil is sandwiched between two brackets spot welded on the ends,so just checking out options.Thanks!Mike L.

You should not have a problem using a 900 or 1000 ohm field coil speaker. The B+ voltages may be a little low but should not be a problem. You could use a permanent magnet speaker and use a resistor in place of the field coil. Just make sure the resistor has adequate wattage. If the higher ohm field is a problem you could shunt it with a resistor to bring it back to the 800 ohm value. You will have to experiment or calculate the value of that resistor. I hope this helps.

MRO

:Hello,I'm trying to replace a speaker for a pre-1942 Motorola console, has a 10 inch speaker that has a 800 ohms field coil thats bad, can this be replaced with a different 10 inch speaker say one with a 900 or 1000 ohms field coil speaker that has the higher resistance and would it work right? I've have'nt found out yet if the original can be re-wound since its field coil is sandwiched between two brackets spot welded on the ends,so just checking out options.Thanks!Mike L.

When shunting a field coil with a resistor, it is good to know what voltage the field is supposed to drop when used in the radio it was originally designed for. This way, if shunting the field produces accurate radio voltages, but too low a field voltage, you can instead shunt for a balance between the two, if you achieve better results. I say this because if the field doesn't drop the voltage it is designed to drop, it won't build up enough magnetism.

T.

.....However, a 900 ohm field will probably work quite well, and a 1000 ohm field probably won't produce a noticeable reduction in quality and power, either.

T.

I've got a 930 ohms speaker I bought,so this may be the closest,and I'll check out what voltage should go to the field,I'm in the learning process still,but thanks for everyones help!Mike L.

930 ohms should be fine. If you cannot find out what radio the 930 ohm speaker originally came from, and what the field is supposed to drop, what you can do is just wire in the speaker without any shunting resistance. Try out the radio. Then shunt the field so that the combined resistance equals 800 ohms. Use a 2 to 5 watt resistor. I forgot the formula for parallel resistors, but you can do it by trial and error, too, starting with 1/4 watt resistors until you find the correct value...then substitute with a 5 watt resistor. Try the radio out again with the shunting resistor. See which way you like better. If you can't tell the difference, pick which one you like, or try shunting to make a resistance that is halfway between 800 and 930 ohms. See if it might work better than the other two. You probably won't notice much of a difference, though.

Thomas

:930 ohms should be fine. If you cannot find out what radio the 930 ohm speaker originally came from, and what the field is supposed to drop, what you can do is just wire in the speaker without any shunting resistance. Try out the radio. Then shunt the field so that the combined resistance equals 800 ohms. Use a 2 to 5 watt resistor. I forgot the formula for parallel resistors, but you can do it by trial and error, too, starting with 1/4 watt resistors until you find the correct value...then substitute with a 5 watt resistor. Try the radio out again with the shunting resistor. See which way you like better. If you can't tell the difference, pick which one you like, or try shunting to make a resistance that is halfway between 800 and 930 ohms. See if it might work better than the other two. You probably won't notice much of a difference, though.
:
:Thomas

The resistance should not be very critical. They designed those old radios with a lot of loose space in the specs for tube aging, so a 10 or even 20 per cent change should work just fine. Twenty per cent resistors were the norm for those days, and as the tubes aged, everything still worked just fine and dandy. Try it, I think you'll like it.

Lewis

Hello Thomas,I'm interested in learning about how to shunt the speaker field coil,like say its original speaker was 800ohms now using another speaker at 930 ohms the difference is 130 ohms so in shunting does this go between the two field coil wires? or only one resistor to one wire? would this be a 130 ohm resistor?& the start side or finish side? or (2) resistors divided out one to each of both wires?,I'm not sure how shunting works or how it is wired.Thanks,Mike L.

Mike,
To derive the net value of 2 resistors in parallel. R1xR2 divided by R1+R2. In your case a std. 5600 ohm resistor yields a net value of 797 ohms when paralled with the 930 ohm speaker field coil. That is connecting each lead of the resistor to each lead of the field coil. I would start with a 1/2 watt resistor. Typically 1/6 of the tube currents will flow thru the resistor, and the other 5/6's thru the coil ie. 5600/930.

marv

:Hello Thomas,I'm interested in learning about how to shunt the speaker field coil,like say its original speaker was 800ohms now using another speaker at 930 ohms the difference is 130 ohms so in shunting does this go between the two field coil wires? or only one resistor to one wire? would this be a 130 ohm resistor?& the start side or finish side? or (2) resistors divided out one to each of both wires?,I'm not sure how shunting works or how it is wired.Thanks,Mike L.

Hello Marv,I read your posting and now understand how to wire it,I also see how the 797ohms is derived I took the 930x5600=5208000. then 930+5600=6530 so dividing the 5208000.by 6530=797.54977 so I thought I would share this with anyone else trying to figure the math on how to shunt a field coil speaker one with higher ohms than the original field coil speaker if you needed to.Thanks for your help!Mike L.

Mike,
Glad I could help. The same method is used for multiple resistors. ie R1+R2+R3 etc.etc.

marv

:Hello Marv,I read your posting and now understand how to wire it,I also see how the 797ohms is derived I took the 930x5600=5208000. then 930+5600=6530 so dividing the 5208000.by 6530=797.54977 so I thought I would share this with anyone else trying to figure the math on how to shunt a field coil speaker one with higher ohms than the original field coil speaker if you needed to.Thanks for your help!Mike L.

The formula for any number of resistors is:

1/Rtotal=1/R1+1/R2+1/R3....

When you use the R1+R2/R1 times R2, you are really doing the same thing as you would with the first formula.

Lewis

Make sure that you try out the field in the three ways I said, though. Don't just connect a resistor across it and call it a day. The radio as a whole might work better without the resistor, or with a total resistance somewhere between 800 and 930. Most likely there will be little difference, but there may be enough for you to prefer one way over the other.

T.