Within the field coil is another winding that is in series with the speaker. I believe this is a "hum bucking" coil.
The speaker is 3.2 ohms. I want to use the original audio output transformer but don't know what speaker to use. I think it should be higher than 3.2 ohms since the hum bucking coil will be removed. Would an 8 ohm speaker be sufficient?
How do you calculate this?
Gary
:
:I need to replace the speaker on a Marconi Model 187.
:There is a field coil, 650 ohms hot (per service info), which I will replace with a 10W, 700 ohm resistor.
:
:Within the field coil is another winding that is in series with the speaker. I believe this is a "hum bucking" coil.
:
:The speaker is 3.2 ohms. I want to use the original audio output transformer but don't know what speaker to use. I think it should be higher than 3.2 ohms since the hum bucking coil will be removed. Would an 8 ohm speaker be sufficient?
:
:How do you calculate this?
:
:Gary
The loudspeaker data in Marconi service manual states:
Voice Coil Impedance @400Hz, 3.2 Ohms
Voice Coil Resistance, 3.1 Ohms
Output Transformer (Primary Res.) 250 Ohms.
The output consists of 2 6F6Gs in parallel.
I am concerned about the effect on the load impedance if the hum bucking coil is removed.
Gary
:Gary, is the speaker voice coil 3.2 ohms (audio freq impedance) or DC resistance? Often, the DC resistance is about half the speaker impedance (which is nominally at 400Hz).
:Doug
:
::
::I need to replace the speaker on a Marconi Model 187.
::There is a field coil, 650 ohms hot (per service info), which I will replace with a 10W, 700 ohm resistor.
::
::Within the field coil is another winding that is in series with the speaker. I believe this is a "hum bucking" coil.
::
::The speaker is 3.2 ohms. I want to use the original audio output transformer but don't know what speaker to use. I think it should be higher than 3.2 ohms since the hum bucking coil will be removed. Would an 8 ohm speaker be sufficient?
::
::How do you calculate this?
::
::Gary
Another question:
I plan on mounting the original output transformer on the new speaker, similar to the original. Since the wires for the old field coil are out at the speaker, what do you think of mounting a terminal strip on the speakers mounting board. This terminal strip would be used to mount the resistor needed to replace the field coil load?
I guess I could just remove the field coil wire all together and mount the resistor under the chassis. This might be safer.
Any suggestions?
Gary
:Gary, is the speaker voice coil 3.2 ohms (audio freq impedance) or DC resistance? Often, the DC resistance is about half the speaker impedance (which is nominally at 400Hz).
:Doug
:
::
::I need to replace the speaker on a Marconi Model 187.
::There is a field coil, 650 ohms hot (per service info), which I will replace with a 10W, 700 ohm resistor.
::
::Within the field coil is another winding that is in series with the speaker. I believe this is a "hum bucking" coil.
::
::The speaker is 3.2 ohms. I want to use the original audio output transformer but don't know what speaker to use. I think it should be higher than 3.2 ohms since the hum bucking coil will be removed. Would an 8 ohm speaker be sufficient?
::
::How do you calculate this?
::
::Gary
:Doug,
:
:Another question:
:I plan on mounting the original output transformer on the new speaker, similar to the original. Since the wires for the old field coil are out at the speaker, what do you think of mounting a terminal strip on the speakers mounting board. This terminal strip would be used to mount the resistor needed to replace the field coil load?
:
:I guess I could just remove the field coil wire all together and mount the resistor under the chassis. This might be safer.
:
:Any suggestions?
:
:Gary
:
:
:
::Gary, is the speaker voice coil 3.2 ohms (audio freq impedance) or DC resistance? Often, the DC resistance is about half the speaker impedance (which is nominally at 400Hz).
::Doug
::
:::
:::I need to replace the speaker on a Marconi Model 187.
:::There is a field coil, 650 ohms hot (per service info), which I will replace with a 10W, 700 ohm resistor.
:::
:::Within the field coil is another winding that is in series with the speaker. I believe this is a "hum bucking" coil.
:::
:::The speaker is 3.2 ohms. I want to use the original audio output transformer but don't know what speaker to use. I think it should be higher than 3.2 ohms since the hum bucking coil will be removed. Would an 8 ohm speaker be sufficient?
:::
:::How do you calculate this?
:::
:::Gary
What is the DC resistance of the hum blocking coil?
Djoug
: Gary, if the resistor will fit near spkr and more room it will be able to dissipate heat better out there then under chasis. PL
:
:
::Doug,
::
::Another question:
::I plan on mounting the original output transformer on the new speaker, similar to the original. Since the wires for the old field coil are out at the speaker, what do you think of mounting a terminal strip on the speakers mounting board. This terminal strip would be used to mount the resistor needed to replace the field coil load?
::
::I guess I could just remove the field coil wire all together and mount the resistor under the chassis. This might be safer.
::
::Any suggestions?
::
::Gary
::
::
::
:::Gary, is the speaker voice coil 3.2 ohms (audio freq impedance) or DC resistance? Often, the DC resistance is about half the speaker impedance (which is nominally at 400Hz).
:::Doug
:::
::::
::::I need to replace the speaker on a Marconi Model 187.
::::There is a field coil, 650 ohms hot (per service info), which I will replace with a 10W, 700 ohm resistor.
::::
::::Within the field coil is another winding that is in series with the speaker. I believe this is a "hum bucking" coil.
::::
::::The speaker is 3.2 ohms. I want to use the original audio output transformer but don't know what speaker to use. I think it should be higher than 3.2 ohms since the hum bucking coil will be removed. Would an 8 ohm speaker be sufficient?
::::
::::How do you calculate this?
::::
::::Gary
The hum bucking coil measured 4.3 ohms. This resistance, in series with the original 3.2 ohm voice coil works out to 7.5 ohms. That's why I was thinking of using an 8 ohm speaker.
Gary
:Gary: The resistor will have B+ on it, so make sure it (and the terminal strip) don't present a shock hazard.
:
:What is the DC resistance of the hum blocking coil?
:Djoug
:
:: Gary, if the resistor will fit near spkr and more room it will be able to dissipate heat better out there then under chasis. PL
::
::
:::Doug,
:::
:::Another question:
:::I plan on mounting the original output transformer on the new speaker, similar to the original. Since the wires for the old field coil are out at the speaker, what do you think of mounting a terminal strip on the speakers mounting board. This terminal strip would be used to mount the resistor needed to replace the field coil load?
:::
:::I guess I could just remove the field coil wire all together and mount the resistor under the chassis. This might be safer.
:::
:::Any suggestions?
:::
:::Gary
:::
:::
:::
::::Gary, is the speaker voice coil 3.2 ohms (audio freq impedance) or DC resistance? Often, the DC resistance is about half the speaker impedance (which is nominally at 400Hz).
::::Doug
::::
:::::
:::::I need to replace the speaker on a Marconi Model 187.
:::::There is a field coil, 650 ohms hot (per service info), which I will replace with a 10W, 700 ohm resistor.
:::::
:::::Within the field coil is another winding that is in series with the speaker. I believe this is a "hum bucking" coil.
:::::
:::::The speaker is 3.2 ohms. I want to use the original audio output transformer but don't know what speaker to use. I think it should be higher than 3.2 ohms since the hum bucking coil will be removed. Would an 8 ohm speaker be sufficient?
:::::
:::::How do you calculate this?
:::::
:::::Gary
I'm still scratching my head about why the old speaker's voice-coil impedance at 400Hz is essentially equal to its DC resistance. But whatever.
Doug
:
:Doug,
:
:The hum bucking coil measured 4.3 ohms. This resistance, in series with the original 3.2 ohm voice coil works out to 7.5 ohms. That's why I was thinking of using an 8 ohm speaker.
:
:Gary
:
::Gary: The resistor will have B+ on it, so make sure it (and the terminal strip) don't present a shock hazard.
::
::What is the DC resistance of the hum blocking coil?
::Djoug
::
::: Gary, if the resistor will fit near spkr and more room it will be able to dissipate heat better out there then under chasis. PL
:::
:::
::::Doug,
::::
::::Another question:
::::I plan on mounting the original output transformer on the new speaker, similar to the original. Since the wires for the old field coil are out at the speaker, what do you think of mounting a terminal strip on the speakers mounting board. This terminal strip would be used to mount the resistor needed to replace the field coil load?
::::
::::I guess I could just remove the field coil wire all together and mount the resistor under the chassis. This might be safer.
::::
::::Any suggestions?
::::
::::Gary
::::
::::
::::
:::::Gary, is the speaker voice coil 3.2 ohms (audio freq impedance) or DC resistance? Often, the DC resistance is about half the speaker impedance (which is nominally at 400Hz).
:::::Doug
:::::
::::::
::::::I need to replace the speaker on a Marconi Model 187.
::::::There is a field coil, 650 ohms hot (per service info), which I will replace with a 10W, 700 ohm resistor.
::::::
::::::Within the field coil is another winding that is in series with the speaker. I believe this is a "hum bucking" coil.
::::::
::::::The speaker is 3.2 ohms. I want to use the original audio output transformer but don't know what speaker to use. I think it should be higher than 3.2 ohms since the hum bucking coil will be removed. Would an 8 ohm speaker be sufficient?
::::::
::::::How do you calculate this?
::::::
::::::Gary
Thanks Doug.
I emailed the schematic to you some time ago.
These numbers are listed on the fisrt page under speaker data.
Gary
:Gary: I think that should be fine.
:
:I'm still scratching my head about why the old speaker's voice-coil impedance at 400Hz is essentially equal to its DC resistance. But whatever.
:Doug
:
:
::
::Doug,
::
::The hum bucking coil measured 4.3 ohms. This resistance, in series with the original 3.2 ohm voice coil works out to 7.5 ohms. That's why I was thinking of using an 8 ohm speaker.
::
::Gary
::
:::Gary: The resistor will have B+ on it, so make sure it (and the terminal strip) don't present a shock hazard.
:::
:::What is the DC resistance of the hum blocking coil?
:::Djoug
:::
:::: Gary, if the resistor will fit near spkr and more room it will be able to dissipate heat better out there then under chasis. PL
::::
::::
:::::Doug,
:::::
:::::Another question:
:::::I plan on mounting the original output transformer on the new speaker, similar to the original. Since the wires for the old field coil are out at the speaker, what do you think of mounting a terminal strip on the speakers mounting board. This terminal strip would be used to mount the resistor needed to replace the field coil load?
:::::
:::::I guess I could just remove the field coil wire all together and mount the resistor under the chassis. This might be safer.
:::::
:::::Any suggestions?
:::::
:::::Gary
:::::
:::::
:::::
::::::Gary, is the speaker voice coil 3.2 ohms (audio freq impedance) or DC resistance? Often, the DC resistance is about half the speaker impedance (which is nominally at 400Hz).
::::::Doug
::::::
:::::::
:::::::I need to replace the speaker on a Marconi Model 187.
:::::::There is a field coil, 650 ohms hot (per service info), which I will replace with a 10W, 700 ohm resistor.
:::::::
:::::::Within the field coil is another winding that is in series with the speaker. I believe this is a "hum bucking" coil.
:::::::
:::::::The speaker is 3.2 ohms. I want to use the original audio output transformer but don't know what speaker to use. I think it should be higher than 3.2 ohms since the hum bucking coil will be removed. Would an 8 ohm speaker be sufficient?
:::::::
:::::::How do you calculate this?
:::::::
:::::::Gary
I have used aluminum encased (think Dale is one producer) power resistors for the purpose of replacing the field of an electrodynamic speaker with a PM type. I used the resistor to mark for screw holes and drilled a couple holes in the basket of the speaker and mounted the resistor. The speaker could also serve as a heatsink for the resistor. I used sleeving on the solder connection to the resistor.
Radiodoc
*******************
:Doug,
:
:Another question:
:I plan on mounting the original output transformer on the new speaker, similar to the original. Since the wires for the old field coil are out at the speaker, what do you think of mounting a terminal strip on the speakers mounting board. This terminal strip would be used to mount the resistor needed to replace the field coil load?
:
:I guess I could just remove the field coil wire all together and mount the resistor under the chassis. This might be safer.
:
:Any suggestions?
:
:Gary
:
:
:
::Gary, is the speaker voice coil 3.2 ohms (audio freq impedance) or DC resistance? Often, the DC resistance is about half the speaker impedance (which is nominally at 400Hz).
::Doug
::
:::
:::I need to replace the speaker on a Marconi Model 187.
:::There is a field coil, 650 ohms hot (per service info), which I will replace with a 10W, 700 ohm resistor.
:::
:::Within the field coil is another winding that is in series with the speaker. I believe this is a "hum bucking" coil.
:::
:::The speaker is 3.2 ohms. I want to use the original audio output transformer but don't know what speaker to use. I think it should be higher than 3.2 ohms since the hum bucking coil will be removed. Would an 8 ohm speaker be sufficient?
:::
:::How do you calculate this?
:::
:::Gary
I mounted the ressistor up near the speaker on a terminal strip and replaced the speaker with an 8 ohm PM type. One thing I didn't anticipate, removing the field coil / hum bucking coil results in unacceptable hum. Imagine that! I suspect the main reason is the field coil is also a filter choke in the power supply filter network. Adding 40uf after my new resistor almost eliminates the hum.
Any suggestions, comments?
I guess I could look for a filter choke to use instead of the resistor.
This must be a common issue when replacing speakers with PM type.
Gary
:Doug,
:
:Another question:
:I plan on mounting the original output transformer on the new speaker, similar to the original. Since the wires for the old field coil are out at the speaker, what do you think of mounting a terminal strip on the speakers mounting board. This terminal strip would be used to mount the resistor needed to replace the field coil load?
:
:I guess I could just remove the field coil wire all together and mount the resistor under the chassis. This might be safer.
:
:Any suggestions?
:
:Gary
:
:
:
::Gary, is the speaker voice coil 3.2 ohms (audio freq impedance) or DC resistance? Often, the DC resistance is about half the speaker impedance (which is nominally at 400Hz).
::Doug
::
:::
:::I need to replace the speaker on a Marconi Model 187.
:::There is a field coil, 650 ohms hot (per service info), which I will replace with a 10W, 700 ohm resistor.
:::
:::Within the field coil is another winding that is in series with the speaker. I believe this is a "hum bucking" coil.
:::
:::The speaker is 3.2 ohms. I want to use the original audio output transformer but don't know what speaker to use. I think it should be higher than 3.2 ohms since the hum bucking coil will be removed. Would an 8 ohm speaker be sufficient?
:::
:::How do you calculate this?
:::
:::Gary
Indeed and well documented.
I have used discarded OP txfmr from AA5 radios to replace chokes. Also 6V transformers and a few 0,1uF caps to help resonate the txfmr.
Or split the resistor value, use two resistors and make a PI filter using two e-caps. A PI filter will never be as efficient as a real choke though meaning you will have to live with a small backround hum using the former solution.
Syl
Syl