Is it possible to convert a field coil type speaker over to a magnet type speaker. And if so, how?

:Is it possible to convert a field coil type speaker over to a magnet type speaker. And if so, how?
-------------
Nom, If you are suggesting a physical conversion - I don't think there is a simple or practical way to do it. It is possible, however, to replace a field coil speaker with a permanent magnet (PM) type by using an appropriate-sized choke as a substitute for the field coil and hooking the new PM speaker to the audio output. Please let us know your particular reason for doing so, and there might be more specific suggestions forthcoming.

You can also substitute a power resistor for the field coil. The resistance should be about the same as the original field coil's DC resistance, often around 1K ohms, or so. The resistor wattage should be at least 2x the expected power disipation, I^2R, where I is the total plate current through the resistor.

With a resistor substituted for the field coil, there might me a little more hum. That can be reduced by jacking up the filter cap values by maybe 25%, but not much more, particularly for the cap nearest the rectifier cathode.
Doug

:Nom, If you are suggesting a physical conversion - I don't think there is a simple or practical way to do it. It is possible, however, to replace a field coil speaker with a permanent magnet (PM) type by using an appropriate-sized choke as a substitute for the field coil and hooking the new PM speaker to the audio output. Please let us know your particular reason for doing so, and there might be more specific suggestions forthcoming.

I have recently acquired a Zenith 7S657R without a wavemagnet or a speaker. I sucessfully bid on a wavemagnet on ebay but a good 12" 1250 ohm field coil speaker is a little harder to come by. Permanennt magnet speakers are easier to acquire and I'm trying to decide what to do. I am grateful to know that conversion is possible. It would be easy to go the resistor route with what Doug told me, but how would you figure the choke value?

:You can also substitute a power resistor for the field coil. The resistance should be about the same as the original field coil's DC resistance, often around 1K ohms, or so. The resistor wattage should be at least 2x the expected power disipation, I^2R, where I is the total plate current through the resistor.
:
:With a resistor substituted for the field coil, there might me a little more hum. That can be reduced by jacking up the filter cap values by maybe 25%, but not much more, particularly for the cap nearest the rectifier cathode.
:Doug
:
:
::Nom, If you are suggesting a physical conversion - I don't think there is a simple or practical way to do it. It is possible, however, to replace a field coil speaker with a permanent magnet (PM) type by using an appropriate-sized choke as a substitute for the field coil and hooking the new PM speaker to the audio output. Please let us know your particular reason for doing so, and there might be more specific suggestions forthcoming.

I don't think the choke's value would be critical as long as it's rated for at least the plate supply current. The DC resistance should be about the same as the original field coil, but if the resistance were less, you could add a power resistor in series with the choke.

If the DC resistance is off, then the B+ voltages will be out of whack. If the inductance is off, then there might be a bit more or less hum.
Doug

:It would be easy to go the resistor route with what Doug told me, but how would you figure the choke value?
:

Doug....You'd still need an output transformer for impedence matching wouldn't you?...Neal

:You can also substitute a power resistor for the field coil. The resistance should be about the same as the original field coil's DC resistance, often around 1K ohms, or so. The resistor wattage should be at least 2x the expected power disipation, I^2R, where I is the total plate current through the resistor.
:
:With a resistor substituted for the field coil, there might me a little more hum. That can be reduced by jacking up the filter cap values by maybe 25%, but not much more, particularly for the cap nearest the rectifier cathode.
:Doug
:
:
::Nom, If you are suggesting a physical conversion - I don't think there is a simple or practical way to do it. It is possible, however, to replace a field coil speaker with a permanent magnet (PM) type by using an appropriate-sized choke as a substitute for the field coil and hooking the new PM speaker to the audio output. Please let us know your particular reason for doing so, and there might be more specific suggestions forthcoming.

Yes. I'm unsure whether the xfmr is on the chassis or was part of the original speaker: http://www.nostalgiaair.org/PagesByModel/383/M0025383.pdf
Doug

:Doug....You'd still need an output transformer for impedence matching wouldn't you?...Neal
:

I thought so thanks doug.

Yes. I'm unsure whether the xfmr is on the chassis or was part of the original speaker: http://www.nostalgiaair.org/PagesByModel/383/M0025383.pdf
:Doug
:
::Doug....You'd still need an output transformer for impedence matching wouldn't you?...Neal
::
:

A choke of about 8 to 10 Henries will be fine as long as the current rating is sufficient.

Thank You Peter.

:A choke of about 8 to 10 Henries will be fine as long as the current rating is sufficient.

:Thank You Peter.
:
::A choke of about 8 to 10 Henries will be fine as long as the current rating is sufficient

Yes, all this reminds me of an old Truetone ( no cabinet) I bought for $5 as a teen. (45 yrs ago )
The original field coil was used and another xfmr for the audio out . What a mess of wires. But it worked .