Field Coil speaker

[angelodp]

This is an offshoot to my Dobro thread. The amp has a field coil speaker that demonstrates a severe buzz on the bass notes when volume is up, but does not buzz when the speaker is facing downward. My guess is that the coil is displaced to some degree and is rubbing in all positions but face down. The speaker has a small tear which I repaired with thin tissue and a rubber based seam glue. I think a re-cone is in order. Any trouble shooting tips on this speaker?

[Stevem]

There could be many things going on with that speaker and all of them could be happening at the same time and all are due to age.

Some of the conditions you may be able to fix, the other calls for a re-cone job.

1) In these age speakers the voice coil was just paper.

Now we all know what happens to 60 year old paper, and now add on top of that a good amount of heat due to the wattage pumped thru the voice coil.

2) these speakers where glued together with animal hide glue which over time dries out and cracks and can turn to power like.

This glue was used to hold the coil bobbin to the cone and the voice coil onto the paper former.

3) the spider is glued to the cone and the speaker frame.

4) the cone around its perimeter is glued to the frame under the mounting gasket.

If you carefully cut the voice coil cover off if you find that the paper voice coil former is coming unglued you can fix that issue and the get a new voice cover and glue that on.

3 and 4 you may be able to fix also, but if the voice coil former is rotting away or if the voice coil is coming unglued from the paper former, then it's re-cone time!

Now you can see how many failure points there are and I have not mentioned that the cone around the

[angelodp]

SteveM, ah yes old age and paper. I have a re-cone scheduled. Like I said, you look at these old amps and they....Thanks for the intel.

[angelodp]

Hi Steve, so it is the case that my problem was not the speaker rather a bad power cap, now fixed and the amp is more or less back to normal. The speaker does have a tear which I repaired with thin tissue and seam-kote and it sounds "not bad". I do want to see about a re-cone but I need a person in LA that has experience with Field Coil speakers. I hear there is a hum cancelling wind on the bobbin?

I'll try reaching out to Scumback.

Best A

[Stevem]

A feild coil type speaker has no magnet.
The magnetization that the speaker works on is made by that coil of wire powered by the D.C. Voltage from the amps power supply.

As a side benefit this coil of wire around the steel core acts like a modern Choke like we are now use to.

This is where the hum canceling comes from that you mention.

This feild coil winding does not need to be replaced when re-coning a this type of speaker if it works.

[angelodp]

Thanks Steve, so does this type of speaker qualify as a standard re-cone job. The Field Coil is not affected or handled beyond getting the new cone in. In your estimation would an experienced re-coner that has not done a Field Coil encounter any problems doing this speaker.

[maxkracht]

I don't believe there is any difference to the person doing a recone if it is field coil or no field coil, but I think some recone places are only set up for certain cone and voice coil sizes and shapes. Might just be an excuse that some shops were making to stick to easier jobs with brands they know, but it would make sense to me that you might need different tooling for different speakers.

You can use a modern permanent magnet speaker, and keep the field coil in as a choke, or replace it with a resistor or inductor. Added pain of relocating some iron, but then you don't have to worry about speaker replacement anymore.

[angelodp]

Thanks Max. Amp plays very well at the moment. I am going to let the owner decide about re-coning. I have provide several reputable places that have experience re-coning Field Coil type speakers.

[johnnyreece]

This is where the hum canceling comes from that you mention.

Just wanted to chime in on this; some field coil speakers do indeed have a separate hum-bucking coil. It's wired in series opposing the speaker voice coil and is wound as part of the field coil. It is meant to counteract the hum produced by the field coil.

Hum_Bucking_k.png (8.48 KiB) Viewed 1948 times

If you end up needing a new field coil speaker, feel free to hit me up. I've got several hundred vintage radio speakers.

[angelodp]

Good info thanks Johnny