Dynamic Speaker to Permanent Speaker

[alphamarquis]

Hi,
I have an old RCA Viola O-50 which I would like to convert to a tube amp. Problem is, I am having difficulty replacing the current speaker which is an electromagnetic dynamic speaker with a permanent magnet speaker.
The site I have looked at so far is:
http://www.radioremembered.org/edspeaker.htm
However I am still a bit confused as to where the resistor goes...

[Chuck H]

In most cases the speakers magnet coil is being used as power supply choke for the amp. There are other more complicated scenarios but they are rare. Most guys will either replace the original unit with a new choke and a new speaker OR remove the inductor and core from the existing speaker and continue to use it as the amps choke and just wire the new speaker standard.

If you can find a schem for your amp it will help you alot to understand what is going on there and how to work around it.

Chuck

[alphamarquis]

There is a transformer now that is sitting right on the body of my speaker...the resistance created in somewhere in the 2000ohm range, putting a speaker onto that kind of resistance doesn't make sense because modern permanent coil speakers read out about 8-64 ohms...

[Enzo]

If there is a transformer mounted right on the speaker frame, I'd be betting it is the output transformer. The secondary wires go right into the speaker, while the wires trailing out are for plate and B+. Is there an output tranny on the amp chassis?

Look at the example drawing in your link. Upper right is the speaker, and it shows the voice coil driven by a transformer, but the transformer is drawn on the speaker, not on the chassis. The lower coil on the speaker is the field coil. The field coil is drawn as a choke for the B+ rail. In some juke box amps I often find the field coil strapped from B+ to ground at the end of the B+ string or sometimes as part of a voltage divider.

Carefully remove the output tranny from the old speaker and mount it onthe chassis or someplace convenient and extend the secondary wires to the new speaker. I'd probably mount it on the chassis and wire it in on the chassis side of the speaker connector, then just run a pair of wires up to the new speaker voice coil from there, rather than from the transformer directly. Wire a choke in place of the old field coil, or even a resistor.

[alphamarquis]

Thanks for your guys's help!
I disconnected the output transformer which was mounted to the speaker and added it into the chassis. Right now, though I have no idea where to put the resistor...(I have two cables coming out of the transformer.) I can't find a diagram for an old '39 phonograph...
Do you guys need some photographs...would that help?
Thanks again for all your input so far.

[Chuck H]

Well, Enzo could probably say off the top of his "experienced" head what the output of that amp probably is...But if I know anything at all I'd say it can't be more than a pair of 6V6s at a moderate plate voltage. So I'd say that you can just install a 100 to 150 ohm 25watt resistor in place of the electromagnetic coil on the speaker. If I'm wrong I hope Enzo jumps in to save my bacon.

Chuck

[alphamarquis]

I have two wires running into the field coil currently, which is the one that needs to be resisted? And don't I need two wires to run into the speaker?

[Chuck H]

"I have two wires running into the field coil currently, which is the one that needs to be resisted? "

Both. The resistor replaces the coil. On the coil now, one wire goes in, and one wire goes out. So... With resistor, one wire goes in, and one wire goes out.

"And don't I need two wires to run into the speaker?"

The output transformer that you removed from the speaker was most likely connected to the speaker lugs it was powering. Those connections are where your speaker leads should come from. You will have to add the wire.

Chuck

[alphamarquis]

I think I am confusing myself...there just seems to be far too many cords coming out of the connector to make a whole lot of sense.
Here's what I'm looking at:

[Steve Conner]

I've seen people just pull the field coil assembly off the back of the old speaker and screw it to the inside of the cabinet, rather than replacing it with a resistor or buying a choke. The field coil assembly is that thick steel frame with the black bobbin inside, where the magnet would be on a modern speaker.

If I counted right, you may well have more cords there than most of the other posters noticed. The round four-pin connector carries the B+ current that goes through the field coil on two pins (these being the pins you should hook your replacement choke/resistor to) and the signal to the O.T. primary on the other two. The O.T. secondary then connects straight to the speaker voice coil.

But what are those two big black and red wires? I assumed they were from the O.T. to the voice coil. But they don't seem to go anywhere near the leads from the voice coil! I'm guessing they must be a humbucking tap on the field coil or something. If it is something like this, then keeping the field coil might be a good idea. The amp may hum more if you just replace the coil for a choke and ignore those leads.

[alphamarquis]

Now it seems sensical to save both the field coil and the O.T., I still am curious as to where my leads are coming from for the P.M. speaker that will take take the place of this speaker.
Thanks for your help by the way!
(Do you guys need more photos?)

[Chuck H]

The pale red and greenish blue leads are probably the OT primary. The red/yellow and red/black leads are almost surely the choke. The bare lead may be the OT secondary - lead. So wheres the OT secondary + lead? And what the heck are those big shiney black and red wires?

Where's Enzo?

Chuck

[alphamarquis]

I am glad I'm not the only confused one.
This bugger is causing me to lose sleep so I'm looking for a solution...

[JJGross]

I am glad I'm not the only confused one.
This bugger is causing me to lose sleep so I'm looking for a solution...

I'm also confused about the whole Field Coil Speaker thing, but this made it a lot easier for me to understand. It's part of a schematic I found for a Sparton radio that I salvaged for parts. I never thought I'd find this, but Google is my best friend these days. Sorry for the crappy picture quality, but you take what you can get when you find stuff on the web and this is how I found them. They're actually smaller, but I zoomed in and took a screen shot to make everything (semi) readable.

The first picture is the schematic of the speaker itself with the voice and power coils + it's 6-pin plug looking at the pins (pins up). The plug is on the upper left of this pic.



The second picture is the section of the power supply and rectifier with the 6-pin speaker plug with the pins facing down. The speaker plug is on the right bottom of this pic. This part allows the first picture to make a lot more sense - at least to me.



Hope this helps you to figure yours out.
It's probably not the same as mine on the wiring, but it might point you in the right direction on yours.

Cheers,
- JJ

[Enzo]

Hey, I work nights here in my shop, and at 1:30PM I expect to be sleeping, thank you.

I see four wires from the chassis plug. Two of them run up to the OT on the speaker. Red and Blue, they are what we would expect. Red from the B+ supply and blue goes back to the plate of the output tube. Like any single ended amp.

Then there are also a pair of wires running to the field coil. They appear to be yellow and red/black. COnnect a choke or resistor where those two wires were/are.

The red and black wires are from the OT to the speaker and are indeed the voice coil wires. I'd disconnect them from the OT and try driving a different speaker with the OT. Note the black and red wires enter the speaker outside the field coil bobbin, they will go down the center to meet the VC. Now the field coil wires enter the field coil within the bobbin.

I got no problem just using that old field coil as a choke, but then you have the problem of mounting it somewhere. So to me a resistor or choke is easier to deal with.

As to the circuit, it is just a power supply, so think of that field coil as the choke in ANY power supply - between the plate and screen B+ nodes. I have no idea what current the thing has flowing, but I don't expect it to be any different from any other amp with similar tube complement. They just use the current through that coil to make a magnetic field for the speaker. Kills two birds with one stone. They didn't have nice cheap permanent magnet speakers back then.