Sourcing a field coil speaker...

[jeyurkon]

Originally Posted by Reece

Use any PM 6 or 8" speaker that you have. The two remaining wires from the transformer go to the two terminals on the speaker. Polarity does not matter in this case. The transformer is fine mounted where it is.

The transformer is used because the tube has a high output impedance. The transformer matches this to the low impedance of the speaker. Ideally your speaker should be rated 3.2 ohms (commonly referred to as 4 ohms) but an 8 ohm will also work, just giving slightly lower volume, but probably not noticeable. This is a pretty hefty radio, power transformer and all, and was surely worth restoring. Congrats!

Reece

I learned something too. Thanks for clearing up when permanent magnet speakers started to be commonly used.

I'm not sure if titanstats caught your comment about all three wires going to the transformer. I take it that this is a push-pull output stage.

Do you have any suggestions for him on how to identify which lead goes to the center tap? That is if it's not still connected. Was there a generally accepted color code for this?

John

[titanstats]

Originally Posted by jeyurkon

I learned something too. Thanks for clearing up when permanent magnet speakers started to be commonly used.

Do you have any suggestions for him on how to identify which lead goes to the center tap? That is if it's not still connected. Was there a generally accepted color code for this?

John

That's the great thing about AK -- I'm still always learning things! Such a great resource, and quick answers, too.

No worries about the connections - the plug and transformer are still connected, leaving just the two leads for the speaker out of the transformer. Going to put one of my spare GEC metal coned speakers in there, I think (they are pretty efficient), but without the coaxial center piece -- don't know that a coaxial would be of any benefit at all to an old radio like this sonically.

Thanks again, guys -- you've been a big help. I'll throw on a couple of proper pics when I get it done.

[Reece]

Don't have the schematic or the tube layout, but I suspect this is a single-ended (1 tube) audio output. There are three wires to the transformer, one goes to output tube plate, one to B+, and the third wire is probably a tap on the primary of the transformer that provides lower voltage to the screen of the output tube and possibly to the plates of the other tubes. If possible, could you post a scan of the output tube section of the schematic?
Reece

[Reece]

Just another thought: don't know what type of speaker you intend on putting in here, almost sounds like a modern day hi-fi speaker of some type. Go ahead and hook it up temporarily before you install it permanently and see if the amplifer can handle it and give you enough volume without distortion. The reason I say this is that some modern speakers with high compliance cones take relatively more power to operate and work best in a tuned or infinite sealed enclosure. Your radio might be "sweating" to operate that speaker. "Normal" replacement speakers are what this radio would have been designed to work with. You used to be able to get them at Radio Shack but I don't think so any more. My best bet lately for replacement PM speakers for radios has been to cannibalize radios, stereos and TV's from the 60's, 70's and 80's, stuff that you see people set out at the curb for the trash or that you can get at a yard sale for a couple of bucks. These are speakers made to operate in open back cabinets like your radio. You have to look harder to find such speakers new. I did find some 8" speakers cheap at the following website, don't know anything about this company, but the $5 and $6 speakers look like they would be OK if yours isn't satisfactory.

http://www.kcnproducts.com/SearchResults.asp

Reece

[Reece]

One more thing occurred to me and then I'll shut up!! I'll bet a 6 x 9" speaker would fit in there too, as an option; they sound good. Lots used in cars, for example.

Reece

[marty59]

So why do they install the transformers onto the speaker? Since nobody has answered it I will give it my best guess! No real reason except it saved design space on a chassis...and sometimes just out of convienence I suppose!
And when you think about it, on older radios that have a field coil speaker one side of the choke(field coil) would be tied to the output transformer B+.

[Reece]

I figure the output transformer on the speaker is mainly to save space. Now, you may find a set with plenty of landscape open on the chassis, and the transformer on the speaker anyway ! Could just be convenience of manufacturing, having one type of speaker that fits a number of sets.

Reece

[electroking]

Hello,

I do have the exact same radio, I recognized it from the pictures. Also, I never
managed to find a proper schematic for it. The radio is in storage now so I cannot
have a look at it, however I am sure I remember it has a single-ended output
stage using a 6v6 tube. If your radio does not have two identical output tubes,
it is just like mine a single-ended unit, and the output transformer does not have
a center tap. The reason the remaining wiring on the speaker plug has three wires
is unclear to me, but I bet only two of those wires actually go to the transformer.
Good luck.

[marty59]

Okay, without a schematic at least verify where the wiring originates from and a tube manual may be handy. It's possible that the screen grid voltage comes from a tap on the primary side hence the three wires? A 6V6 plate/pin 3, screen grid-G2/pin 4.
There is some standardization as to color codes on transformer wiring but only use as a guide and verify. A red would be B+, blue is output tube plate, and not sure for screen grid offhand...primary side speaking of course! Output side since it mounts to the speaker are maybe just short, bare stubs that hooked directly to the speaker terminals right in line with each other.If there was any length to those wires proper hookup would be a green to the dot or positive phase of the speaker and a black lead to the other.
For those of you interested on how to check for speaker phasing all you need is a 9 volt battery preferably one that is a little low on voltage. Just go across the terminals and observe movement. Forward (outward) throw- positive/proper phasing...rearward- negative!

[Reece]

Right on, Marty: single-ended output transformer should have red to B+, blue to plate, and if there is a third primary wire, "other" color to screen. I have seen "other" as white, orange, brown....

Reece

[jeyurkon]

This might be straying a little off topic but while looking at a schematic for a 1949 Television there is a four pin plug to connect the speaker. Two of the wires go to the speaker mounted transformer and connect to the B+ and plate in the set as you would expect.

The other two pins just have a jumper across them. When you pull the plug it disconnects the screen bias.

Is this to protect the output tube?

John

[Reece]

Yep. The Westinghouse in question appears to have a sticker next to the speaker socket, can't read it, but think it says not to operate the set without the speaker plugged in.

Reece

[gadget73]

Sometimes a third wire back from the output transformer to the radio is for a negative feedback connection. The jumper to cut B+ without the speaker in place is very common. My Scott actually has a jumper to cut the 120v line power to the main transformer if the speaker isn't plugged in.

[Reece]

On something as powerful as a Scott, they wanted to be sure that the speaker was connected: operating without it would be very damaging to the output tubes.

Reece