Sourcing a field coil speaker... - Videokarma.org TV - Video

titanstats · #1 12-04-2008, 05:45 PM

Hi all. Recently reveneered and almost finished restoring an old Westinghouse radio that was my parent's. It looks beautiful now (covered it with Koa wood, kept it kinda blonde), but I'm having trouble finding a speaker for it. It used a 6" field coil speaker originally, that I cleverly removed when I was about 16 (and tried to restore it for the 1st time) because the cone had a rip in it.

Question is, when looking for a speaker, what matters? I know little about these types of speakers -- anything that I have to consider? Is it possible to replace it with a more contemporary fixed-magnet speaker? Seems that 6" field coils are a bit of a bugger to find...wish I'd known that way back when I was 16!

Thanks, all.

jeyurkon · #2 12-04-2008, 08:28 PM

The field coil usually also acts as a choke in a pi filter in the power supply. If you use a PM speaker you should replace the field coil in the circuit, assuming you no longer have it, with an appropriately rated choke. It should have about the correct resistance also. You can add a series resistor if necessary.

What the model number of your Westinghouse? There's probably a schematic for it on http://www.nostalgiaair.org

John Y.

titanstats · #3 12-04-2008, 09:11 PM

Strangely enough, I still have the original field coil.

What now?

The radio is nothing special dollar-wise, but I'm a sentimental fool -- was one of the folks' wedding presents, way back when. It looked like hell a few weeks ago, but I'm fairly pleased with it now. I believe it is a model 6T-104. Sorry, the grainy pics don't do the wood much justice.

totem · #4 12-04-2008, 10:56 PM

Nice job, adds a touch of class to a classic.

Looks like you are enjoying the Veneer end of things.

jeyurkon · #5 12-04-2008, 11:21 PM

Wow! That looks great!

I think it looks at least as good as this one: http://radioatticarchives.com/radio.php?radio=7894

I couldn't find a schematic for your set. Looking at other Westinghouse sets from around that period they used a 3.2 ohm voice coil impedance. I think a choke that could be used to substitute for the field coil is a 5H 50ma Hammond choke carried by tubes and more. Their catalog number is P-T155H. It should provide adequate filtering and has a similar resistance.

This is only an educated guess so don't hold me responsible.

If you look at the photos in radioatticarchives you can see that they replaced their speaker also with a permanent magnet one. It appears that they kept the field coil and hid it under the top. You can see the wires going from the plug up to it.

Another worry is weather the audio transformer was mounted on the speaker or if it's in the chassis. Hopefully it's in the chassis or you have to add one.

John

gadget73 · #6 12-05-2008, 10:02 AM

If you've got the original field coil, wire it in, and install a 4 or 8 ohm 6" PM speaker. Problem solved. If its the type with the transformer on the speaker, you'll need a substitute. If its a common single 6bq5 or 6aq5 output, the transformer isn't very critical, a common 12v filament transformer will do the job or there are cheap AA5 types available from various sources. The power transformer quite probably will sound better than a cheap transformer though.

jeyurkon · #7 12-05-2008, 10:29 AM

I missed where you said you had the original field coil. So, yeah "what he said."

John

toxcrusadr · #8 12-05-2008, 02:14 PM

If you have the original coil why not have the speaker rebuilt?

Or is the coil burned out? That's the most common failure mode.

There's a guy up in Cleveland or Akron at Play Things of Past who has a storehouse full of parts and junque. He may have the right speaker.

http://oldradioparts.com/pg2.htm

jeyurkon, I'm a bit confused about your post - I'm with you to the 3.2 ohm voice coil impedance - but field coils varied from 500 to several thousand ohms impedance. Without knowing what the radio used, substitution of a choke for the original field coil is dicey, isn't it?

jeyurkon · #9 12-05-2008, 03:28 PM

Quote:

Originally Posted by toxcrusadr

If you have the original coil why not have the speaker rebuilt?

Or is the coil burned out? That's the most common failure mode.

There's a guy up in Cleveland or Akron at Play Things of Past who has a storehouse full of parts and junque. He may have the right speaker.

http://oldradioparts.com/pg2.htm

jeyurkon, I'm a bit confused about your post - I'm with you to the 3.2 ohm voice coil impedance - but field coils varied from 500 to several thousand ohms impedance. Without knowing what the radio used, substitution of a choke for the original field coil is dicey, isn't it?

That's why I said it was an educated guess and didn't want to be held responsible. I looked at various Westinghouse radios of the same year and they all seemed to be a little over 300 ohms.

But, since he has the field coil he can use it. Or, he could measure it's DC resistance and see if the choke is close enough. I should have read his response more carefully because I had missed where he said he still had the field coil.

One thing we should probably be asking is whether any other restoration has been done like recaping it. I hope I haven't overlooked that information somewhere too.

I'm new to this forum so I don't know who's an expert or novice.

John

titanstats · #10 12-05-2008, 05:21 PM

Well, I'm no expert, that's for sure, so I appreciate all/any advice on things like this -- I'm more at home with solid-state devices and "regular" speaker drivers.

My ignorance has become apparent with this post; I thought that the field coil referred to the transformer, and never thought beyond that. It becomes obvious with a bit of thinking that this isn't the case, however! The original driver is gone, as far as I can tell (my folks are packrats, but the driver can't be found), but the transformer that was mounted to the speaker basket remains. Like I said, I was 15 or so at the time, and suppose I didn't have a clue as to what I was doing -- but apparently I knew that I shouldn't throw the transformer away, so I mounted it to the cabinet wall inside the radio.

In the link that jeyurkon provided, it looks suspiciously like the original driver in the radio -- is that possible? I don't have Photoshop right now (I'm at work), but if I did I'm suspecting that you'd see a small transformer mounted to the speaker basket up above, where the 3 wires from the radio are headed. That's the same configuration as was originally on my radio. Why I can remeber that, and not whether it was a permanent magnet on the speaker, I don't know.

If it was a permanent magnet speaker, why the transformer mounted to the speaker?

Dammit -- must go home and lighten up that picture!

Thanks, all -- this has been a big help, believe it or not.

Tom Bavis · #11 12-05-2008, 07:17 PM

I didn't find a similar Westinghouse model number in Sams or RCC (Radio College of Canada). But it may have been an export model - these seldom made it into the service literature. A post WWII radio probably used a PM speaker - transformers were often mounted to the speaker.

jeyurkon · #12 12-05-2008, 07:46 PM

Quote:

Originally Posted by titanstats

Well, I'm no expert, that's for sure, so I appreciate all/any advice on things like this -- I'm more at home with solid-state devices and "regular" speaker drivers.

My ignorance has become apparent with this post; I thought that the field coil referred to the transformer, and never thought beyond that. It becomes obvious with a bit of thinking that this isn't the case, however! The original driver is gone, as far as I can tell (my folks are packrats, but the driver can't be found), but the transformer that was mounted to the speaker basket remains. Like I said, I was 15 or so at the time, and suppose I didn't have a clue as to what I was doing -- but apparently I knew that I shouldn't throw the transformer away, so I mounted it to the cabinet wall inside the radio.

In the link that jeyurkon provided, it looks suspiciously like the original driver in the radio -- is that possible? I don't have Photoshop right now (I'm at work), but if I did I'm suspecting that you'd see a small transformer mounted to the speaker basket up above, where the 3 wires from the radio are headed. That's the same configuration as was originally on my radio. Why I can remeber that, and not whether it was a permanent magnet on the speaker, I don't know.

If it was a permanent magnet speaker, why the transformer mounted to the speaker?

Dammit -- must go home and lighten up that picture!

Thanks, all -- this has been a big help, believe it or not.

I don't know why transformers are often mounted to speakers, but they are. I have a 9T1 Motorola TV that is that way and the speaker has a permanent magnet.

The plug on the back of the radio for the speaker should have about 4 wires. Two would go to the transformer and two to the field coil. Do you have the plug and if so, is it still connected to anything? If it is, could you post a photo?

John

Reece · #13 12-06-2008, 11:39 AM

The radio in the Radio Attic Archives has a PM speaker in it. As stated, almost anything from around 1946-47 and on, which includes your radio, would have a PM speaker. Consumer radios weren't built during the war. In 1946 or so when they started making them again, the manufacturers used up whatever stock they could find, the demand was so great. That's why you'll find some radios from that era with field coils and some of the same model with PM's, or with varying tube numbers within the same model. But the one in the Archives picture is definitely a PM speaker. There are only three wires coming from the speaker plug. There would have to be more wires if there were a field coil involved. The three wires from the plug go to the speaker transformer, and then two wires from the transformer to the voice coil on the speaker. Use any 6" PM speaker. It looks like there would be room for an 8" speaker in that cabinet, which is what I would use. It would be better to cut the speaker board larger, but if it's too much trouble to take out, the 8" would still sound better and give even better bass than the 6". Might have to put a deeper front cardboard or other gasket on it to space the cone out from hitting the board. Nice job on the radio!

Reece

titanstats · #14 12-06-2008, 11:43 AM

Well it looks like I was wrong about the speaker -- it was likely a permanent magnet to begin with. Being a 1949 model seems to make it that way; wartime improvements in permanent magnets had made their way into the post-war consumer market by then. Here are the pics of what is there -- quality is grainy, but you can see what is left. Three wires out of the radio to the transformer that was mounted on the speaker basket; two of them into the transformer, and one to the speaker's (-) terminal, I suppose.

So what is with the transformer then? Why is it there? Have to excuse my ignorance -- haven't poked my head into too many old radios. What would be a good guess as to the impedance? I have a couple of pretty neat old drivers hanging around that I haven't found a use for yet in several years. Reece, you are correct -- there's room inside for an 8" driver to fit with very little modification, as long as the basket clears the tube at the back (which I think most will). Thanks again for the help; I love this place -- I've learned a bit in this exercise.

Totem, I am indeed enjoying the veneering side of things -- it's pretty cool to be able to turn a POS into something that looks nice. I find that I'm enjoying the DIY side of this hobby more than anything else, now that I'm not continually trying to upgrade my gear. Just another great thing that I've learned about here on AK; I'd have never done anything like this without help from this site.

If you have anything that sounds great, but looks like crap, I'd really recommend giving the reveneering a go; it isn't that hard to do.

EDIT: Reece was posting the same time as I was

Reece · #15 12-06-2008, 11:57 AM

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