[dirkbuddy]

The field coil in my speaker is Toast, here are the specs on my speaker
Part #  P-19  36-1101
Field Coil-  1140 Ohms
Voice coil impedance 1 Ohm
Output transformer Primary- 6200 ohms
                            Secondary .89 Ohms

I have found the same size speaker with a Field coil of 1.02 K ohms, the voice coil impedance is 3.2 Ohms.
I know the Voice coil impedance do not match and the Field coil is close, could i safely use this speaker as a replacement?

Thanks

[morzh]

Dirk

If your speaker contains the transformer:

1. If you graft the transformer to the new speaker (the FC is OK, 10% difference is not very critical) it will result in 3 times the impedance and your output performance will be degraded quite a bit.
2. If however the speaker has its own transformer AND it was made to work with a 42 tube output (or a tube with the similar requirements for a load impedance) then the VC impedance no longer is important.


If your transformer is separate, then you are going to have same #1 problem. I think in the 45 it is separate and is installed on the chassis.

[dirkbuddy]

Thanks for the reply Morzh

You are correct, the output transformer is mounted inside the chassis, so would I have to change the output transformer to match the impedance of the voice coil?  
I have several here,  what primary and secondary resistances should I be looking for?

Thanks for your help as always.

[Radioroslyn]

If you found a speaker out of an 80, 81, or 610 they would be a suitable electrical match for your set. I think all of those have the transformers mounted the speaker's frame so they would have to be moved to the chassis.
Dc resistance of either the pri or sec of the opt isn't going to tell you much about it's plate load resistance when connected to a tube. Nor will it give you any idea about the turns ratio which are two key factors. If you know what circuit (tube (s) used) the transformer came from that can be a big help.

[morzh]

OK.

Let's assume we want to see 7,000 ohm of impedance as it is a 42 tube. Your original VC is 1 Ohm.
The ratio therefore is 7000 and the square root of that is 84, which is your required turn ratio for the transformer that is there right now.

You could check it by feeding 84V from a Variac to the Primary and measuring the output that should be around 1V.


Now you want it for 3.2 Ohm. The turn ration required for your now needed transformer is then 47.
In the same fashion, take any of your transformers and start feeding say 50V into the primary and measuring the secondary; then record it all.
Calculate the ratios. Pick the one closest to 47. If the difference is too large, you will have to find another one. This is where the Hammonds 125-SE series is suitable.

[dirkbuddy]

Thanks for the detailed explanation, I think I may even have a Hammonds 125-SE series. anyway first thing I will do is because the speaker is useless to me right now, and I have nothing to loose I will remove the field coil and see if I can find a break, if it cannot be repaired, I will go with your option of testing transformers I have here. thanks for your help guys!

[Arran]

Don't toss the original speaker out though, they can be rebuilt, and original 5'' field coil speakers are getting harder to find.
Regards
Arran

[dirkbuddy]

Ok just tossing things around here, as seen in this pic, there is a mounting screw that attaches the square frame around the field coil to the chassis, the frame and chassis are separated by a rubber grommet, the grommet of course has been long dried up, inside the chassis there should also be a rubber grommet separating the screw head from the metal chassis, this grommet was missing, and someone had used a lock washer and plain flat washer, this resulted in making the screw "longer" which is why it was driven into the field coil, (approx 1/8 " deep) I did not remove the speaker completely but if you look at the pic you can clearly see the threads in the field coil frame which the screw went up through. I did not notice this on initial survey, but when I brought the radio up with a variac I could see the amp draw rise to almost 1A at 60 volts I then shut it down. I suspect numerous windings are damaged. In order to repair or rewind the coil, the cone and voice coil would have to be removed, the center core pressed out with an arbor press, the coil removed and repaired or rewound, and the speaker reassembled and likely reconed. Getting the core reinstalled and centered properly without the jigs likely originally used would be a challenge! and beyond my abilities I am afraid.
Therefore I am also considering replacing the FC speaker with a PM magnet speaker, this leads me to the following questions:
1) Could I use a 1140 ohm or close 10W resistor in place of the field coil?
2) would my current output transformer work?
3) Since my original FC speaker voice coil was 1 ohm, what impedance value of PM speaker would I need? 2ohm, 4 Ohm etc?

The schematic is also attached here as reference.

Any help would be greatly appreciated!

[OZ4]

dirkbuddy 
Yes the resistor will work. Just check the voltage levels and adjust the value of resistor to correct. I would poke around in the field coil and see just how bad it is. Maybe you can rewind just the outer part that is bad.

[OZ4]

Another idea would be to bypass the shorted part and connect to the part that is left still good and add a resistor to compensate. It would depend on how deep the screw went.

[Chas]

Getting the core reinstalled and centered properly without the jigs likely originally used would be a challenge! and beyond my abilities I am afraid.

Can be done without a jig using flexible shims to align the voice coil. Yes, some "machine shop" equipment is required to work with a pressed/welded electrodynamic speaker.

The field coil is always a better choice than a "filter" resistor.

IMHO not to worry about the apparent mismatch of the resistance of the "new" voice coil vs the old one...

From what I see, (I should reference the schematic) is the original speaker has a hum bucking coil, if the replacement has that coil fine. If it does not. there may be a faint residual hum that can be dealt with using a 10 to 25% large cap after the "2nd" cap in the filter array. Also try reversing the current flow in the filed coil regardless if there is faint hum.

There is tolerance in values used in an everyday radio that will allow for variations in component values to some degree. Lash it up and try it before committing to a permanent installation...  chas

[OZ4]

I know it's cheap and dirty but as far as the rubber grommet goes I would use a cut off piece of rubber hose.

 (something from the auto parts store should be tough and last a long time) 

To get a nice smooth end I use a bench grinder first then cut.  Just watch your fingers.

[dirkbuddy]

Thanks OZ4

I have zero experience with ripping apart a speaker to check the field coil, i have come to the conclusion to leave this to the experts that have the proper tools to complete this task, therefore I am in the hunt for someone in my country (Canada) that has experience with re-winding field coils, as I do not want to throw the speaker away as another member had suggested.
Now regarding replacing with a PM speaker, i have done some reading and it looks like using two equal value resistors in series that would make up the 1140 ohms needed for the replacement of the FC would be better for heat dissipation with an electrolytic connected to their junction with the - going to the - of the other 2 electro's would be great for reducing hum and may sound like a plan, am I on the right track here?
and if I can use the current output transformer (primary of 6,200 ohm and Secondary of .89 ohm)  what type of impedance of PM speaker would I need?

Thanks in advance!

[dirkbuddy]

So Chas you are saying that impedance on the new PM speaker is not critical?  could i use a 4 ohm?

[OZ4]

I use to wonder about the impedance matching Then I learned that with matched impedance you get maximum power transfer. 
Mismatched impedance just means less transfer of power. Also the mismatch will be reflected back to the output tube and may cause some distortion but you probably won't be able to tell. As a kid I used to hook up all kinds of speakers and never had any problems.