[Danny]

Got my 2nd model 20 working somewhat but volume is a bit low. Ok lets
look at the plate voltages on the two 71's. Yep, there it is...no voltage on one of the plates.
Bad output transformer ? Wait a second, why is there only three wires on this speaker plug ?
Did some checking and from the tech
section on this great site I find "The Speaker Problem in early Philco
Cathedrals ". With this info and my Fluke 23 I find that I've got
445 ohms on the output trans primary side. Now couple this with the only 3
wires from plug I'm wondering is it possible. Do I have a model 70 speaker
with a model 20 chassis ? Anyone have a model 20 spkr they want to trade for a model 70 ?

[Ron Ramirez]

Hi Danny

Keep your speaker, change the audio output transformer from a single ended output (what your speaker now has) to a push-pull audio output. You'll have to add a wire - while you're at it, might as well replace all of the wires with new cloth-covered wire if yours is dried out, which they probably are.

Your Model 20 will sound much better with the 70 speaker and its open spider, as opposed to the 20 speaker and its closed, stiff spider.

If you have any questions on rewiring, please ask.

[Steve Davis]

Danny, A model 20 speaker will have a nut on the back of the field coil frame. The model 70 (or model 90) speaker is pressed together and has no nut. With the proper output transformer in stalled, the model 20, 70 and 90 speakers will interchange.
I think there is a model 20 speaker listed on ebay right now.
Steve

[Danny]

Ron
How does one rewire this to make it push-pull. Guess I'll need to replace this
output transformer. So what do I replace it with ?
Thank you for all the help.

Steve
No nut on the field coil so looks like it indeed is a model 70 spkr...
Cheers !

[Ron Ramirez]

Here is a drawing showing how a push-pull audio output transformer should be connected to your speaker.

[Image: http://i326.photobucket.com/albums/k420/.../spkr2.jpg]

A Hammond HX-125B (5 watt output), available from http://www.radiodaze.com should work well on your speaker. You will need a universal type push-pull output to match to the speaker voice coil, regardless of where you purchase one.

[Danny]

Ok Ron, I ordered the HX-125B from Radio Daze and it will be delivered in a few Daze. In the tech section on your site [table I] documents .7ohms for the voice coil impedance. On the RD site Schematic & Hook Up Data (HX-125A - HX-125E) lists 1.5 ohms as lowest impedance on this model. Don't want to burden you with a whole lot but I'm confused. It would help if I understood the table used in the hookup data page on RD site. Anyone got the time to walk me through it ? Thanks....

[Ron Ramirez]

The primary impedance of the original output transformer was 6700 ohms per Philco data; secondary impedance 0.7 ohm.

Using these figures...the closest match you are going to get will be by connecting the speaker voice coil leads to either terminals 1 and 3 or 5 and 6.

Universal audio transformers do not offer exact matching of output to speaker voice coil; instead, they offer "close enough" matching to enable their use in many different radio receivers.

These transformers have six terminals for the secondary connection to your speaker voice coil. You will use the two terminals that most closely match the output load impedance to your speaker voice coil impedance.

Use whichever combination sounds the loudest and best: Terminals 1 and 3, or 5 and 6.

[Danny]

Ok, think I've got it...Ball park will work. Found some great info concerning choosing a replacement audio output transformer. It's been a long time since school and I never did any troubleshooting at the component level out in the field. It was always board level or whole black box swap out. Here is a great site that will provide all the ins and outs of selecting a replacement transformer and it's simplified to the point that even I can understand.
http://www.radioremembered.org/outimp.htm

Thanks for the help Ron, I do appreciate it.