[artim2205]

My friend and next door neighbor inherited a 1931 Philco Model 70 when his mother-in-law passed away recently. He doesn't know anything about it other than his wife loved to listen to it when she was young and now its missing the speaker that used to be in it. Neither of them have any idea where it went. I'm somewhat familiar with electrical/electronics and offered to do the research on it and as you probably all know, speakers for it are very hard to find. 

So, I'll start by asking if there is a way of adapting a modern 4 ohm or 8 ohm speaker to it. I've seen the tables about the matching transformers and tech info on the speakers so I'm thinking that the replacement transformers for the radio would not necessarily work with a modern speaker since I've seen that the ones for this radio are only 0.7 ohm speakers. I'm looking for help in figuring out a path to help get this old radio to play again. The missing speaker is only problem number one right now and I don't have any idea if there are more problems with the electronics yet since there is no speaker to check to see if there is any sound coming out of it. 

He asked me for help because he has no idea what questions to ask and would like to see if he can get it working for his wife as something to ease her pain of losing her mother. 

This is my first post to this forum so please forgive me if I've put it in the wrong place or made some other mistake in this post. 

Allan

[Paul Philco322]

Welcome aboard and you landed it in the right place.

Paul

[admin]

Hello and welcome.

Asking questions such as these - as Paul said, you are in the right place.

If you wish to seek an original 70 speaker, you will need to place a "Wanted" ad in the Wanted Ads section.

[artim2205]

I've done internet searches for an original speaker for this radio and came up empty. What my question now is to this group is there a way to adapt a modern speaker to these old radios that someone knows about?

[Phlogiston]

Overall, the answer is Yes. But, do you really want to?

First thing is to consider the "speaker" as 3 parts, the speaker, the power supply choke and the output transformer.

A replacement would have to take into consideration the parts and function lost with the original speaker. It would not be too hard to find a speaker that would mount in the original location, maybe with a gasket/adapter. Then find an OPT to match the radio's OP and the speaker's voice coil.

The biggest problem is the loss of the field coil that provided a magnet to the speakers voice coil but also provided a great deal of smoothing to the hum inherent to the poorly filtered power supply. Some would have you mount a large resister in place of the field coil. The resistor is going to get warm so it has to be of several watts rating. It will help a little with hum and it will drop the B+ to the proper voltage, but , since it is not a choke, the smoothing will be greatly diminished. Some would increase the size of the filter caps - then recalculate the drop needed in B+ and get a different resister. But this will only go so far. A 10uf cap in the first filter is about the limit. You could add another resistor and bump up the 2nd cap to maybe 30uf (this might be a little trial and error).  Finding a place to mound a large choke would be the best for performance, but this is a tough install without mounting it to the cabinet or a spot on the chassis, drilling some "extra " holes in one or the other.

Anyway - do you really want to do that. I certainly would resist making so many changes to an old family radio. Unless it is done very neatly it is probably going to affect the value some. It also isn't going to sound like it did with the original speaker - It will probably sound better.

If it were me, I would continue the search for an original speaker. You're right, they are getting harder to come by. Which is another good reason to find one now rather than later. Somebody here might have one or one that is repairable (new cone or such).

Good Luck

Russ

[Radioroslyn]

Hi Allan and welcome,

The 70 speaker is not all that rare they do turn up on epay maybe twice a month. The look pretty much identical to the 20,21,and 90 models also. Electrically some of them the output transformers are different. After 90yrs the cones are brittle and it's probably will need it replaced. The cone is easy to remove just several screws no glue.

If you have an interest I have a 21 speaker but it has no cone, vc, and the opt is open along with the fc. Essentially it's the frame (basket+fc). You can have it for the shipping. I use a shop in NJ to recone and they do the vc and spider too. The fc can be rewound. There has been a couple of guys here that done it. And an opt.

If if get one from epay it could have a open fc or opt.

The 8" pm spkr won't fit properly it's a bit bigger than the original. You'll need the 4pin Philco speaker plug. It's not a 4 pin tube base. A 3100 ohm 20 ohm resistor, 10 and 47 mfd 450v caps. Resistors make lousy power supply filters. Chokes are great! And a se to vc opt.

[artim2205]

Well thank you both for this information. Being unfamiliar with these old radios, the modern speaker idea was my first thought since we don't have a speaker at all and it seemed like a good idea at the time. 

From what you tell me, that's a non-starter since it is a family heirloom. I'll talk to my friend and explain that it might take a little more time, but the search for the original 70 speaker might be the best way to go to get this radio running again. 

You saved me from a lot of work on a project that I don't know much about. The fate of that radio would probably not have been good without your help. 

Thanks to you both for your quick and knowledgeable replies. 

Allan

[John Bartley]

The Philco Radio site has a pretty good summary page on exactly this topic, which speakers can be used where, as well as the FC resistances, etc. I don't know if that info can be reposted here (or anywhere), so maybe someone more familiar with the site can let us know?

I think it would help Allan with his search.

[Ron Ramirez]

I assume you are referring to the file speakers.zip, which is indeed available for download from this site.

It may be found in this thread:
https://philcoradio.com/phorum/showthread.php?tid=215

or, you can just click HERE to download it.

It must be unzipped before you use it, and then requires Microsoft Excel or OpenOffice/LibreOffice Calc to open and view.

[John Bartley]

Hi Ron,

Thank you for the answer about copying from the Philco radio site.

The page I saw was this one, which seemed to be a good guide for Allan :

https://philcoradio.com/library/index.ph...-21-70-90/

cheers

[Ron Ramirez]

Ah. Ok. Yes, that link is more focused on the speakers which the OP would need.

[artim2205]

As part of my searching on the site I found the article about the speaker problems in Cathedral radios in the library. That was the information that made me start to think that the idea of using the 4/8 ohm speakers may not work with the transformers. That 0.7 ohm impedance was the first time I'd seen one like that.

[artim2205]

I just took a quick look at the links in your message Ron. I'll spend some time looking them over this afternoon. I had no idea that all of this type of info was available going back in time so far. It reminds me of someone finding the dead sea scrolls.   I've been restoring a 1965 Thunderbird and its taken me years to dig up technical info on some of the details about this old car. You guys are like the keepers of the archeology of these radios.